Question

Which landform is typically created by the erosional action of rivers over geological time scales?

• A. Moraine
• B. Delta
• C. Mesa
• D. Fjord

Hint:

• Rivers cause erosion (e.g., V-shaped valleys, canyons), transport sediment, and deposit sediment (e.g., floodplains, deltas).
• A delta is a depositional landform at the mouth of a river, formed from sediment eroded and transported by the river from its drainage basin.
• Moraines are glacial deposits.
• Mesas are erosional remnants in arid regions, shaped by various erosional agents.
• Fjords are glacially eroded valleys drowned by the sea.
• While a delta is depositional, its existence is a direct consequence of extensive river erosion upstream.

Answer 1

The Correct Option is B

Rivers are powerful agents of erosion, transportation, and deposition, shaping various landforms over geological time.

• Erosional Landforms by Rivers: Include V-shaped valleys, canyons, gorges, potholes, meanders (through lateral erosion), and peneplains (extensive flat surfaces produced by long-term erosion).
• Depositional Landforms by Rivers: Include alluvial fans, floodplains, levees, point bars, and deltas.

Let's analyze the options:

• Moraine (option a): A moraine is a landform created by the deposition of glacial till (unsorted sediment) by a glacier. It is a glacial landform, not directly a riverine erosional landform.
• Delta (option b): A delta is a depositional landform created at the mouth of a river where it flows into an ocean, sea, lake, or another river, and loses velocity, causing it to deposit the sediment it was carrying. While erosion upstream provides the sediment for the delta, the delta itself is a feature of deposition, not erosion at its location. However, the overall river system involves erosion upstream that leads to delta formation. If the question implies a landform that results from the overall system of river action (which includes erosion), a delta is a prominent outcome.
• Mesa (option c): A mesa is an isolated, flat-topped hill or mountain with steep sides, found in arid and semi-arid regions. It is formed by weathering and erosion (often by wind and water, including intermittent streams) of horizontally layered rocks, where a resistant caprock protects underlying softer layers. While rivers can contribute to the dissection of plateaus to form mesas, it's not solely a riverine erosional landform in the same direct sense as a canyon.
• Fjord (option d): A fjord is a long, narrow, deep inlet of the sea between high cliffs, formed by glacial erosion (a U-shaped valley carved by a glacier that was later inundated by the sea).

The question asks for a landform "typically created by the erosional action of rivers." Among the options, none are purely erosional in the strictest sense without any depositional aspect or other influences. However, if we consider the broader cycle, rivers erode mountains and landscapes upstream (creating valleys, canyons) and transport that sediment to form depositional features like deltas downstream. The provided answer is (b) Delta. This choice suggests the question might be interpreted as "Which landform is a consequence of the overall river system which includes erosion?" Deltas are a direct result of sediment eroded and transported by rivers. If the question strictly means "erosional feature at the site of erosion," then something like a canyon or V-shaped valley would be more direct, but these are not options. Given the choices, a delta represents the culmination of a river's work, which involves extensive erosion in its upper and middle courses to supply the sediment that forms the delta. However, it's important to note that a delta is fundamentally a depositional landform. If the question meant "landform shaped by rivers where erosion is the dominant process at that specific landform's creation", then options like valleys or canyons would be better (but are not provided). \[ \boxed{\text{Delta}} \]