Glacial Depositional Landforms

Did you know that glaciers covered 32% of the land during the last ice age? If you look at the landscapes that the glaciers used to cover, there are clues literally scattered around. Retreating glaciers leave behind soil and rocks; from finding and looking at the different types of landforms, we can tell where the glaciers came from, how they moved and where they melted. From the clues in the landforms, we can tell the planet looked very different to the present. Let's explore the various landforms that the glacial deposition process creates and leaves behind.

Glacial Depositional Landforms creation

Glacial depositional landforms are created when the glacier retreats and leave behind crushed rock and sediments (also known as glacial drift) that they are carrying.

Glaciers are not just blocks of pure ice but layers and layers of compacted snow. This means that every now and then, materials such as soil, rocks and other sediments fall in and become part of the glacier. This can happen from a nearby surface being weathered by processes such as freeze-thaw weathering, meaning rocks would break off, or landslides can drop material onto a glacier. They can also collect material by picking up rocks and soil that they pass by and over from an erosional process called plucking. So when the glacier retreats, it will start to deposit some of the material it was carrying.

Glacial Depositional Landforms causes

Glacial depositional landforms are created from the glacier retreating, so they are often found in lower altitudes in lower valleys where the temperature is warmer. As the glacier melts, it loses the capacity to carry and deposits some of its material.

Glaciers advance and retreat naturally during their lifetime; there may be a slight summer glacial retreat which is then made up in the winter.

However, glacial retreat is accelerating because of the Earth's warming. The rising sea levels due to the glaciers melting and the appearance of more glacial depositional landforms are both signs of the acceleration of glacial retreat.

Glacial Depositional Landforms types

There are many types of glacial depositional landforms, drumlins, erratics, moraines, outwash plain, esker, kettle and kame. The three major glacial depositional landforms would be drumlins, erratics and moraines.

Drumlins

Drumlins are elongated hills made from glacial deposits and are compared to shapes such as an inverted spoon or a partially buried egg. It has a high-rounded back and a narrow tail. The long axis indicates the direction that the glacier was moving in. They can reach 4.3 mi/7km in length, 1.2 mi/2km in width and 98 ft/30m in height. Drumlins are found on flat lowland areas, extending parallel to the glacial flow. Often found in groups, they are called a drumlin swarm or a basket of eggs. It is said that drumlins are formed from the material deposited underneath a glacier and as the glacier moves over it. However, glaciologists disagree on how drumlins are formed.

Fig. 1 - Drowned drumlin in Clew Bay, County Mayo, Ireland

Erratics

Erratics are rocks that have been deposited by the glacier after travelling often over hundreds of miles/kilometres. They are often large rocks that are entirely different to the rock type found in the area and are found alone rather than in piles. By tracing the parent bedrock, it's possible to tell the ice flow route of the glacier.

Fig. 2 - Glacial erratic boulder in Eryri, Snowdonia, Wales

Moraines

A moraine is a fragment of rock transported by the glacier and deposited when it has melted. It is one of the most commonly found glacial depositional landforms, often found wherever a glacier exists. The size of the fragments can vary from fine powder to large boulders. However, it is often angular as it has formed from being frozen and then shattering. There are different types of moraines:

• Terminal moraine - is found at the terminus, the glacier's snout as it retreats.
• Lateral moraine - is found on the side of glaciers.
• Medial moraine - is deposited at the junction of two glaciers.
• Ground moraine - is a disorganised pile of rocks of different sizes and types.

Fig. 3 - Medial moraine in Nuusuaq Peninsula, Greenland

Outwash plain

An outwash plain is formed from fine material carried by melted ice (also known as meltwater).

The meltwater outwash is at the terminus of the glacier. They are also called sandur, sandr or sandar. Outwash plains are often found in Iceland as geothermal activity accelerates the melting of ice flows and the deposition of sediments by meltwater and other glaciated areas such as Svalbard.

Fig. 4 - Outwash plain at Skeiðarársandur, Iceland, viewed from the terminus os Svínafellsjökull glacier

Esker

An esker is a long winding ridge made from stratified gravel and sand deposited by meltwater travelling through tunnels underneath glaciers or the meltwater channels on top of glaciers. Gradually the channel or tunnel becomes filled with sediments, and then the ice retreats leaving the ridge behind. Eskers are often several miles/km long. Due to the ease of access, they are quarried for their gravel and sand for construction.

Fig. 5 - Esker at Fulufjället in Western Sweden

Kettles

Kettles are holes formed by glacial outwash that comes from a detached piece of ice that is partially or wholly buried, and when the ice melts, it leaves a void. Sometimes they become filled with water and become kettle lakes. They are usually round in shape as ice tends to become round as it melts. Kettles range in sizes from 16 ft/5 m to 8 mi/13 km in diameter and up to 148 ft/45 m in depth.

Fig. 6 - Kettle in the highlands of Isunngua, Greenland

Kame

A kame is an irregularly shaped hill or mound made of sand and gravel that has accumulated in a depression on a retreating glacier and was deposited at the glacier's terminus. A group of kames can be called kame field or kame complex and can be found with kettles or kettle lakes.

Fig. 7 - Kame field in Washington, US

Glacial Depositional Landforms diagram

Some photographs can capture glacial depositional landforms. However, diagrams can clarify and show what each landform looks like. The diagram below shows a drumlin swarm; it clearly presents the elongated hills with a high-ended back and a narrow tail.

Fig. 8 - Diagram of drumlin swarm

Below is a diagram of an esker used as a hiking route. Because of their uniform shape, they are said to look like railway embankments.

Fig. 9 - Diagram of esker used as a hiking path

This is a diagram with all the glacial depositional landforms mentioned in the article concerning a receding glacier. While in reality, they don't all occur in one place, it helps to see them situated in relation to each other.

Fig. 10 - Diagram of glacial depositional landforms

Glacial Depositional Landforms examples

Let us look at some examples of depositional landforms in detail.

Horicon Marsh, Wisconsin, US

Horicon Marsh in Wisconsin, US, was created during the Pleistocene era when Wisconsin was glaciated. It has one of the highest numbers of drumlins in the world. The glacier created many drumlins whilst advancing, and many have become islands in the marsh. The marshes were created by a glacier retreating and forming a moraine. This moraine became a natural dam which collected melted glacier water and made Glacial Lake Horicon. The Rock River eroded the moraine and drained the water. The marshes formed as the peat, silt, and clay accumulated on the lakes basin.

Fig. 11 - Drumlins around Horicon Marsh, Wisconsin, US

Big Rock, Alberta, Canada

Big Rock is a large erratic which has broken in two with the measurement of 134.5 by 59 ft and 29.5 ft high (41 by 18 m and 9 m) in the Canadian Prairies in Alberta. It is one of the erratics on the Foothills Erratics Train, which is a 578 mi/930 km long linear scatter of erratics. Big Rock was part of the Athabasca River valley; it travelled through a landslide between 12,000 and 17,000 years ago and was carried on top of a glacier and ice stream.

Fig. 12 - Big Rock on the Candian Prairies