Geological Structures Questions and Answers

Answer: C

Correct answer: a break in the geological record

An unconformity is a surface representing time not recorded - a period of non-deposition, erosion or both, between the rocks below and those above. It is where the record has a gap, and the length of the gap is established from the ages of the beds on either side.

Answer: D

Correct answer: at different angles of dip

The angular unconformity is the most obvious kind and tells the fullest story: the lower beds were deposited, tilted or folded, eroded to a surface, and then buried by younger beds laid down at a different angle. Hutton’s unconformity at Siccar Point is the classic example.

Answer: A

Correct answer: igneous or metamorphic rock

In a nonconformity the sediments rest directly on crystalline basement. Since the igneous or metamorphic rock formed at depth, the surface implies uplift and the removal of everything that once lay above - usually kilometres of it - before the sediments were laid down.

Answer: B

Correct answer: an erosion surface between parallel beds

A disconformity has beds parallel above and below, so there was no tilting, but the contact is a recognisable erosion surface, often channelled or with a lag of pebbles. The break is real and visible; only the angular discordance is missing.

Answer: C

Correct answer: the beds are parallel and the surface shows no erosion

A paraconformity is a break with nothing to see: the beds are parallel, the surface is planar and unremarkable, and the contact looks entirely conformable. It is detected only from the fossils or the radiometric ages, which reveal that a great span of time is missing from an apparently continuous sequence.

Answer: D

The first sediment laid on a newly submerged land surface is the coarse waste lying on it, so the clasts are commonly of the underlying rock. Recognising them confirms the surface as an unconformity rather than a fault.

Answer: A

Because the surface bevels across a tilted sequence, the same bed above may lie on one formation here and on another a kilometre away. It is direct evidence that the lower rocks were tilted before the upper were laid down.

Answer: B

Correct answer: younger rock entirely surrounded by older rock

An outlier is a remnant of a formation left standing after erosion has removed the rest, so the younger rock survives as an island in the middle of older ground - typically capping a hill. Its opposite is an inlier, where erosion has cut through younger cover to expose older rock beneath.

Answer: C

Boundaries between formations are drawn as they would appear with the soil stripped away, with dip readings and structural symbols added. A section is the vertical complement, constructed from the map and the boreholes.

Answer: D

Correct answer: a vertical slice through the ground

A map shows what crops out in plan; a section shows what would be seen in the wall of a trench cut along a chosen line. The two are complementary, and drawing a section is the standard way of testing whether an interpretation of the map is geometrically possible.