What the Ice Left Behind
Glaciers flow slowly and erode rock beneath and beside them. They can widen valleys into U-shaped troughs and leave sediment, transported boulders and hollows that later hold lakes. Many landscapes in northern Europe and North America retain traces of past ice.
What a learner can do afterwards
- Say how a glacial valley differs in cross-section from a river valley, and why
- Name two things a glacier leaves behind when it melts
- Explain why lakes are common in landscapes that were once under ice
1 · Read
A glacier flows slowly as its ice deforms. Some glaciers also slide over their beds. Rock fragments carried at the base scrape the valley floor and sides. Ice can also pull away blocks. These processes can widen and deepen a valley into a broad U-shaped trough.
A river cutting down into upland ground often forms a V-shaped valley as the sides also wear back. A glacier may widen it into a U shape with steep sides and a broad floor. These are clues, not rules for every valley. A river can later occupy a valley shaped by ice.
Glaciers carry mixed rock fragments. Material left directly by ice is often unsorted, with different sizes together. An accumulation of glacial debris is called a moraine. Some moraines form ridges beside or beyond the ice. Moving fragments can scratch bedrock. Ice can also carry boulders far from their source rock.
Glacial erosion can leave hollows that collect rain, meltwater or stream water. Moraines can also block drainage and form lakes. A lake alone does not prove that ice shaped the land. Compare several clues: a U-shaped cross-section, scratched bedrock, moraines and transported boulders. Together they can support a glacial explanation.
Moving ice can widen valleys, scratch rock and leave sediment. Use several clues together to investigate a glacial landscape.
Lesson, question and flashcard revisions written for this dataset.
Readings written and flashcards revised for this dataset. Video references: How to grow a glacier - M Jackson; My glacier cave discoveries - Eddy Cartaya. Video licensing terms have not been verified.
More to read
Moving ice and rock
A glacier flows because ice changes shape under stress. Some glaciers also slide over their beds. Rock fragments carried at the base scrape the ground, and ice can remove blocks. These processes can deepen and widen a valley. Cold temperature alone is not the explanation: moving ice and the material it carries do the erosional work.
Testing landscape clues
Record what you observe before deciding how it formed. A ridge of mixed debris might be a moraine, but look for supporting evidence such as scratched bedrock and transported boulders. A river can occupy a valley previously widened by ice. The process visible today need not have made every feature in the landscape.
Water after the ice
Glaciers can create places where water collects. Erosion may leave bedrock basins, while deposited debris may block drainage. After the ice disappears, rain and streams can keep supplying these lakes. Other lakes have different origins. Investigate the basin and nearby deposits before attributing a lake to glacial activity.
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2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.