Questions: Erosion Agents: Fluvial, Glacial, and Coastal Processes

Question 1

A geologist examines ancient deposits and finds material with boulders, pebbles, sand, and clay all mixed together with no size sorting, and some clasts have parallel scratches on their surfaces. What is the most likely depositional agent?

• A. A high-energy river during a major flood event
• B. A coastal storm surge depositing material from the seafloor
• C. A glacier: till is characteristically unsorted, and the parallel scratches are glacial striations
• D. A debris flow triggered by heavy rainfall on a steep slope

Answer: (C)

The two diagnostic features here — complete lack of size sorting and parallel scratches (striations) on clast surfaces — point unambiguously to glacial transport. Glaciers carry all sizes of material indiscriminately within the ice and deposit everything at once when the ice melts, producing unsorted till. The striations form when rock fragments frozen into the glacier's base scrape across bedrock. Rivers, even in floods, sort sediment by grain size because water's carrying capacity depends on flow velocity. Debris flows produce unsorted deposits but do not produce striations.

Question 2

Why does a river carve a V-shaped valley while a glacier carves a U-shaped valley?

• A. Rivers chemically dissolve rock in a V pattern; glaciers mechanically freeze and break rock in a U pattern
• B. Rivers concentrate erosive energy on the streambed, cutting downward; glaciers fill their valleys and erode walls as effectively as the floor
• C. V-shapes form in soft rock and U-shapes in hard rock, regardless of the erosional agent
• D. Rivers flow faster than glaciers, producing sharper walls; glacial valleys are rounded by slower movement

Answer: (B)

A river's erosive power is concentrated at the narrow channel where water flows — it cuts downward into the bed, creating steep walls that are not eroded laterally. The result is a V-shaped cross-section. A glacier is as wide as its valley and made of solid ice that erodes everything it contacts — the floor, the walls, and the corners. This produces the characteristic U-shape: flat or rounded floor and steep, near-vertical walls. The shape is not about rock type or speed; it is about where the erosive agent contacts the rock.

Question 3

Well-sorted, rounded sand grains in an ancient deposit indicate glacial transport, because glaciers are efficient at sorting and rounding sediment.

• T. True
• F. False

Answer: False

This is precisely backwards. Glacial deposits (till) are characteristically unsorted — boulders, gravel, sand, and clay all deposited together because ice carries material regardless of size and releases it all when it melts. Well-sorted, rounded sand strongly suggests water transport: rivers sort sediment by grain size as heavier particles fall out first, and repeated transport rounds angular grains by abrasion. Beaches and river channels are the classic settings for well-sorted, rounded sand. Recognizing sorting and rounding as indicators of water transport (not glacial) is a fundamental skill in reading the sedimentary record.

Question 4

Longshore drift transports sediment laterally along coastlines and can build depositional features like spits and barrier islands.

• T. True
• F. False

Answer: True

When waves approach the shore at an angle, they push sediment in the direction of wave travel as they wash up (swash), but gravity pulls sediment straight back down the slope (backwash). The net effect is a zigzag motion that moves sediment along the coastline — longshore drift. Over time, this accumulates enormous volumes of sediment in specific locations. Where longshore drift carries sediment past a headland into open water, it builds a spit extending from the shoreline. Where sediment accumulates offshore parallel to the coast, barrier islands form. Understanding longshore drift is essential for coastal engineering and hazard assessment.

Question 5

Explain how you would distinguish a glacially deposited sequence from a fluvially deposited sequence when examining outcrops in the field. What specific features would you look for, and what do they indicate about the transport process?

Model answer: Glacial deposits (till) are characterized by: (1) complete lack of size sorting — boulders, gravel, sand, and clay all mixed together in the same layer; (2) angular to sub-angular clasts, since glacial transport doesn't round material the way water does; (3) striated clasts — parallel scratches on rock surfaces from grinding against the glacier's base; (4) bullet-shaped or faceted clasts from pressure grinding. Fluvial deposits show: (1) size sorting — coarser material at the base of a layer, grading to finer material at the top (graded bedding), or lateral sorting by channel energy; (2) well-rounded clasts from repeated abrasion in water; (3) cross-bedding from migrating ripples and dunes; (4) channel structures. The most diagnostic contrast is sorting: glacial transport is blind to grain size, while water transport systematically separates particles by size according to the flow's carrying capacity.

These diagnostic criteria allow geologists to reconstruct paleoenvironments — ancient climate conditions — from the rock record. Glacial deposits in tropical latitudes indicate ancient ice ages; the presence of tills and striations far from present-day glaciers was some of the earliest evidence for continental-scale glaciations in Earth's past, including the Pleistocene ice ages and the Snowball Earth events.