Fold terminology (Geological Maps and Structures)

Question 1

What is the hinge zone?

Answer 1

The region of the folded surface with the highest curvature

Question 2

What are limbs?

Answer 2

Two areas of lower curvature along a folded surface, on either side of the hinge zone

Question 3

What is a hinge / hinge line?

Answer 3

The line of maximum curvature along a single folded layer. Hinge lines may be curved or straight

Question 4

What is a fold axis?

Answer 4

A straight hinge line

Question 5

What is the hinge point?

Answer 5

The intersection of a hinge line with another surface (a cross section of the Earth’s surface)

Question 6

What is the axial surface / plane?

Answer 6

The surface connecting the hinge lines of multiple folded layers

Question 7

What is the axial trace?

Answer 7

The intersection of a fold’s axial surface with another surface. The axial trace is a line.

Question 8

What is the crest line?

Answer 8

The line connecting the highest points on an antiform?

Question 9

What is the trough line?

Answer 9

The line connecting the lowest points on a synform

Question 10

What is a culmination/depression?

Answer 10

Highest/lowest points on a crest/trough line.

Question 11

What is a dome/basin?

Answer 11

Area around a culmination on a crest line / depression on a trough line.

Question 12

What is the inflection line?

Answer 12

Points on a single folded layer where the curvature of a limb is minimised / equal to zero

Question 13

What is the inflection point?

Answer 13

The intersection of an inflection line with another surface (a cross section or the Earth’s surface)

Question 14

What is the inflection surface?

Answer 14

The surface connecting the inflection lines of multiple folded layers

Question 15

What is a cylindrical fold?

Answer 15

A fold with a straight hinge line

Question 16

What is an antiform?

Answer 16

A fold which closes upwards

Question 17

What is a synform?

Answer 17

A fold which closes downwards

Question 18

What is a neutral fold?

Answer 18

A fold which closes sideways (folds without a crest/trough line)

Question 19

What is an anticline?

Answer 19

Fold with oldest layers in the core

Question 20

What is a syncline?

Answer 20

Fold with youngest layers in the core

Question 21

What is an interlimb angle?

Answer 21

The angle between two fold limbs. Used to define the tightness of a fold.

Question 22

What are the ways in which we describe the tightness of an interlimb angle?

Answer 22

Gentle
Open
Close
Tight
Isoclinal

Question 23

What is an isoclinal fold?

Answer 23

A tight fold with roughly parallel limbs

Question 24

What is roundness in terms of folds?

Answer 24

A measure of the curvature of a fold hinge.

Question 25

What are the ways in which we describe the roundness of a fold?

Answer 25

Round/blunt Sub-angular Sharp

Question 26

What is a horizontal fold?

Answer 26

A fold with a horizontal fold axis

Question 27

What is a plunging fold?

Answer 27

A fold with a non-horizontal fold axis

Question 28

What is an upright fold?

Answer 28

Fold with a vertical axial surface?

Question 29

What is an upright fold?

Answer 29

A fold with a vertical axial surface

Question 30

What is an inclined fold?

Answer 30

A fold with a non-vertical axial surface

Question 31

What is an overturned fold?

Answer 31

An inclined fold in which both limbs dip in the same direction

Question 32

What is a recumbent fold?

Answer 32

A fold in which the axial surface is horizontal

Question 33

What is a symmetric fold?

Answer 33

Folds where the fold axial plane bisects the limbs. Limbs are equal lengths

Question 34

What is an asymmetric fold?

Answer 34

Fold in which one limb is shorter than the other

Question 35

What is the vergence?

Answer 35

The direction of apparent shear of the upper, longer limb of an asymmetric fold.

Question 36

What is the wavelength?

Answer 36

Distance between two adjacent hinges

Question 37

What is amplitude?

Answer 37

The distance between hinge point to inflection point, measured in the plane of the axial surface perpendicular to the hinge line.

Question 38

What is a non-cylindrical fold?

Answer 38

A fold with a hinge line that is not straight