Major Geomorphic Theories Flashcards

Question

Geomorphic Theory of Davis: Axioms?

Answer 1

1. Landforms are a product of interplay of endogenetic and exogenetic forces 2. Evolution of landforms takes place in an orderly manner, as a sytematic sequence of landforms develop in response to env changes 3. Streams erode their valleys reapidly downward until graded condition is achieved. 4. A short period, rapid upliftment precedes Davis' Geomorphic cycle 5. Erosion by fluvial agents does not start until the upliftment is completed.

Answer 2

1. Starts after the upliftment ends 2. Valley Deepening: Youth stage is characterised by rapid vertical incision, deepening the valley, due to (i) steep channel gradient which lends high Kinetic Energy to the running water and (ii) potholle drilling by high calibres boulders and pebble carried by the stream dur to the high carrying capacity 3. Summits remain largely unaffected beacuse the streams are small and widely spread. however, they do engage in stream lengthening by headward erosion. 4. As shown in the Figure, UC remains largely unaffected (point 3 above) while LC moves down rapidly (point 2). Hence, Absolute relief largely remains the same while relative relief increases rapidly, with the peak attained by the end of youth stage. 5. Valleys are V-shaped- narrow and deep with steep, convex side slopes. Gorges and Canyons are characteristically found in youth stage. 6. Rapids and waterfalls are found along the longitudinal profile of the river, which practically disappears by the end of youth stage,

Answer 3

1. Mature stage is heralded by marked lateral erosion and well integrated drainage network. 2. Valley deepening is reduced to a large extent due to lower channel gradient, flow velocity and carrying capacity of the river 3. Summits of water divideds are also eroded. In Fig this can be seen in fall of UC indicating lowering of absolute relief. 4. Relative relief lowers due to decrease in valley deepening while summits are downgraded. 5. The lateral erosio leads to valley widening which transforms the V-shaped valleys of youth stage into U-shaped valleys- wide valleys with uniform or rectilinear side slopes. 6. graded conditins spread over a larger area and most of tributaries are graded to base level of erosion by the end of mature stage.

Answer 4

1. Old stage is characterised by almost absence of valley incision while lateral erosion and valley widening remain active 2. Water divides are eroded more rapidly by both downwasting and backwasting. In the figure, UC falls more rapidly indicating rapid fall in absolute relief. 3. Relative relief falls as well due to lack of valley deepening. Valley floor experiences only lateral erosion and almost no vertical erosion owing to very low channel gradient, low KE and maximum entropy. 4. Valleys become almost flat with concave valley side slopes. 5. Old stage duration is many times as long as youth and mature stage combined together. 6. Gradually entire landscape is dominated by graded valley sides, broad, open and gently sloping river valleys with extensive flood plains, well-developed meanders and residual convexo-concave monadnocks. The late old stage is charcterised by the whole landscape turning into an extensively undulating plain of extremely low relief called peneplain.

Answer 5

(1) Davis' model of geographical cycle is highly simple and applicable presented in a very lucid, com pelling and disarming style using very simple but expressive language (2) Davis based his model on detailed and careful field observations. (3) Davis' model came as a general theory of landform development after a long gap after Hutton's 'cyclic nature of the earth history." (4) This model synthesized the current geological thoughts. In other words, Davis incorporated the concept of 'base level' and genetic classi fication of river valleys, the concept of 'graded streams' of G.K. Gilbert and French engi neers' concept of 'profile of equilibrium' in his model. (5) His model is capable of both predictions and historical interpretation of landform evolution (retrodictions).

Answer 6

1. Davis'concept of upliftment is not acceptable. He has described rapid rate of upliftment of short duration but as evidenced by plate tec tonics upliftment is exceedingly a show and long continued process. 2. Davis' concept of relationship between upliftment and erosion is erroneous. Accord ing to him no erosion can start unless upliftment is complete. Can erosion wait for the comple tion of upliftment? It is a natural process that as the land rises, erosion begins. Davis has answered this question. He admitted that he deliberately excluded erosion from the phase of upliftment because of two reasons- (i) to make the model simple and (ii) erosion is insignificant during the phase of upliftment. 3. The Davisian model requires a long period of crustal stability for the completion of cycle of erosion but such eventless long period is tectonically not possible as is evidenced by plate tectonics according to which plates are always in motion. Davis has also offered explanation to this objection. Accord ing to him if crustal stability for desired period is not possible, the cycle of erosion is inter rupted and fresh cycle of erosion may start. However, Davis fails to account for climate change interruptions of his geomorphic cycle. 4. Walther Penck objected to over emphasis of time in Davis' model. In fact, Davisian model envisages 'time-dependent series' of landform development whereas Penck pleaded for 'time independent series' of landforms. According to Penck landforms do not experience pro gressive and sequential changes through time. He, thus, pleaded for deletion of 'time' (stage) from Davis' 'trio'. Further, AN. Strahler, JT. Hack and R.J. Chorley and several others have rejected the Davisian con cept of 'historical evolution' of landforms. They have forwarded the dynamic equilibrium theory for the explanation of landform development. 5. Though Davis has attempted to include strac ture, process and time in his model but he overemphasized time. His interpretation of geomorphic processes was entirely based on empirical observation rather than on field in strumentation and measurement. Though Davis decribed the structural control on landforms but he failed to build any model of lithological adjustment of landforms. 6. Critics point out that the concept of balance between available energy and the work to be done has not been properly explained by Davis. Davis attempted to explain the concept of grade in terms of ability to work (erosion and deposition) and the work that needs to be done It is evident from the essays of W.M. Davis that in the initial stage of landform develop ment (in terms of cycle of erosion) the avail able energy is more than needed to transport the eroded sediment. Thus, the river spends additional available energy to erode its valley. As the river valley is deepened the sediment supply (the work needed to be done increases) for transportation increases but available en ergy decreases. Ultimately, required energy and available energy become equal and a con dition of equilibrium is attained. there are two shortcomings in this concept viz. (1) erosion in itself depends on the mobility of sediments and erosion is never effective in the absence of sediments, (ii) such condition when the whole energy is spent in transporting the sediments and erosion becomes totally absent is practically not possible.

Answer 7

W. Penck's work was posthumously published in the form of 'Die morphologische Analyse' in 1924. He proposed his 'Entwickelung' (development) model of 'Time-independent' landform development as the counter to 'time-dependent cyclin model of WM Davis.'

Answer 8

Walther Penck postulated that, 'geomorphic forms are an expression of the phase and rate of uplift in relation to the rate of degradation. Landforms, thus, reflect the ratio between the intensity of endogenetic processes (Le rate of upliftment) and the magnitude of displace ment of materials by exogenetic processes (the rate of erosion and removal of materials).

Answer 9

The reference system of Penck's model is that the characteristics of landforms of a given region are related to the tectonic activity of that region.

Answer 10

to find out the mode of development and causes of crustal movement on the basis of exogenetic processes and morphological characteristics.

Answer 11

1. Landscape Develpment is time-independent and is a result of competing tectonic and denudational forces 2. Shape of hillslopes depend on relative rates of valley incision by rivers and removal of debris from hillslopes 3. upliftement and erosion are always co-existent 4. primarumpf is initial geomorphic unit for the beginning of the develop ment of all sorts of landforms. Penck definf primarumpf as peneplain type of featureless land surface representing either newly emerged surface from below sea level i.e. a fastenbene or a land surface converted into featureless landmass by upliftment 5. Landscape development begins with upliftment of primarumpf. Penck is supposed to have assumed varying rates of upliftment of primarumpf for the development of landforms. In the beginning the uplift is characterized by exceedingly slow upheaval of long duration and thereafter the rate of uplift is accelerated and ultimately it stops after passing through the intermediate phases of uniform and declerating rates of upheaval.

