14.13: Deposition by Glaciers
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How could those rocks on the glacier modify the landscape?
Glaciers modify the landscape by erosion. They also modify the landscape by deposition. Glaciers carry an enormous amount of material and dump it. The features they leave behind show where they were and what happened as they were melting away.
Deposition by Glaciers
As glaciers flow, mechanical weathering loosens rocks on the valley walls. These rocks falls onto the glacier. Glaciers can carry rocks of any size, from giant boulders to silt. The glacier may carry the rocks for many kilometers over many years. Glaciers deposit the sediment when they melt. They drop and leave behind whatever was once frozen in their ice.
Giant rocks carried by a glacier are eventually dropped. These glacial erratics, like the one pictured below (Figure below), are noticeable because they are huge. Also, they are usually a different rock type from the surrounding bedrock.
These glacial erratics at Yosemite National Park was deposited by a glacier.
The material dropped by a glacier is usually a mixture of particles and rocks of all sizes. This unsorted pile is called glacial till. Water from the melting ice may form lakes or other water features. The figure below shows some of the landforms glaciers deposit when they melt.
- A moraine is sediment deposited by a glacier. A ground moraine is a thick layer of sediments left behind by a retreating glacier. An end moraine is a low ridge of sediments deposited at the end of the glacier. It marks the greatest distance the glacier advanced.
The hikers are standing on a moraine. What erosional feature is the hikers looking at?
- A drumlin is a long, low hill of sediments deposited by a glacier. Drumlins often occur in groups called drumlin fields. The narrow end of each drumlin points in the direction the glacier was moving when it dropped the sediments.
- An esker is a winding ridge of sand deposited by a stream of meltwater. Such streams flow underneath a retreating glacier.
- A kettle lake occurs where a chunk of ice was left behind in the till of a retreating glacier. When the ice melted, it left a depression. The meltwater filled it to form a lake. You can see examples of kettle lakes below (Figure below).
Kettle lakes are found where ice sheets once covered the land. These are in Siberia.
Several types of stratified deposits form in glacial regions but are not formed directly by the ice. Varves form where lakes are covered by ice in the winter. Dark, fine-grained clays sink to the bottom of the lake in winter. Melting ice in the spring brings running water that deposits lighter colored sands. Each alternating dark/light layer represents one year of deposits.
- Glaciers dump glacial till. Glacial moraines outline a glacier's extent.
- Drumlins, eskers, and kettle lakes are features made of glacial till.
- Varves form in lakes covered by ice. Varves are useful to scientists for understanding climate.
- Describe how these depositional features form: moraine, drumlin, esker, and kettle lake.
- Why are varves important to scientists?
- Why does the presence of glacial till mean there was a glacier in the area?