When a glacier loses the ability to transport material depositional landforms are formed. As ice melts the material it is transporting is deposited on the ground. As a glacier moves material being transported in its base that has been plucked from the bedrock can be dropped. This could be because it gets stuck behind obstacles on the floor, such as a band of hard rock. Most of the material that is deposited is till. Where ice has melted and retreated we see drumlins and moraines.
The different types of moraine
Moraines are piles of rock debris. There are several types of moraine, each with its own shape. These are:
- Lateral moraine – lateral means towards the sides. A lateral moraine is a ridge of material found along the edge of a u-shaped valley. This material is the result of freeze-thaw weathering along the sides of the valley. Frost shattered rock falls onto the glacier and as the ice melts the material is deposited on the valley floor.
- Medial moraine – medial means in the middle. When two glaciers meet side-by-side the lateral moraines join to form a ridge of rock. These are visible as a dark line running down the centre of the merged glacier. When the ice melts the material is deposited in the centre of a u-shaped valley.
- Ground moraine – material that is frozen to the base of the glacier and gets stuck and deposited on the valley floor is known as ground moraine. Large areas of the UK were covered in ground moraine at the end of the last ice age.
- Terminal moraine – terminal means end. As its name suggests this material is found at the end or the snout of the glacier. It is material that has been bulldozed by the glacier. It is usually a very large ridge of material that indicates the distance a glacier has travelled.
The main features of a drumlin
A drumlin is an egg-shaped formation made up of till. They are formed when material is deposited as ground moraine. This is then shaped by the movement of ice. Drumlins show the direction of ice movement. They can be 100 metres long or more. A group of drumlins is known as a basket of eggs. Some drumlins have a large piece of rock in their centre.
What do ‘glacial erratics’ tell us about past ice movements?
An erratic is a rock or boulder that differs from the surrounding rock and has been transported by ice over a long distance and deposited by a melting glacier. They provide evidence of the route a glacier has taken.