Answer 12

pleaded for the rejection of Davision model of geographical cycle based on time-depend ent series of landform development. According to Penck landform development should be interpreted by means of ration between diastrophic processes (endogenetic, or rate of uplift) and erosional processes (exogenetic, or rate of ver tical incision) There are three states of adjustment be tween crustal movement and valley deepening viz. (1) if crustal upliftment remains constant for longer period of time, the vertical erosion by the river is such that there is balance between the rate of upliftment and erosion, (ii) if the rate of uplift exceeds the rate of valley deepening, then the channel gradient con tinues to increase till the rate of valley deepening matches with the rate of upliftment and the state of equilibrium is attained when both become equal, and (iii) if the rate of valley deepening exceeds the rate of crustal upliftment, then the channel gradient is lowered to such an extent that the rates of upliftment and erosion become equal and the state of equilib rium is attained. The landforms observed at any given site give expression to the relation between the two factors (uplift and degrada tion) that has been or is in effect, and not to a stage in a progressive sequence" "Penck found escape from the concept of cyclic change marked by the stages youth, maturity and old age". In the place of 'stage' he used the term entwickelung meaning thereby 'development'. Thus, in the place of youth, mature and old stages he used the terms aufsteigende entwickelung (waxing or accelerated rate of development), gleichformige entwickelung (uniform rate of development) and absteigende entwickelung (waning or decelerating rate of development.

Answer 13

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Answer 14

Two processes at play giving rise to Three rates of changes: .1. a= rate of upliftment of summit = rate of valley floor since the whole landscape is being lifted 2. b= rate of valley deepening 3. c= rate of downward erosion of water divide summit In Aufsteigende, a >> b and c is almost 0 In Gleichformige, three sub-phases Phase of uniform Development, a>b and a>c thus both summit valley floor continue to rise though at a lower rate; b=c thus relative relief remains constant Phase of constant relief: a=b=c thus, absolute relief remains constant; relative relief remains constant as well Phase of waning : a=0 and b=c; thus absolute relief decreases while relative relief remains constant in Absteigende, a=0 and b= almost 0 thus both absolute and relative relief decreases

Answer 15

1. Aufsteigende entwickelung means the phase of waxing (accelerating) rate of landform development 2. Initially, the land surface rises slowly but after some time the rate of upliftment is accelerated. 3. Because of upliftment and consequent increase in channel gradient, flow velocity and kinetic energy and of course increase in discharge (not due to uplift) the rivers continue to degrade their valleys with accelerated rate of down-cutting (valley deepening or incision) but the rate of upliftment far exceeds the rate of valley deepening 4. the altitudes of divide summits as well as the altitudes of valley bottoms continue to increase as the rate of upliftment far exceeds the rate of vertical erosion. but the relative or available reliefs continue to increase due to ever-increasing rate of vertical erosion or valley deepening 5. Continuous active downcutting and valley deepening results in the formation of deep and narrow V shaped valleys. As the rate of uplift (aufsteigende entwickelung) continues to increase the V shaped valleys are further deepened and sharpened. The radius of convexity of slopes is reduced with passage of time due to parallel retreat of the steeper slope segments. With the passage of time and more accelerated uplift and degradation the primary peneplain or say primarumpf is surrounded by a series of benches called as piedmont treppen. Each of such benches develops as a piedmont flat, called in German as piedmontflache on the slowly rising margins of the dome.

Answer 16

Gleichformige entwickelung means uniform development of landforms. It is called so because the relative relief remains constant, since rate of erosion of divide summits matches the rate of valley deepening. This phase may be divided into 3 subphases on the basis of rate of uplift and degradation 1. Phase of rising absolute relief ->characterized by still accelerated rate of uplift -> absolute height continues to increase, although at a lower rate than previously, because rate of vertical erosion of both the summits and valley floor is less than rate of upliftment. Maximum altitude (absolute relief) is attained -> valley sides are characterized by straight slopes 2. Phase of constat absolute relief -> Upliftment still continues ->but Altitude (absolute relief) neither increases nor decreases i.e. remains constant due to matching of upliftment by the lowering of divide summit due to denudation. ->he slopes of valley sides are still straight as in phase 2a because of parallel retreat 3. Phase of lowering absolute relief -> upliftment stops cmpletely ->Absolute reliefs or altitudes of summit divides start decreasing because of absence of upliftment but continued erosion of summits of divides. -> Relative reliefs also remain constant because the rate of the lowering of divide summits equals the rate of valley deepening

Answer 17

1. means wanning development of landscape during which the landscape is progressively downgraded. 2. landscape is progressively dominated by the erosional process of lateral erosion and consequent valley widening and marked decrease in the rate of valley deepening through vertical downcutting. 3. Absolute relief (altitude from sea level) decreases remarkably because of total absence of upliftment but continued downwasting of divide summits. 4. Relative relief also decreases because the divide summits are continuously eroded down and lowered in height while downcutting of valley floor decreases remarkably due to decrease in channel gradient and kinetic energy. 5. Parallel retreat of valley side slopes still continues. 6. Now the valley side slope consists of two segments. The uppermost segment is steep-sloped, called gravity slope or boshungen while the lower segment, made of talus materials is gently inclined and is called haldenhang or wash slope. the intersection of boschungen and haldenhang produces sharp knick (break in slope). 7. Boschungen maintains its steep angle inspite of continuous lowering of ridge crests. 8. Haldenhang develops at the base of the valley sides due to rapid parallel retreat of gravity slope or boschungen and consequent elimination of much of the convex waxing slopes. 9. Divide summits are continuously lowered by the intersection of the retreating boschungen of adjoining valleys. Haldenhang or wash slope continues to expand at the cost of upper gravity slopes. 10. In the advanced stage of the phase of absteigende entwickelung the gravity slopes or boschungen are reduced to steep-sided conical residuals called inselbergs.Eventually, inselbergs are also consumed and the whole landscape is dominated by a series of concave wash slopes or haldenhang. Such extensive surface produced at the end of absteigende entwickelung is called ‘endrumpf’, which may be considered equivalent to Davis’ peneplain.

Answer 18

1. Penck followed a deductive approach and did not restrict himself to any particular condition. 2. Compared to the Davisian cycle, Penck's approach was forward looking. 3. Penck, quite appropriately, emphasised the mutual relation between uplift and the deepening of valleys. This indicates Penck's respect for geological evidence. Penck's third stage is evident in the Middle Alps

Answer 19

1. Penck gave too much importance to the role of endogenetic forces 2. The orderliness in landform changes, as assumed by Penck, may be difficult to achieve 3. Inadequate knowledge about the initial pristine landscape does not allow much verification Penck’s model of landscape development, could not be correctly interpretted because of its publication in obscure German language and wrong interpretation of his ideas by English translators. Penck’s morphological system was severely criticised in the USA in the same way as the ‘geographical cycle’ was criticised in Germany. Penck’s concepts of parallel retreat of slope and continued crustal movements became the most sensitive points of attacks by American geologists. It may be pointed out that earlier translation of Penck’s work in English revealed that Penck believed in parallel retreat of slopes but subsequent English translations showed that Penck believed in slope replacement wherein each upper slope unit of hillslope and valley sides was considered to be replaced by lower slope unit of gentler slope. His concept of long continued upliftment and tectonic speculations could not find any support but his concepts of slope development and weathering processes are definitely of much geomorphological significance.

