U-Shaped Valley Formation and Glacial Landforms
This page discusses the formation of U-shaped valleys by glaciers and introduces two important glacial depositional features: terminal moraines and drumlins.
U-Shaped Valley Formation
U-shaped valleys are formed when glaciers erode and modify pre-existing V-shaped river valleys. The process involves several key mechanisms:
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Glacial flow: The weight of the glacier causes it to flow downhill, following existing V-shaped valleys.
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Erosion of valley sides:
Plucking: Ice freezes onto rocks and pulls them away from the valley sides, steepening them.
Abrasion: Angular rocks embedded in the ice scrape the valley floor, deepening it.
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Truncation of spurs: The glacier cuts off interlocking spurs as it flows downhill, leaving truncated spurs.
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Hanging valleys: Smaller tributary glaciers create shallower valleys that "hang" above the main U-shaped valley.
Vocabulary: Plucking - The process where ice freezes onto rocks and pulls them away from the valley sides.
Vocabulary: Abrasion - The process where rocks embedded in the ice scrape and erode the valley floor.
The final shape and width of the glacial trough depend on factors such as rock type and the intensity of glacial erosion. Some sections of the valley floor may be over-deepened due to differential erosion, potentially forming ribbon lakes.
Example: Langdale Valley is an example of a U-shaped valley.
Terminal Moraine
A terminal moraine is a ridge of glacial deposits that forms across a valley, marking the furthest extent of a glacier's advance. Key characteristics include:
- Composition: Unsorted glacial till or boulder clay
- Formation: As the glacier moves downhill, it pushes sediment in front of its snout
- Deposition: Occurs when the glacier melts and loses power at lower altitudes
- Post-glacial feature: Can form a natural dam, creating a ribbon lake after ice retreat
Definition: Terminal moraine - A ridge of glacial deposits marking the furthest point reached by a glacier.
Drumlins
Drumlins are elongated hills composed of unsorted glacial deposits. Their formation and characteristics include:
- Shape: Streamlined, with a steep slope stoss facing up-valley and a gentler slope lee facing down-valley
- Formation: Created when a glacier becomes overloaded with sediment and deposits it, reshaping it as it flows over
- Distribution: Often found in groups called "swarms" or in a "basket of eggs" topography
- Modification: May be reshaped by further ice movements after initial deposition
Highlight: The unique shape of drumlins, with a steep up-valley face and gentler down-valley slope, provides evidence of the direction of past glacial movement.
Understanding these glacial landforms helps geographers and geologists reconstruct past ice movements and climate conditions, providing valuable insights into Earth's geological history.
