Question 4 of 9: Structural Geology Terms

Question 4: Structural Geology Terms (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Strike. The compass bearing (trend) of the horizontal line formed by the intersection of an inclined planar surface (bedding, a fault, a foliation) with a horizontal plane; always measured perpendicular to the true-dip direction and reported as a bearing (e.g. N30°E).

b) Graben. An elongate, down-dropped crustal block bounded on both long sides by normal faults dipping toward each other (or toward the block's centre), formed under regional extension; the complementary uplifted block bounded by the same fault pair is a horst. Grabens form classic rift valleys (e.g. the East African Rift).

c) Disconformity. An unconformity in which the beds above and below the erosional (or non-depositional) surface remain parallel to one another, so the time gap is not obvious from attitude alone – it is recognized instead from an erosional surface with relief/channeling, a buried soil horizon, or a jump in the fossil/age record across the contact.

d) Hanging wall. On a non-vertical fault, the hanging wall is the block of rock that lies structurally above the fault plane (the block you could "hang a lantern from" while standing in a mine tunnel on the fault); the block below is the footwall. On a normal fault, the hanging wall moves down relative to the footwall; on a reverse/thrust fault, it moves up.

e) Joint. A fracture in rock along which there has been no observable shear displacement parallel to the fracture surface (distinguishing it from a fault); joints commonly form in sets with a consistent, regionally systematic orientation from cooling contraction (columnar joints in lava), unloading/exfoliation, or regional tectonic stress, and strongly influence rock-mass permeability and engineering behaviour (block size, slope stability, groundwater pathways).