Answer 20

geomorphic system of L.C. King comprises a set of cyclic models such as the landscape cycle, the epigene cycle, the pediplanation cycle, hillslope cycle etc. essentially based on the land scape characteristics of arid, semi-arid and savanna regions of South Africa as studied by him. King during his extensive studies in arid, semi-arid and savanna regions of Africa identified remarkable surfaces of planation, surmounted by isolated hills (inselbergs) and piles of rock boulders (castle koppies). King propounded an entirely new 'cyclic model of pediplanation' in 1948 to account for these unique landscapes as he was convinced that Davisian model of arid cycle of erosion was not competent to explain these landscapes.

Answer 21

There is uniform Development of landforms in varying environmental conditions carried out through the twin processes of scarp retreat and pedimentation

Answer 22

There is insignificant ifluence of climate changes in development of fluvially originated landforms. Major landscapes in all continents have been evolved by rhythmic global tectonic events

Answer 23

1. UNiform development of landform in varying environmental conditions. King formulated his model (theory) on the basis of information of landform characteristics derived through his personal studies of landscape scenery of South Africa having arid, semi-arid and savanna environment and then asserted that his model may be practicable in other parts of the globe. 2. Landscape (African landscape on which his model of pediplanation was based) consisted of three basic elements- rock pediments, scarps and inselbergs. 3. an ideal hillslope profile consists of all the four elements of slope viz. summit, scarp, debris slope and pediments and such hillslopes develop in all regions and in all climates where there is sufficient relief and fluvial process is dominant denudational agent. 4. Each cycle of pediplanation begins with rapid rate of upliftment of previously formed pediplain followed by long period of crustal stability. Thereafter it passes through youth, mature and old stages

Answer 24

After extensive study of South African land scape scenery King was convinced that the African landscape consisted of three basic elements (1) rock pediments flanking river valleys and having concave slope varying in angle from 1.5° to 7° cut into solid rocks, (2) scarps having steep slopes bounding the uplands and varying in angle from 15º to 30° and experiencing parallel retreat due to backwasting by weathering and rainwash. (3) The third element comprises steep sided residual hills known as inselbergs (bornhardts) which vary in size and shape. The size of inselbergs is determined by the magnitude of erosion, less eroded inselbergs are large in size (e.g. mesa) while intensely eroded ones are small in size (e.g. buttes). The shape of these inselbergs depends on the nature of underlying struc ture.

Answer 25

The stage of youth is characterized by initiation of rapid rate of active down cutting of valleys by the rivers consequent upon upliftment. The valleys are so deepened that they assume the form of gorges and canyons Thus, the long profile of the rivers is punctuated by a series of nick points which move upstream. With the march of time active down cutting of valleys is slowed down and as a consequence of which the valley side slopes are characterized by constant slope angles. The form of valley side slope is controlled by physical processes operating on the slopes and lithological characteristics. As downcutting becomes less active, small pediments will begin to appear in the valley bottoms. These will become more extended as interfluve and upland areas are consumed by scarp retreat' (R.J.: Small, 1970). By the late youth most of the interfluves are narrowed down due to scarp retreat and are converted to steep sided hills which are called as inselbergs. The rounded inselbergs are called as bornhardts and castle koppies.

Answer 26

The beginning of mature stage is heralded by the absence of active valley deepening and initiation of lateral erosion. There is backward retreat of valley side slope because of valley widening and hence valley sides are distanced from the channel but there is no significant change in the angle of valley side slope. Extensive pediments varying in slope angles from 5° to 10" are formed at the base of valley side slope. The pediments are of concave slope plan. Continuous erosion and weathering results in pro gressive decrease in the number of inselbergs. Many of the inselbergs are so greatly weathered that they are converted to castle koppies. Gradually, many of the inselbergs and castle koppies finally disappear while there is continuous extension of pediments consequent upon gradual parallel retreat of scarps (upper segment of valley side slope). Eventually. many pediments coalesce to form extensive flat surface termed by King as pediplain which is char acterized by uneven surface with low reliefs and subdued intersecting concave surfaces. The pediplain surface is still characterized by the presence of a few remnants of inselbergs and mounds.

Answer 27

By old stage most of the residual hills (inselbergs) disappear. "The whole landscape will now be dominated by low-angled pediments; the multi-concave surface is the ultimate form (pediplain) of the cycle, the pediplain itself' (R. J. Small, 1970).

Answer 28

According to King the rem nants of original pediplains developed during each cycle are preserved and exist on all summits. 'Particularly where formed in resistant rocks, pediplains and pediplain remnants are believed to achieve great antiquity, so much so that the highest pediplain remnants are believed by King to have formed be fore the break-up of the southern hemisphere conti nental plates in the Jurassic'. King has identified a few antique pediplanation surfaces in Africa, S. America and Australia viz. (i) African Gondwana pediplain (formed in Jurassic period) of 1300 m height having its counterpart at the elevation of 700-1000m in Brazil; (ii) African pediplain (formed in Creataceous period) at two elevations i.e. 600-800m (in the coastal areas of Africa) and 1000-1600m (in the interior of South Africa) which is comparable to Australian pediplain at the elevation of 400-500m.

Answer 29

Similarities 1. both the models are compatible to some extent as both envisage cyclic develop ment of landscape wherein cycle of erosion begins with rapid rate of upliftment of short period followed by long period of crustal stability (tectonic stability or tectonic inactivity). Eventually, the landmass is eroded down to peneplain (Davis) and pediplain (King). 2. Both the landscapes (peneplain and pediplain) have common similarity in that both have antique characteristics, extensive areas and subdued reliefs. 3. Both the models are based on the assumption of completion of all the three stages (youth, mature and old) of the cycle. Differences 1. Besides these similarities, both the models also differ from each other viz. Davis' peneplain is formed due to downwasting while King's pediplain is formed due to coalescence and integration of several pediments which are formed due to parallel scarp retreat. 2. Davis' peneplain, once formed, does not experience further development (growth) until it is,reuplifted. When uplifted, new cycle of erosion is initiated and the rivers are rejuvenated. On the other hand, King's pediplain, once formed, further grows headward. New scarp is initiated at the far end of the previously formed pediplain which is progressively consumed by the retreat of new scarp and thus second pediplain is formed while the former pediplain experiences decrease in its extent. The process continues and a series of intersecting pediplains are formed which extend headward. Thus, King's pediplains, so formed, are analogous to W. Penck's piedmont treppen.

Answer 30

Pros: 1. explains all three stages of landform development 2. based on his field studies in Africa 3. his concept of antique pediplanation similar to polycyclic relief ideas Critique: (a) King’s model was limited to the African experience. (b) It is doubtful to assert that there is uniform development of landscapes in different environmental conditions. (c) King’s concept of antique pediplanation remains questionable.

Answer 31

Based on the two basic issues of slope development i.e. 1. parallel retreat and constant slope angle - dynamic equilibrium, and 2. progressive slope decline with time, there are two distinct schools of thoughts which are based on contrasting views of W. Penck and W.M. Davis All the theories of slope development may be grouped in broad categories- (i) slope decline theory (ii) slope replacement theory (iii) parallel retreat theory

Answer 32

W.M. Davis Davis' theory of slope decline has its roots in his essays on 'the convex profile of badland divides" (1892), 'the grading of mountain slopes' (1898). the geographical cycle' (1899), 'piedmont bench lands and primarumpfe' (1932) etc.

Answer 33

the steepest part of the slope progressively decreases in angle, accompanied by the development of a convexity and concavity'.

