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Flashcards in The Basics Deck (23)
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1
Q

Formation?

A

A distinct, recognisable assemblage of rocks that can be represented at the scale of geological mapping

2
Q

Fault?

A

A fracture in rock along which there has been an observable amount of displacement

3
Q

Joint?

A

A fracture with no observable displacement

4
Q

Footwall?

A

Rocks underneath fault plane

5
Q

Hanging Wall?

A

Rocks above the fault plane

6
Q

The Earth is…

A

“The Earth is made up from volumes of rock that are bounded by planar and curved surfaces”

7
Q

Geological Maps?

A
  • The colours represent the outcrop of a formation; where that formation intersects the Earth’s surface
  • The lines represent the intersection between a geological surface and the Earth’s surface
8
Q

Shape of an outcrop?

A

where a formation intersects the Earth’s surface) depends on both the:
• Shape of the Earth’s surface (flat plain, valley, hill)
• Orientation (tilt or dip) of the geological surfaces

9
Q

Joint and Cleavage planes?

A

Joint planes: Aka fracture planes

Cleavage planes: Aka foliation planes; caused when rock is metamorphosed or deformed

10
Q

Foliation?

A

Grains have random orientations initially, then pressure is applied and the grains align and become perpendicular to the applied force

11
Q

Plane definitions?

A

Defined by:
• A line and a point not on that line
• Three non-collinear points (points that do not fall on the same straight line)
• Two (or more) parallel lines
• Two distinct but intersecting lines (conventionally at right angles to each other) (Just like strike and dip)

12
Q

Strike and Dip?

A

Strike line = horizontal
Dip line = perpendicular to strike (steepest line)

Apparent dip = any line that is oblique to the dip line

  • Strike is a 3 figure number
  • Dip is a 2 figure number less than or equal to 90 degrees
  • The dip direction should be written
  • 230/40 S
13
Q

British National Grid?

A
  • OS maps based on a transverse Mercator projection with a central meridian at 2 degrees West
  • Allows use of a Cartesian coordinate system
  • “False origin” lies southwest of Scilly Isles
  • Divides GB into 500 km and 100 km grid squares
  • Each 100 kilometre is designated by two letters or two numbers based on the distance east and north of the False origin
14
Q

Precision of BNG?

A
•	Can then divide 100 km grid squares into 10km and then 1 km
•	Precision: 
o	2 figures to nearest 100km
o	Four figures to nearest 10km
o	Eight figures to nearest 100 meters
o	Ten figures to the nearest 10 meters
15
Q

Outcrop patterns and contours?

A

• Valley cut by a vertical surface: Intersection is straight and cuts across the topographic contours
• Valley cut by a horizontal surface: Intersection is curved and parallel to the topographic contours
• Surface dips towards the foot of a valley: Intersection is curved and cuts across the topographic contours
o The intersection closes (“vees”) down the valley towards the dip direction
• Surface dips towards head of valley: Intersection is curved and cuts across the topographic contours
o Intersection closes (“vees”) up the valley towards the dip direction
• Conical hill
o Intersection is curved and cuts across the topographic contours
• Intersection opens towards the dip direction

• Outcrop pattern depends on the orientation of the geological surface and on the topography

16
Q

Structure contours?

A
  • Lines of constant elevation on a geological surface
  • A horizontal line
  • To determine: look at relationship between geological outcrop and topographic contours
  • Parallel to strike, perpendicular to the dip
  • Planar surfaces have straight, equally spaced structure contours
17
Q

Outcrop patterns from one point method?

A

• Determine or use given strike, dip and dip direction
• Draw the structure contour of the known elevation
• Determine an appropriate vertical interval for the topographic contours to give 50m distances
• Calculate the positions of those structure contours using Tand (dip)=V1 /h1
o So Horizontal distance = V1/Tand
• Draw on the 50 meter structure contours
• Calculate the horizontal distance between the 50m contours and then add structure contours at 50m distances
• Draw the points of intersection between structural contours and topographic contours of equal height
• Join the points together
• Colour the geological map

18
Q

Definition of structure contour?

A

A line of constant elevation on a geological surface that is parallel to the strike direction of the geological surface

The structure contours are straight and equally spaced only if the geological surface is planar

19
Q

Pros and cons of single point method?

A

+ Simple
+ Does not require large amounts of data – single measurement needed
- Does not alter for faults or folding
- Margin of error due to drawing
- Measurement may not be representative of the geological surface overall
- Method assumes the geological surface is planar

20
Q

Two point method?

A

Two or more structure contours are drawn directly from the intersections between the outcrop pattern and the topographic contours

21
Q

Two point method pros and cons?

A

+ Depending on map accuracy it is mathematical = accurate
+ Method is based on an interpretation of the outcrop pattern, which is likely more representative and can be more accurate than a single measurement
- Does not alter for faults
- Relies on accuracy of map
- Mapped outcrop pattern may not be accurate if the exposure is poor and/or sparse
- There may be an insufficient number of intersections between the outcrop and topographic contours to define parallel sets of structure contours

22
Q

Three point method?

A

• Enables us to calculate the orientation of a plane based on the known locations and elevations of three isolated points on the plane

  1. Label the elevations of the three points that lie along the same geological plane in order of highest elevation
  2. Join the three points to form a triangle; calculate the horizontal distances between the highest-intermediate points (Dab) and the highest-lowest points (Dac)
  3. Calculate the intersections of the highest structure contour with the two sides Dab and Dac
    a. X = (ElevationA – intersection/ ElevationA – Elevation B) x Dab
    b. Y = (ElevationA – intersection/Elevation A – Elevation c) x Dac
  4. Repeat for remaining structure contours
  5. Join points of equal elevation to create the structure contours
  6. Now calculate the strike and dip of the plane
23
Q

Three point method, pros and cons?

A

+ Only requires 3 points
+Can be applied to isolated boreholes
- Depends on accuracy of triangle
- - Depends on accurate locations and elevations of points
- Time consuming
- Method assumes the geological surface is planar