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As glaciers move and retreat, they push and drop rocks and sediments in a process known as glacial deposition. Learn about this glacial process and the interesting landforms that result from it, including moraines, erratics and drumlins, in this lesson.

Glacial Deposition

I have a little quiz for you. Which area of the United States was at one time covered by a glacier? Was it:

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  • A: Long Island, New York
  • B: The state of Wisconsin
  • C: The Yosemite Valley located in the mountains of California
  • D: All of the above?

Well, if you picked D, then you got it right. In fact, at different times over the long history of our planet, glaciers covered much of North America.

We know this because as glaciers move out of an area, they leave behind clues in the form of glacial deposition.Glacial deposition is simply the settling of sediments left behind by a moving glacier. For example, Long Island was formed by rocks and sediment pushed there by a couple of glaciers. Wisconsin contains many interesting sediment deposits that were carried there by glaciers from Canada, and the Yosemite Valley was carved by a glacier. Glacial deposition and the landforms that result from this process are the subjects that we will cover in this lesson.

Glacial Till & Moraines

Glaciers are kind of dirty. This is because as they move over the land surface, they pick up debris, sand, soil and rocks, and these particles get stuck within the ice and travel with the glacier, often for long distances.

When the ice melts, the mixture of unsorted sediment deposits carried by the glacier, known collectively as glacial till, is dropped, or deposited.These sediments often get formed into piles known as moraines, which we can define as piles of till deposited along the edges of past glaciers. Because moraines form in lines, their locations give us important clues as to where the borders of a glacier were once found. Lateral moraines are deposits found along the sides of a past glacier, and terminal moraines are deposits found at the farthest point that a past glacier reached.It might help your understanding of moraines if you think of a glacier as a bulldozer pushing through a mound of dirt.

As the bulldozer pushes forward, some dirt is left behind in piles along the sides of its path; these would be the lateral moraines. If the bulldozer backs up and leaves a pile of dirt at the farthest point it reaches, this pile would be the equivalent of a terminal moraine. This goes back to our discussion of how Long Island, New York, was created. A couple of glaciers pushed rocks and soil south, forming a terminal moraine. Then the glacier retreated, meaning it melted, leaving behind Long Island.

Erratics

Glaciers are kind of like litter bugs because they drop materials wherever they please and sometimes they leave behind boulders so large that they could never be moved there by human power. These are called glacial erratics, and they are described as big rocks deposited in unusual places due to glacial movement.For example, you could find a huge limestone boulder sitting in an area where most of the bedrock is made up of granite. These misplaced boulders were aptly named because the term ‘erratic’ is used to describe something that is unusual. In fact, you could say that erratics are big rocks deposited in erratic places.

Drumlins

A drumlin is another example of a deposit left behind by a glacier. It’s described as an elongated hill composed of glacial till.

Unlike moraines, which form in rows of sediment, drumlins look like the backs of whales as they breach the surface of the ocean, and where there is one drumlin, you will likely find others. This is because they are usually found grouped together in drumlin fields.These fields do not only leave behind clues that the glacier once passed through the area, they also tell us which direction the glacier was moving. This is because all drumlins in a drumlin field align themselves in the direction of ice flow. Bunker Hill in Massachusetts is a famous drumlin.

In fact, you might find it easier to recall this term if you picture a soldier in the Battle of Bunker Hill playing a drum as he walks up the drumlin.

Lesson Summary

Let’s review. Glacial deposition is the settling of sediments left behind by a moving glacier. As glaciers move over the land, they pick up sediments and rocks. The mixture of unsorted sediment deposits carried by the glacier is called glacial till.

Piles of till deposited along the edges of past glaciers are called moraines.Lateral moraines are deposits found along the sides of a past glacier and terminal moraines are deposits found at the farthest point that a past glacier reached. Glaciers are big enough to pick up boulders and move them to different locations. These big rocks deposited in unusual places due to glacial movement are called glacial erratics.

A drumlin is an elongated hill composed of glacial till. Drumlins are grouped together in drumlin fields and are found aligned in the direction the ice flowed through the area. Their appearance resembles the backs of whales.

Learning Outcomes

The information contained in this lesson could help you to:

  • Understand what glacial deposition, glacial till and moraines are
  • Characterize glacial erratics
  • Describe the appearance of drumlins

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