Answer 34

Like the cyclic development of landscapes Davis' hillslope and valleyside slope also undergo the process of cyclic development wherein there is progressive decline in slope angle and sequential change in slope form from youth (convex form) through mature (rectilin ear or uniform slope form) to old (concave form) stages. Steep convex slope evolves during youth stage of cycle of erosion because of active downcutting resulting into valley deepening and weathering processes. There is very limited slope retreat and practically there is no decline in slope angle rather it is increased . Lateral erosion dominates over vertical erosion and divide summits are eroded (downwasting of water divides) during mature stage. Thus, downwasting of water divides results in decrease in slope angle (and hence slope decline) and slope profile of smooth curve is formed. Slope becomes graded because at each point on the slope profile the gradient is such that weathered debris may be transported (removed) downslope. Because of continued lateral erosion and downwasting of water divides (and hence marked lowering of relief) there is marked slope decline in old stage so that the general slope form becomes concave and nowhere slope angle becomes more than 5 degree.

Answer 35

Davisian model of slope evolution includes three aspects viz. rounded convexity of hill tops and interfluves, graded waste sheets on slope profiles and graded valley sides. Davis tried to explain their significance and origin. 1. According to Davis summital rounded convexity results from the action of soil creep ( This is the slow movement of soil materials down slopes under the influence of gravity, usually accelerated by rainwash) in humid climate. Soil creep is motivated because of rainwash (the washing away of soil or other loose material by rain) the intensity of which increases downslope. Reasoning on priori grounds that surface wash increases in volume downslope, he supposed that, near drainage divides, the ratio of creep to wash is large. Creep (soil creep) produces 'rounded contours, and is responsible for convex profile of di vides (https://1drv.ms/u/s!AvN_8sA-Zf0djknUgOclAJUdktAU). 2. Graded Waste Sheets: 2.1. The weathered materials existing on slope. profiles (both hillslope and valley side slopes) has been termed as waste sheet which constantly moves downslope by the agents of transportation under the influence of gravity force. 2.2 when the available energy for transportation of de bris and the work to be done (debris to be transported down slope) are equally balanced, the layer of waste sheet of debris on slope profile is called 'graded waste sheet' 2.3. In youthful stage upper slopes are so steep that available energy (transporting capacity) far exceeds the work to be done (debris to be moved downslope) and hence debris is quickly removed and transported downslope because of steep gradient of slope. 2.4. It may be pointed out that condition of graded waste sheet begins from the base of the slope and gradually proceeds upslope. 2.5. The entire slope profile is graded (i.e. transporting capacity equals the total amount of debris to be transported downslope throughout the profile length) by the attainment of old stage of cycle of erosion and the whole slope profile from hill top to the base is covered with sluggishly moving rock waste. There is balance betn supply and removal of debris at all points of a graded slope profile 2.6. in the initial stage graded slope profile has steeper gradient and thin veneer of coarse debris (waste) but as the stage of cycle of erosion advances the gradient of graded slope profile declines, debris becomes finer and debris thickness increased. 3. Graded Valley sides 3.1. gradient of valley side slope also decreases as the stage of cycle of erosion advances with time 3.2. valleyside slope is dominated by summital convex ity (in the upper part of the valley sides) and basal concavity. 3.3. With the advancement of cyclic time (stages) the radius of the curvature of valley side slope profile increases because of gradual increase in the length of convex and concave segments. This again denotes flattening of slope profile and result ant decline in slope angle

Answer 36

by W. Penck Three sub-parts 1. Process-Response model of Slope evolution 2. Parallel Retreat and Slope replacement model of Penck 3. model of slope development on the basis of rate of stream erosion

Answer 37

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Answer 38

a steep rock face left to itself, moves back upslope, maintaining its original gradient; and a basal slope of lesser gradient develops at its expense'. The mechanism is as follows 1. For the explanation of evolution of hill slope Penck selected a steep rock cliff of homogeneous composition. 2. The upper surface of the slope unit is surmounted by level surface (fig. 15.11, A-F). 3. There is a river at the foot of the slope which is neither eroding nor depositing but is capable of removing all the materials coming at the foot-hill from upslope segments (fig. 15.11(A)) 4. 'In a unit time a superficial layer of rock, of a definite thick ness the same everywhere, is loosened and removed. 5. For this to happen the gradient must be too great to allow the little pieces of rock, just loosened by weathering but not further comminuted and reduced, to remain at rest. This gradient is available for each unit of rock face except the lowest. Thus, the slope face except the lowest segment (A-B, fig. 15.11 (A) undergoes parallel retreat due to uniform rate of weathering and instantaneous removal of weathered materials from slope segments. The lowest segment does not experience parallel retreat because its slope angle is not such that it may attain required mobility which may help in the removal of weath ered materials. 6. Thus, the lowest segment of initial cliff slope face (A-A', fig. 15.11(A) is replaced by a new (young) unit of gentle slope angle (A-B) and the cliff slope profile now consists of two slope units viz. A-B and B- B'. 7. The process repeats and slope profile shifts from AA' to BB' to CC' to DD' to FF'. At the same time, the basal slope develops along A-B-C-D-E-F. The summit is also consumed due to the parallel retreat of the slope. 8. If the parallel retreat is occurring on both the sides of an interfluve, then after the removal of free face or steep segments on both the sides the interfluve is subjected to downwasting and thus there is successive lowering of altitude 9. If we look at Bhander plateau (M.P) it clearly appears that the upper free face segment of massive sandstones of Bhander escarpments is under the process of parallel retreat and the lower segment of rectilinear slope (of shales) is extending upslope. The parallel retreat is complete over Sharda Pole hill (fig 3.8) and hence downwasting has resulted into the development of convexo-com cave profile. 10. https://1drv.ms/u/s!AvN_8sA-Zf0djktrZmULm7-UmFVD

Answer 39

1. Penck has also applied his model to explain the development of debris covered slope. 2. Reduction begins at the basal slope and continues till the entire basal slope attains such mobility that the debris resting on the slope becomes mobile. 3. The required mobility (for the removal of debris) on basal slope is much higher than on the cliff slope because the gradient of the former is very low in comparison to the latter. 4. With the attainment of this situation (re quired mobility at the basal slope) the rock debris begins to move downward on all segments of slope profile. except the lowest slope segment because of lack of sufficient slope gradient. Thus, the basal slope having its original inclination extends upslope. 5. A new slope unit of still gentle gradient develops at the base of initial basal slope. Later on, this slope unit is also replaced by new slope unit of further gentle gradient. This mechanism repeats itself and thus a new slope unit of gentle slope is produced at the base of slope segment. Thus, 'flattening of slope always occurs from slope base and proceeds upslope'.

