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Depositional Landforms

Depositional Landforms

A depositional landform is a landform that is created from glacial deposition. This is when a glacier carries some sediment, which is then placed (deposited) somewhere else. This could be a large group of glacial sediment or a single significant material.

Depositional landforms consist of (but are not limited to) drumlins, erratics, moraines, eskers, and kames.

There are many depositional landforms, and there is still some debate on which landforms should qualify as depositional. This is because some depositional landforms come about as a combination of erosional, depositional, and fluvioglacial processes. As such, there is no definite number of depositional landforms, but for the exam, it is good to remember at least two types (but aim to remember three!).

Types of depositional landforms

Here are some brief descriptions of different types of depositional landforms.

Drumlins

Drumlins are collections of deposited glacial till (sediment) that form under moving glaciers (making them subglacial landforms). They vary greatly in size but can be up to 2 kilometers long, 500 meters wide, and 50 meters in height. They are shaped like half a teardrop rotated 90 degrees. They are usually found in large groups known as drumlin fields, which some geologists describe as looking like ‘a large egg basket’.

Terminal moraines

Terminal moraines, also known as the end moraine, are a type of moraine (material left behind from a glacier) that form at the edge of a glacier, a prominent ridge of glacial debris. This means that the terminal moraine marks the maximum distance a glacier travelled during a period of sustained advance.

Erratics

Erratics are usually large stones or rocks left behind/dropped by a glacier either due to chance or because the glacier melted and started retreating.

What distinguishes an erratic from other objects is the fact that the composition of the erratic doesn’t match anything else in the terrain, which means that it is an anomaly in the area. If it is likely that a glacier carried this anomalous object, it is an erratic.

Depositional Landforms, Drumlin terminal moraine erratic, StudySmarterFig. 1 - A diagram highlighting glacial depositional landforms

Using depositional landforms to reconstruct past glacial landscapes

Are drumlins a useful depositional landform to reconstruct past glacial landscapes?

Let’s see how useful drumlins are in reconstructing past ice movement and ice mass extent.

Reconstructing past ice movement

Drumlins are very useful depositional landforms for reconstructing past ice movement.

Drumlins are oriented parallel to the movement of the glacier. More importantly, the drumlin’s stoss end points upslope (direction opposite glacial movements), whilst the lee end points downslope (direction of glacial movement).

Note that this is opposite to roches moutonnées (see our explanation on Erosional Landforms). This is due to the different processes that created the respective erosional and depositional landforms.

Since the drumlin is made up of deposited glacial sediment (till), it is possible to conduct till fabric analysis. This is when the movement of the glacier influences the sediment it runs over to point in the direction of its movement. As a result, we can measure the orientations of a large number of till fragments to inform the reconstruction of the direction of glacial movement.

One more way drumlins help reconstruct past ice mass movement is by calculating their elongation ratio to estimate the potential rate at which the glacier was moving through the landscape. A longer elongation ratio suggests faster glacial movement.

Depositional Landforms Glacial Drumlin State Trail in the USA StudySmarterFig. 2 - The Glacial Drumlin State Trail in the USA. Image: Yinan Chen, Wikimedia Commons/Public Domain

Reconstructing past ice mass extent

When it comes to using drumlins for reconstructing ice mass extent, there are some problems.

Drumlins suffer from what is called equifinality, which is a fancy term for: ‘we don’t know for sure how they came about’.

  • The commonly accepted theory is the constructional theory, which suggests that drumlins are formed by sediment deposition from subglacial waterways.
  • The second theory suggests that drumlins form by erosion by a glacier via plucking.
  • Due to the conflict between the two theories, it is not appropriate to use drumlins to measure ice mass extent.

Another issue is that drumlins have been altered and damaged, mostly due to human actions:

  • Drumlins are used for agricultural purposes, which will naturally alter the position of loose rocks and sediment on the drumlins (disabling the possibility for till fabric analysis).
  • Drumlins also undergo lots of construction. In fact, Glasgow is built on a drumlin field! It is almost impossible to perform any studies on a drumlin that has been built upon. This is because studies would disrupt urban activity, and the drumlin has likely been damaged as a result of the urbanisation, meaning it wouldn’t give any helpful information.

Are terminal moraines a useful depositional landform to reconstruct past glacial landscapes?

Very simply, yes. Terminal moraines can give us a great indication of how far a past glacier travelled in a given landscape. The position of the terminal moraine is the final boundary of the glacier’s extent, so it can be an excellent way to measure the maximum past ice mass extent. However, two potential issues can impact the success of this method:

Issue one

Glaciers are polycyclic, and this means that in their lifetime, they will advance and retreat in cycles. It is possible that after a terminal moraine is formed, a glacier will once again advance and surpass its previous maximum extent. This leads to the glacier displacing the terminal moraine, forming a push moraine (another depositional landform). This can make it tough to see the extent of the moraine itself, and so it is difficult to determine the maximum extent of the glacier.

