What the Ice Leaves Behind
Written by Studio AM.
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A glacier forms where snow builds up over many years. New layers compress older snow into dense ice, although some air remains trapped inside. Under its own weight, a thick mass of ice can flow. Its movement may be too slow to notice during a short visit.
A moving glacier can erode the land beneath it. Stones held in its base scrape grooves into bedrock. Ice can also freeze onto fractured rock and pull blocks away as it moves. These processes can widen and deepen a mountain valley. Many glacial valleys have a broad, U-shaped cross-section, unlike the narrower V-shaped profile common in river-cut valleys.
When the ice retreats, its deposits remain. Ridges of mixed rubble can mark former glacier edges. Large boulders may be left far from rocks of the same type. Geologists compare those stones with possible source areas and look for other evidence, such as grooves and deposits, to reconstruct the ice's route.
No single feature tells the whole history. Together, the valley's shape, scratched rock and transported stones can reveal a landscape once covered by moving ice.
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Questions
Choose an answer. The explanation appears after you answer.
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Question 1 of 4
Which statement best expresses the main idea?
The answer is B: Glaciers form from accumulated snow, erode land and leave evidence of their movement.
The passage follows glacier formation, erosion and the clues left after retreat. The different clues work together to help reconstruct its history.
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Question 2 of 4
What cross-section is common in valleys shaped by glaciers?
The answer is B: a broad U
Many glacial valleys have a broad U-shaped cross-section. The comparison concerns the valley's whole profile, not just its floor.
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Question 3 of 4
A boulder matches rock from a distant mountain. What would strengthen the case that a glacier carried it?
The answer is C: Finding glacial grooves and deposits along a possible route.
The matching rock suggests a source area. Grooves and deposits provide additional evidence for glacial transport.
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Question 4 of 4
In this passage, "bedrock" means
The answer is A: the solid rock lying underneath the loose surface material
Stones in the base of the glacier scratch lines into the bedrock beneath, so it is the firm rock under the ice and the loose material.
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