Answer 40

Penck has also proposed a model of development of slope forms on the basis of rate of stream erosion. In the case of accelerating rate of stream erosion, the valleyside slope becomes convex while decelerating rate of erosion causes the development of concave valley side slope. On the other hand, constant rate of stream erosion results in the devel opment of rectilinear slope https://1drv.ms/u/s!AvN_8sA-Zf0djkz52Ld65YVAN2P9

Answer 41

1. A. Young (1972) has pointed out two errors in this second model of Penck (i) the application of the mechanism of rockfall and instantaneous removal of debris from cliff slope to the regolith covered slope is unrealistic, and (ii) Penck's assump tion that all parts of basal slopes or its later replace ment slopes are equally exposed to weathering is erroneous. It may be mentioned that period of exposure is maximum at the lowest segment of slope profile and decreases upslope and becomes zero at the intersection point of upper slope segment. 2. It is not clear from the close perusal of the interpretations of Penck's model by some scientists as to whether slope development takes place in discrete stages, or whether this was just a device to be adopted for explanatory purposes. If this replacement is not in discrete stages but continuous, then no part of the concave slope retreats parallel to itself. Also not clear whether slope profile, during its evolution ary stages, consists of several intersecting rectilinear segments or it becomes concave profile Textual passages can be found in support of either interpretation. 3. H. Mortensen (1969) has expressed doubt about the parallel retreat of cliff rock face by uniform rate of weathering. It may be mentioned that Penck's model becomes valid where there is continuous and instantaneous re moval of regoliths from slope but the model be comes invalid where removal of regolith occurs in stages

Answer 42

If we look at Bhander plateau (M.P) it clearly appears that the upper free face segment of massive sandstones of Bhander escarpments is under the process of parallel retreat and the lower segment of rectilinear slope (of shales) is extending upslope. The parallel retreat is complete over Sharda Pole hill (fig 3.8) and hence downwasting has resulted into the development of convexo-com cave profile.

Answer 43

1. rejected the basic. tenet of climatic geomorphology that 'the varia tions in slope forms and slope elements depend on climatic types 2. According to King a standard (ideal) hill slope consists of all the four elements e.g. summital convexity, free face, rectilinearity and basal concavity. Such ideal hillslope is the natural products of normal processes of slope evolution involving fluvial process (flowing water) or massmovement or both. 3. He further maintains that full development of slope elements depends on local conditions i.e. resistant and strong bedrocks and bold and sufficient reliefs. If either of these condi tions are absent, free face (scarp) and debris slope do not develop and thus a waning convexo-concave slope is formed. 4. If scarp is present then pediplanation cycle. othrwise a completely different process at play for slope development

Answer 44

According to King pediplain formation is the result of twin processes i.e. scarp retreat and pedimentation. In the initial stage of hillslope development, the scarp face experiences parallel retreat due to backwasting caused by weathering of exposed rocks. This parallel retreat of scarp controlls the evolution of entire hillslope. According to King the debris slope just below the scarp or free face does not extend upslope and hence it is neither capable of obscuring nor destroying the free face element. It means that there is a balance between the supply of debris from upslope and removal of debris downslope on this section i.e. debris slope. Similarly, there exists balance between debris supply and debris removal at hill crest and hence it remains constant. The gradual parallel retreat of free face and rectilinear elements (debris slope) results in the formation of pediment of concave plan at the base of the hillslope. As the parallel retreat of free face and debris slope continues, their lengths decrease and pediments extend upslope at the cost of rectilinear and free face elements. Ultimately, upper slope elements mainly free face disappear and pediments extend upto hill crest. Thus, an extensive erosion surface of concave slope is formed. This surface is called pediplain, which is, in fact, the product of coalescence of several pediments (fig. 15.15). Thus, the whole process of development of pediplain and the entire process of slope evolution during this period is called 'pediplanation cycle'and hillslope evolution cycle'. https://1drv.ms/u/s!AvN_8sA-Zf0djk0fqcSRauSRS9Ks?e=eDZ8J6 https://1drv.ms/u/s!AvN_8sA-Zf0djk5HNkDXCAZX7PiK?e=S42HBt

Answer 45

King has further maintained that if the scarp is absent due to lack of resistant rocks and bold relief then the process of slope evolution would be entirely different from the process of parallel retreat and pediplanation. There will be regular decrease in maximum slope angle and extensive convexo-concave slope is produced. According to King the hillslope with scarp is considered normal because scarp (free face) is the active element of slope retreat. Wherever scarp is absent, the present slope form is not the result of present geomorphic processes. Based on this assumption King has maintained that extensive convexo concave slopes of the temperate regions of the north ern hemisphere are, in fact, relict forms and are not the result of present day processes rather they are the result of past periglacial processes. These have not been formed due to scarp retreat rather these have developed because of weathering and erosion of weak rocks.

Answer 46

Pros: validated in some examples in india Critique: 1. King did not survey slopes in the field and did not measure slope making and controlling processes but based his model on field observations alone. Lacked measurements of either process or forms. King has used assertions rather than evidences in support of his model. 2. Rejected any influence of climate on hillslope development 3. King's model of slope evolution is, in fact, not original concept of his own rather it is amalgamation of views of earlier workers like A. Wood, T.J. Fair, R.E. Horton etc. 'Evidence from the work of others is selectively mustered in support of preconceived views.

Answer 47

close observations and selective measurement of slope angles in certain parts of foreland of Indian peninsula viz. Chotanagpur highlands (Bihar), Rohtas plateau (Bihar), Kaimur ranges, Rewa plateau, Bhander pla teau (M.P.) etc. give evidence of precipitous scarps (free face element) which are undergoing the process of parallel retreat due to backwasting caused by physico chemical weathering of well jointed massive sandstones (of scarps) resting over shales and siltstones forming debris or rectilinear slope. Bhander plateau having imposing escarpments on its eastern, south eastern and southern sides consists of all the four slope elements (crestal convex, free face, rectilinear and basal concave elements) whrein scarp or free face element is subjected to parallel retreat. A few of the detached hills having sandstone capping are the examples of mesas characterized by flat top but steep scarps on all sides. These scarps are experienc ing parallel retreat due to backwasting.

Answer 48

Penck's model is more of a slope replacement model. The scarp or free face of slope does undergo parallel retreat but in the process is replaced by the gentler slope frm below. Further the backwasting is not limited to free surface but the debris slope is further consumed by even gentler slope. Thus, a series of gentler slopes replaces the previous slope from below. In comparison, King's slope's free surface is the the active element of slope retreat. The debris slope does not replace the free surface but also undergoes parallel retreat and is instead replaced by the pediment -basal slope.

Answer 49

similar to Davis's cycle of Erosion, but still Complete: https://1drv.ms/b/s!AvN_8sA-Zf0djlIbOvj9G7GHpSfy?e=Qb0Yfl Youth stage: https://1drv.ms/u/s!AvN_8sA-Zf0djk9kKZmM9ay8-SPC?e=CyZzl3 Mature stage: https://1drv.ms/u/s!AvN_8sA-Zf0djlAQUXdjCibY9SNH?e=m0CRZB Old Stage: https://1drv.ms/u/s!AvN_8sA-Zf0djlEfoV4niIubTQee?e=cscZvo

Answer 50

Any sort of obstacle in normal functioning of CoE is called Interruption The completion of normal cycle of erosion depends on tectonic stability of longer period of time, which is seldom possible in the nature as the earth is unstable. Thus, the cycle of erosion is liable to frequent. The basic causes of interruption may be cli matic or tectonic or both. cycles punctuated by interruptions are called as interrupted cycles which lead to occurrences of several cycles in a region: Such cycles are called polycycles. If the polycycles (multi-cycles) occur in succession, they are called successive cycles of erosion and the landscapes resulting therefrom are called polyclic or multi-cyclic landscapes. Applachian Regions of the U.S.A. and Chotanagpur Region of India present typical examples of polyclic landscapes where several cycles have been com pleted. Paired terraces, valley in valley topography, and incised meanders of the Damodar Valley at Rajroppa in Hazarbagh (Bihar) and valley in valley topography and paired terraces, Dhunwadhar falls and incised meander of the Narmada river at Bheraghat near Jabalpur (M.P.) are indicative of rejuvenation and polycyclic reliefs.