Issue two

Moraines are susceptible to weathering. The edges of terminal moraines can undergo intense weathering due to harsh environmental conditions. As a result, the moraine can appear shorter than it was originally, making it a poor indicator of past ice mass extent.

Depositional Landforms, Terminus of Wordie Glacier in northeast Greenland, StudySmarterFig. 3 - The terminus of Wordie Glacier in northeast Greenland with a small terminal moraine. Image: NASA/Michael Studinger, Wikimedia Commons

Are erratics a useful depositional landform to reconstruct past glacial landscapes?

If we can identify the origin of the erratic, then it is possible to trace the general direction of the past glacier that deposited the erratic.

Suppose we mark the origin of an erratic point A on a map and its current position as point B. In that case, we can draw a line between the two points and align it with either a compass direction or bearing in order to find a very accurate direction of past ice mass movement.

However, this method in the example doesn’t capture the exact movements the glacier may have taken, but for practical purposes, these movements don’t matter much.

Unlike the other depositional landforms mentioned here, erratics face few issues when reconstructing past ice mass movement. But what if we can’t identify the origin of the erratic? No problem! We can argue that if we can’t identify the origin of an erratic, then it’s likely it wasn’t deposited by a glacier – meaning it wouldn’t be suitable to call it an erratic in the first place.

Depositional Landforms glacial erratic in Alaska StudySmarterFig. 4 - Glacial erratic in Alaska, Wikimedia Commons/Public Domain

Depositional Landforms - Key takeaways

  • A depositional landform is a landform that was created due to glacial deposition.
  • Depositional landforms consist of (but are not limited to) drumlins, erratics, moraines, eskers, and kames.
  • Depositional landforms can be used to reconstruct former ice mass extent and movement.
  • Each landform has its unique indicators for reconstructing former ice mass extent.
  • Depositional landforms generally come about as a result of glacial retreat, but this is not the case for drumlins.
  • There are limitations to each landform’s usefulness for ice mass reconstruction. This should be considered when using the discussed techniques.

Frequently Asked Questions about Depositional Landforms

Depositional landforms consist of drumlins, erratics, moraines, eskers, and kames.

A depositional landform is a landform that is created from glacial deposition. This is when a glacier carries some sediment, which is then placed (deposited) somewhere else.

There are many depositional landforms, and there is still some debate on which landforms should qualify as depositional. This is because some depositional landforms come about as a combination of erosional, depositional, and fluvioglacial processes. As such, there is no definite number of depositional landforms.

Three depositional landforms (which are very useful to learn for discussing the possibility of reconstructing past ice mass movement and extent) are drumlins, erratics, and terminal moraines.

Final Depositional Landforms Quiz

Question

Why is there an issue with using drumlins to recreate past ice mass extent?

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Answer

Due to equifinality: we don’t know the exact process by which drumlins are formed, so we can’t make definite claims as to how a drumlin is indicative of past ice mass extent.

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Question

Describe how two depositional landforms may be used to recreate ice mass extent of movement

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Answer

 Mapping the journey of an erratic and finding the endpoint of a terminal moraine.

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Question

Explain why terminal moraines could be misleading when using them to reconstruct past ice mass extent.

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Answer

Due to the polycyclic nature of glacial movement, a terminal moraine could be the endpoint of just one period of glaciation. In another period, a glacier could move past it and form a push moraine.

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Question

What is an erratic? How can we differentiate an erratic from any other stone or rock in an area?


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Answer

An erratic is a rock or other piece of matter that is uncommon in the rest of the surrounding geology (i.e. it does not make sense that it would have formed in the location it is found). As a result, it was likely deposited in that area by a glacier, making it an erratic. 

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Question

Name two ways drumlins can indicate past ice mass movement.


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Answer

Drumlins can indicate past ice mass movement via their stoss and lee slopes. Till fabric analysis can also be done. 

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Question

In which directions do a drumlin's stoss and lee slopes point?


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Answer

The lee slope points in the direction of glacial flow (downslope) and the stoss slope is against the glacial flow (upslope).

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Question

Name the limitation of using erratics to reconstruct precise ice mass movement.


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Answer

Erratics only indicate the likely starting point and endpoint of the erratic's journey without considering any specific movements in the journey between the two points. 

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Question

What does the position of a terminal moraine indicate?

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Answer

The maximum extent of a glacial advance.

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Question

What is till fabric analysis?


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Answer

It is the analysis of the clasts of a landform to check if they are all orientated in a particular direction.

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Question

What is a depositional landform?


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Answer

A landform that is created from glacial deposition. This is when a glacier carries some sediment, which is then placed (deposited) somewhere else. 

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