Lanforms of Glacial Deposition

Question 1

Describe the formation of an erratic

Answer 1

An erratic is an individual piece of rock, varying in size from a small pebble to a large boulder. What makes them distinctive is that they are composed of a different geology from that of the area in which they have been deposited. They were eroded, most likely by plucking, or added to the supraglacial debris by weathering and rockfall, in an area of one type of geology and then transported and deposited into an area of differing rock type. A good example is the erratic blocks of Silurian shale deposited on carboniferous limestone at Norber in the Yorkshire Dales. These have protected the underlying rock from carbonation weathering, leaving them perched on a pedestal.

Question 2

Describe the formation of a terminal moraine

Answer 2

A terminal moraine is a ridge of till extending across a glacial trough. They are usually steeper on the up-valley side and tend to be crescent shaped, reaching further down-valley in the centre. These landforms mark the position of the maximum advance of the ice and were deposited at the glacier snout. Their crescent shape is due to the position of the snout; further advance occurs in the centre of the glacier, as there is no friction with the valley sides. The steeper up-valley side is the result of the ice behind supporting the deposits and making them less likely to collapse. The Franz Josef glacier in New Zealand has left a terminal moraine 430 m high.

Question 3

Describe the formation of a lateral moraine

Answer 3

A lateral moraine is a ridge of till running along the edge of a glacial valley. The material accumulates on top of the glacier having been weathered from the exposed valley sides. As the glacier melts or retreats, this material sinks through the ice to the ground and is deposited. A lateral moraine left by the retreating Athabasca glacier in Canada is 1.5 km long and 124 m high.

Question 4

Describe the formation of a Recessional Moraine

Answer 4

Recessional moraines are a series of ridges running transversely across glacial troughs and which are broadly parallel to each other and to the terminal moraine. They are found further up the valley than the terminal moraine (Figure 2.16). They form during a temporary still-stand in retreat. These temporary pauses are rarely prolonged; thus recessional moraines seldom exceed 100 m in height.

Question 5

Describe the formation of a Drumlin

There are different theories

Answer 5

A drumlin is a mound of glacial debris that has been streamlined into an elongated hill. Often they are prominent landforms, sometimes more than a kilometre in length and 100 m high. In plan they are typically pear-shaped and aligned in the direction of ice flow. The higher and wider stoss, or blunt, end faces the ice flow, while the lee side is more gently tapered.

Their formation is not fully understood. They may be formed by:
*Lodgement of subglacial debris as it melts out of the basal ice layers
*reshaping of previously deposited material during a subsequent re-advance
*accumulation of material around a bedrock obstruction - these are known as rock-cored drumlins
*thinning of ice as it spread out over a lowland area, reducing its ability to carry debris.

Drumlins tend to occur in large groups or swarms, forming a so-called ‘basket of eggs’ topography. Good examples can be seen in central Ireland, New York State and in the hills of Elslack, North Yorkshire

Question 6

Describe the formation of Till Sheets

Answer 6

• A till sheet is formed when a large mass of unstratified drift is deposited at the end of a period of ice sheet advance, which smooths the underlying surface. They may not be very conspicuous in terms of relief, but they are significant landforms because of their extent.
• The till itself is variable in composition, depending greatly on the nature of the rocks over which the ice has moved. If there is a high clay content, compaction by the weight of the overlying ice sheet may lead to relatively hard deposit. In East Anglia the till is quite chalky in content, due to the rocks over which the ice passed. It is typically 30-50 m deep although it can be up to 75 m in places.
• It was formed by several different ice sheet advances between 480,000 and 425,000 years ago. In Minnesota, USA, extensive till sheets were formed by deposition during the retreat of the Laurentide ice sheet.