Answer 51

The widespread volcanic fissure flow causes upwelling and pouring of immense volume of basaltic lava which covers larger area and obliterates surface drainage and reliefs which results in the permanent interruption and closure of the existing chapter of existing fluvial cycle of erosion. The fresh cycle of erosion may start only when the fissure flow ceases, lavas are cooled and solidified, new surface is formed and new sets of streams are initiated. It may be pointed out that such interruptions in the fluvial cycle occurred over Indian peninsula during Cretaceous period when Deccan lava flows covered vast areas of the Deccan plateau including Chotanagpur plateau and even southern parts of the Vindhyan upland causing closure of the Jurassic cycle of erosion. The new Tertiary cycle was initiated only after the Deccan trappean lavas were cooled and solidified and monsoon climate set in.

Answer 52

Climatic interruptions (accidents) occur due to major changes in the climate of the concerned region. for example, if the fluvial cycle of erosion in a humid region is passing through mature stage, and there is sudden climatic change leading to onset of either extreme dry conditions or extreme cold condi tions, then the cycle of erosion is interrupted to such an extent that the chapter of current cycle of erosion is closed and another set of cycle either arid cycle (if the climate becomes extreme hot and arid) or glacial or periglacial cycle (if the climate becomes glacial/ periglacial or sub-glacial) sets in. It may be men tioned that if there are minor changes in the climate, then the chapter is not closed, rather it (cycle) is either augmented or slowed down. Suppose, if the climate becomes more humid leading to increased amount of rainfall, then the surface runoff and stream discharge will automatically increase which would cause local interruption in the cycle by accelerating the rate of erosion (case of rejuvenation), the effects of which may spread over larger areas. Can also lead to base level changes due to glaciation or deglaciation. glaciation will rejuvenate the river while Deglaciation will speed up the cycle

Answer 53

subsidence or upliftment of landmass

Answer 54

Rejuvenation simply means acceleration of erosive power of the fluvial process (rivers) caused by a variety of factors. Rejuvenation lengthens the period of cycle of erosion. For example, if the cycle of erosion is passing through senile stage (old stage) characterized by gentle channel gradient, sluggish river flow and broad and shallow alluvial valleys, after rejuvenation (caused either due to substantial fall in sea-level or due to upliftment of landmass) the cycle is interrupted and is driven back to juvenile (youth) stage characterized by steep channel gradient and acceler ated valley incision. One ancient example of rejuvenation is the Nile, which was rejuvenated when the Mediterranean Sea dried up in the late Miocene. Its base level dropped from sea level to over 2 miles below sea level. It cut its bed down to several hundred feet below sea level at Aswan and 8000 feet below sea level at Cairo. After the Mediterranean re-flooded, those gorges gradually filled with silt 1. types 2. causes 3. topographic expressions

Answer 55

Rejuvenation is of 3 types 1. Dynamic Rejuvenation Causes : (i) upliftment in the landmass (ii) tilting of land area (ii) lowering of outlet 2. Eustatic (widespread) Rejuvenation Causes : changes in sea-level due to (i) diastrophic events (subsidence of sea-floor or rise of coastal land) (ii) glaciation causing fall in sea-level 3. Static Rejuvenation Causes : (i) decrease in the river load (ii) increase in the volume of water and conse quent scream discharge due to increased rainfall or melt-water (iii) increase in water volume of the main river due to river capture

Answer 56

1. negative change in the base level of erosion (which is determined by the sea-level) : caused by a host of factors. -> Negative change of base level of erosion is always related to negative change in sea-level (fall in sea level) which is also called as eustatic movement as it is widespread and global phenomenon. -> fall in sea-level steepens the channel gradient resulting into increased kinetic energy of the fluvial process which resorts to valley incision with renewed vigour. -> The eustatic negative change in sea-level is caused during glacial ages when most of seawater is locked on the continents as thick cover of ice sheets. The consequent lowering of sea-level causes steepening of channel gradient of streams which are, in fact, rejuvenated and are engaged in active downcutting of their valleys. -> The Pleistocene glaciation of the northern parts of N. America and Eurasia caused widespread rejuvenation in the tem perate and tropical zones. Four river terraces of the Red river of the U.S.A have been related to four periods of advancement of ice sheets i.e. Nebraskan, Kansan, Illinoin and Wisconcin glacial periods of the Pleistocene Ice Age. -> Negative change in sea-level causing rejuvenation locally and regionally is also caused because of subsidence of sea-floor in relation to coastal land due to tectonic factors. 2. Local or regional upliftment of landmass -> causes interruption in the fluvial cycle of erosion and reju venates the fluvial processes (streams). -> Such type of regional rejuvenation caused by secular rise in the landmass has been reported from several parts of the Chotanagpur Highlands of Bihar which experienced 3 phases of upliftment in response to three episodes of upliftment of the Himalayas during Tertiary pe riod. The Patlands of the Ranchi plateau and Plamau uplands (Bihar) were subjected to an upliftment of 305 m resulting in the interruption of fluvial cycle of erosion and rejuvenation of the N. Koel river and its tributaries. The nick points and resultant waterfalls on Burha river (Burhagaugh falls 142 m, Gutamghaugh falls, 36.57 m and Gharaghughra falls, 7.62 m) a tributary of the N. Koel river, on Pandra river (Ghagri falls of 43 m), on the Sankh river (Sadnighaugh falls of 61 m), on the Jori river (Jalmighaugh falls of 37 m), on the Ghaghra river (Nindighaugh falls of 45.72 m) etc. indicate rejuvenation. 3. Lowering of outlets of streams also causes rejuvenation due to release of extra volume of water in the concerned river. 4. Such rejuvenation (due to increase in the volume of water) also occurs when the water supply suddenly increases due to river capture (supply of extra water of the captured stream to the captor stream).

Answer 57

1. Topographic dissonance 2. Valley-in-valley or multi-valley topography 3. uplifted peneplains 4. incised meanders 5. paired terraces 6. nick points

Answer 58

Topographic discordance refers to the creation of older topographic forms above and younger forms below. In other words, when the topographic accordance or uniformity from the top of the river valley to its bottom is not maintained rather is dis turbed due to interruption in fluvial cycle of erosion caused by rejuvenation, the resultant topographic expression is called topographic unconformity or discordance wherein the upper part of the valley reveals the sign of senile or mature stage whereas the lowest part of the valley belongs to youth stage. Topographical discordance leads to formation of Valley in Valley topography' or paired terraces. example: The Damodar river developed its broad and flat valley of senile stage before the onset of Tertiary upliftment. The river was rejuvenated due to upliftment of landmass during Tertiary period caused by the side effects of the Himalayan orogeny and thus the Damodar exca vated its new deep and narrow valley of youthful stage within its broad and flat valley of senile stage. also observable in the alley of the Narmada river at Bheraghat (down stream from Dhunwadhar falls, 15 km away from Jaballpur city in M.P.).

