Depositional Landforms

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.

Fig. 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).

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.

Fig. 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.

Fig. 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.

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.

Fig. 4 - Glacial erratic in Alaska, Wikimedia Commons/Public Domain