Answer 59

It refers to the peculiar tpography wherein a relatively younger, narrower valley is formed within an older broader valley of a river because of rejuvenation of the river's life cycle. example: The river develops flat and shallow valley at the end of mature and beginning of senile stage but if there is sudden negative change in the base level of erosion caused either by fall in sea-level or upliftment of landmass, the river is rejuvenated and begins active downcutting of its valley due to increased erosive power. Thus, a deep and narrow valley is formed within flat and broad valley. This new deep and narrow valley is flanked by terraces on its either side which represent earlier older valley. Such topography is called 'valley in valley topography' or 'two storeyed valley' or 'two-cycle valley'. By the march of time the rejuvenated river deepens its valley to the new base level and thus forms second broad and flat valley within the first broad and flat valley formed during first cycle of erosion. Suppose, if there is again upliftment of landmass or fall in sea-level, the cycle is again interrupted and the river is rejuve nated. Consequently, again a narrow and deep valley is formed within previously formed two storeyed valley. Now, there are two paired terraces on either side of new deep and narrow valley. Thus, the resultant form of the valley is called three storeyed valley. Such topographic forms, where there are more than on valley in a single river's cross profile, are called 'multi-storeyed or multi-cyclic valleys'. Examples: The Damodar valley at Rajroppa in Hazaribagh (Bihar) is a typical example of poly cyclic valley or topographic discordance which is characterized by two-storeyed valley. The Damodar river developed its broad and flat valley of senile stage before the onset of Tertiary upliftment. The river was rejuvenated due to upliftment of landmass during Tertiary period caused by the side effects of the Himalayan orogeny and thus the Damodar exca vated its new deep and narrow valley of youthful stage within its broad and flat valley of senile stage. The Bhera river coming from over the Ranchi plateau makes a water fall while joining the Damodar river and thus presents an example of a hanging valley. Such topographic discordance is also observable in the alley of the Narmada river at Bheraghat (down stream from Dhunwadhar falls, 15 km away from Jaballpur city in M.P.). Three storeyed valleys have been located in the Himalayas where the rivers were rejuvenated thrice due to three major episodes of upliftment of the Himalayas during Tertiary orogeny. Nearly all the major rivers of the U.P. Himalayas are flanked by three sets of terraces on their either side. https://1drv.ms/u/s!AvN_8sA-Zf0djlNXd7hcRFuWDaFg?e=X1rxGs

Answer 60

Terraces of the same elevation on opposite sides of either a stream or river are called paired terraces. They occur when river downcuts evenly on both sides and terraces on one side of the river correspond in height with those on the other side. Paired terraces are caused by river rejuvenation. Unpaired terraces occur when either a stream or river encounters material on one side that resists erosion, leaving a single terrace with no corresponding terrace on the resistant side Nearly all the major rivers of the U.P. Himalayas are flanked by three sets of terraces on their either side.. https://1drv.ms/u/s!AvN_8sA-Zf0djlNXd7hcRFuWDaFg?e=X1rxGs River terraces are particularly useful for settlements as they provide flat areas above the present floodplain. Oxford, Cambridge and London all developed on the river terraces of the Isis, Cam and Thames respectively.

Answer 61

Uplifted peneplains are formed due to interruption caused by rejuvenation consequent upon regional upliftment. The uplifted peneplains are represented by their remnants of accordant summit levels which rise above the general ground surface of the present-day planation surface. Uplifted peneplains are, in fact, the results of successive cycles of erosion wherein several fluvial cycles of erosion are completed in succession. Three uplifted peneplains have been identified in the Applachians (which are indicative of successive phases of upliftment, consequent rejuvenation and cycles of erosion) e.g. (from older to younger) viz. Schooley peneplain (after Schooley Mountain), Harrisburg peneplain (after Harrisburg Mountain), and Sammerville peneplain. The Patland of the Ranchi Plateau is a typical example of uplifted peneplain which is higher than the central Ranchi plateau (610 m a.m.s.l). The granitic-gneissic surface (910 m a.m.s.1) of the western highlands has a capping of 154 m thick basaltic lava (now weathered to laterites) cover of Cretaceous period. Prior to Cretaceous lava flow the entire Ranchi plateau, including the present western highlands, was peneplained by Jurassic pe riod, the western part of which received lava cover of 154 m thickness during Cretaceous period. This western part (610 m + 154 m lava) was uplifted by 305 m in Tertiary epoch and thus the granitic gneissic surface of 915 m height lying below 154 m thick lateritic basalt is an example of uplifted peneplain (fig. 16.5). The North Koel and its numerous tribu taries have dissected patlands and segmented them into several small tableaus locally known as 'pats' (which are fine examples of mesas and buttes) such as Netarhat pat, Khamar pat, Rudni pat, Jamira pat, Raldami pat, Bangru pat etc. https://1drv.ms/u/s!AvN_8sA-Zf0djlQCqyxvHCAHMT3q?e=6PlUBs

Answer 62

AKA incised meanders, entrenched meanders, intrenched meanders, inclosed meanders and ingrown meanders. Incised meanders are the representative features of rejuvenation and polycyclic reliefs and are developed through vertical erosion leading to valley incision consequent upon renewed erosive power due to rejuvenation. The narrow and deep meanders formed due to accelerated rate of valley incision caused by rejuvenation within simple broad meanders (having wide and shallow valleys) developed by lateral erosion during 1st cycle of erosion are called incised meanders which are further divided into (i) entrenched mean ders (having uniform slopes of both the valley sides of meander loops). entrenched meanders are symmetrical and form when the river downcuts particularly quickly. Due to the speed which the river downcuts, there is little opportunity for lateral erosion to occur giving them their symmetrical shape (ii) ingrown meanders which have unequal slopes of valley sides wherein one side of the valley representing concave side is deeply undercut and the outer side (convex side) is charac terized by gentle valley slopes. Ingrown meanders are asymmetrical. They form when the river downcuts at a less rapid pace, giving the river opportunity to erode laterally as well as vertically. The meandering valley of the Karo river downstream from Pheruaghaugh falls at the southern margin of the Ranchi plateau has been considerably incised due to rejuvenation and hence presents an ideal example of incised meander. The Damodar gorge near Rajroppa is typical example of incised meander. Similarly, Bheraghat gorge of the Narmada near Jabalpur (M.P.) is fine example of incised meander.

Answer 63

Knick point, simply called as nick point or only 'nick' represents breaks in slope in the longitudinal profile a river caused by rejuvenation (https://1drv.ms/u/s!AvN_8sA-Zf0djlUBnRPFYHH5Ryy3?e=CngMKY). This is why nick point is also called as head of rejuvenation which registeres gradual recession upstream. These breaks in channel gradient or nick points denote sudden drops of elevation in the longi tudinal profile of the rivers and allow water to fall down vertically giving birth to waterfalls of varying dimensions. These are called as nick point falls or simply nickfalls. Hundru falls (76.67 m) on the Subarnarekha river (near Ranchi city), Johna or Gautamdhara falls at the confluence of the Raru and Gunga rivers (to the east of Ranchi city), Dassam falls (39.62 m and 15.24 m) on the Kanchi river (east of Ranchi city), Burhaghaugh falls (148 m) on the Burha river, a tributary of the North Koel river, Dhunwadhar falls on the Narmada river (near Jabalpur, M.P.), major falls of Rewa plateau-M.P. (e.g. Chachai falls - 127 m on the Bihar river, Kevti falls -98 m on the Mahana nadi, Tons or Purwa falls - 75 m on the Tons river, Odda falls - 145 m on the Odda nadi etc.) are the examples of nick points caused by rejuvenation.

Answer 64

Denudation chronology simply means reconstruction of denudational history of a given region. The primary goal of this ap proach is to reconstruct the chronological history of denudation of a given region known as denudation chronology and 'to identify, date and interpret planation surfaces developed in past cycles and subcycles of erosion' This is primarily based on 'historical or chronological approach of landform study which involves the basic concepts 'that there is sequential change in landforms through time', 'principles of uniformitarianism,' 'cyclic nature of earth's history', 'palimpsest topography' and Davisian model of 'cyclic evolution of landforms'. involves description of landform evolution through successive stages of geological time or say cyclic time involving longer geological time and larger spatial scales based on the assumption that evidence of the past character of the landscape is still apparent in its present form' In fact, denudation chronology is based on the concept of 'palimpsest topography' which means such a surface which bears the imprints of geomorphological processes during past geological periods after partially erased initial imprints (features) in the beginning. Palimpsest refers to that manuscript which has been written, erased. and rewritten several times. Similarly, palimpsest topography represents complex topographic features of a region which have been written (character ized by topographic features) by geomorphological processes, erased (previous geomorphological fea tures partially destroyed by succeeding processes) and rewritten (production of new reliefs on older surfaces) several times. An attempt is made to reconstruct the geomorphic history of the region concerned on the basis of present and remnant (relict) landforms following the dictum of 'present is key to the past'. The denudation chronology approach suffers from certain perceptible weaknesses. (i) This approach is highly deductive because unknown events and their responses are described on the basis of very limited known information and evidences. In fact, the past geomorphic history is reconstructed on the basis of very small parts of the earlier landforms. (ii) historical approach is highly deductive and speculative because the old erosion surfaces and remnant forms have been so greatly modified by subsequent processes that it becomes difficult or say impossible to find out their original forms and initial heights. (iii) the dating of erosion surfaces is also highly speculative as valid geological evidences are not available.

Answer 65

The almost plain topographic surfaces having undulating ground surface and remnant low reliefs caused by dynamic wheels of denudational processes and cutting across geological formations and structures are generally called erosion or planation surfaces. Erosion surfaces form significant elements of landscape of a given region and provide prominent clues for the reconstruction of denudation chronology (erosional and depositional history) of that region. peneplains, panplains. pediplains and planes of marine erosion are all erosion surfaces in the accepted sense of the term (R.J. Small, 1970). Besides, etch plains, cryoplains (by periglacial processes) etc. are also major erosion surfaces. Besides, there are some minor erosion surfaces viz. valley side benches (terraces), river terraces, marine benches (platforms), marine flats, marine terraces, raised beaches etc. It may be mentioned that erosion or planation surfaces widely differ from rock-cut structural benches, as erosion surfaces cut across geological formation and structures while structural surfaces are structurally controlled. For example, if horizontal soft (weak) rock beds overlie horizontal resistant (hard) rock beds ( https://1drv.ms/u/s!AvN_8sA-Zf0djlZbXIfDQs60QyqS?e=HoZlAs ), soft rock beds are more or less uniformly corroded and thus are almost entirely removed and underlying relatively hard rock beds are exposed to environmental processes. The surface of such lithological formation is called structural surface. It is, thus, apparent that structural surfaces are formed due to removal of overlying weak strata by denudational processes and parallel to the exposed underlying strata and their dip angle. On the other hand, erosion surfaces are formed due to erosion of different rock types (soft and hard) and different geological structures (folded, faulted. uniclinal etc.) alike .

Answer 66

it is very difficult for erosion surfaces to be found very close to present sea-level (base level of erosion) because, after their formation (peneplana tion), these have been largely affected by tectonic movements (uplifting, subsidence, upwarping etc.). Most of the erosion surfaces have been uplifted and thus there is every likelihood that they must exist relatively at higher elevations. If the erosion sur faces are found close to the present sea-level, it clearly means that it has experienced subsidence resulting in the lowering of its height. It is also important to note that sea-level in itself is not stable rather it has experienced several phases of positive (rise) and negative (fall) changes due to climatic changes (glacial and interglacial ice ages) and tec tonic movement (upliftment and subsidence of coastal land and sea floor) in the past geological and geomorphic history of the earth e.g. Carboniferous and Pleistocene ice ages punctuated by interglacial periods. Even Quaternary period registered several phases of sea-level changes which effected changes in base levels of erosion. 'At the onset of Quaternary era, the sea-level was some 600 ft (182.88m) higher than present It may be mentioned that post-Quaternary true erosion surfaces are difficult to be found be cause these could not be formed due to lack of sufficient time required for the completion of such surfaces. That is why erosion surfaces younger than Tertiary era are not found but partial erosion sur faces might have developed. It is, thus, evident that no post-Tertiary ero sion surface may be possible. The surfaces may be of Tertiary period or older than Tertiary. If this is the case, the erosion surfaces might have undergone substantial changes due to longer period of their formation and existence. The identification and determination of erosion surfaces are accomplished on the basis of certain morphometric techniques e.g. altimetric frequency histograms and curves and superimposed profiles and field checks. The frequency maxima of spot heights, summit levels, benches and shoulders as revealed by frequency histograms and curves of these variables separately and together using running-sum-class intervals indicate different levels of surfaces but this technique does not reveal the fact as to whether these are structural and tectonic surfaces or are erosion surfaces. The nature of surfaces (whether structural or erosional) may be determined by field observa tion and ground checks. The altimetric frequency histogram and curve (fig. 17.2) of the Belan basin (U.P.) reveal three frequency maxima viz. (i) at the height of 1000-1200 ft (305 m-366 m) (X 1), (ii) at the height of 800 feet (240 m) (X 2) and (iii) at the height of 500 feet (150 m) (X 3). Besides, a fourth (though weak) frequency peak is observed at the height of 1350-1400 ft. (427 m). These surfaces (from higher to lower elevations) are, in fact, Kaimur surface (427 m), Panna surface (1000-1200 ft or 305 m-366 m), Rewa surface (800 ft or 240 m) and Trans-Ganga-Yamuna surface (500 feet or 150 m). The crowding of several serial profiles at the same level denotes a surface, the nature of which (i.e. whether of structural or erosional origin) may be determined on the basis of minute field observations and field checks. The nature of modification of erosion surfaces depends on a number of factors viz. age of the surfaces, thickness of later deposits on the surfaces, relative hardness of rocks of which the surfaces have been formed, number of spacing of streams (i.e. stream frequency and drainage density) etc. It is simple assumption that, older the erosion surfaces, greater the modifications and changes i.e. older erosion surfaces are subjected to more changes and modifications by environmental factors than younger surfaces. The changes and modifications of old surfaces may be negated or minimized only when these are protected by overlying thick layers of later deposition of sediments brought down by denudational processes. The degree of changes and modifications suffered by erosion surface depends on the relative resistance of rocks forming those surfaces. For ex ample, if the erosion surface consists of resistant rocks, then after its upliftment relatively longer period of time would be required for the completion of next cycle of erosion because of slow rate of denudation, while the erosion surface formed of weak rocks, when uplifted, would be peneplained in relatively short period of time due to faster rate of denudation. Based on this basic premise it may be inferred that if two erosion surfaces are found side by side at the same elevation but their lithological characteristics are different i.e. one surface has de veloped on resistant rocks while the other has devel oped on relatively soft and weak rocks, then the former erosion surface would be much older than the latter surface. The erosion surfaces having high stream frequency and high drainage density suffer more modifications and are destroyed much earlier than the surfaces having poor stream frequency and low drainage density. There is variation in the nature and form of erosion surfaces. The relatively younger surfaces (i.e. late Tertiary and pre-Quaternary surfaces) and formed on resistant rocks are found in the form of extensive plateaus having accordant heights and flat-topped interfluves. This is because of the fact that such surfaces are least dissected by streams due to lack of requisite period of time. On the other hand, if the surfaces are of pre-Tertiary or Mesozoic age, these are found in much dissected condition wherein the surfaces are segmented into numerous parts which exist in the form of accordant summit levels. If the surfaces are older than Mesozoic, then there is every possibility that the surface might have been obliterated by the dynamic wheels of denudational processes.

Major Geomorphic Theories Flashcards

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