6. Fluvioglacial processes - Deposition

Question 1

Describe what Eskers are/their appearance.

Answer 1

Eskers are believed to be deposits made by subglacial streams. Eskers have the following main features:

• they are long ridges of material running in the direction of ice advance
• they have a sinuous (winding) form, 5-20m high
• they consist of sorted coarse material, usually coarse sands and gravel
• they are often stratified (layered)

Question 2

Explain the formation of an Esker.

Answer 2

Eskers are believed to be deposits made by subglacial streams. The channel of the stream will be restricted by ice walls, so there is considerable hydrostatic pressure which enables a large load to be carried and also allows the stream to flow uphill for short distances. This accounts for the fact that some eskers run up gentle gradients. The bed of the channel builds up above the surrounding land, and a ridge is left when the glacier retreats during deglaciation.

In some areas, the ridge of an esker is combined with mounds of material, possibly kames. Such a feature is known as a beaded esker.

Question 3

Draw a diagram of an Esker.

Question 4

1. Explain what Kames are and their formation.
2. Explain what Kames terraces are and their formation.

Answer 4

1. Kames are mounds of fluvioglacial material (sorted, and often stratified, coarse sands and gravel). They are deltaic deposits left when meltwater flows into a lake dammed up in front of the glacial snout by recessional moraine deposits. When the ice retreats further, the delta kame often collapses.
2. Kame terraces are frequently found along the side of a glacial valley and are the deposits of meltwater streams flowing between the ice and the valley side. They look like lateral moraine, but they’re sorted into layers - meltwater streams deposit their heaviest load first, so kame terraces have gravel at the bottom and sand on top.

Question 5

Draw a diagram to show the formation of a Kame terrace.

Question 6

Explain what Outwash plains (sandur) are and their formation.

Answer 6

Outwash plains (sandur) are found in front of the glacier’s snout and are deposited by the meltwater streams issuing from the ice. They consist of material that was brought down by the glacier and then picked up, sorted and dropped by running water beyond the position of the ice front. The coarsest material travels the shortest distance and is therefore found near to the glacier; the fine material, such as clay, is carried some distance across the plain before being deposited. The deposits are also layered vertically, which reflects the seasonal flow of meltwater streams.

Question 7

Explain what Kettle holes are and their formation.

Answer 7

Meltwater streams that cross the outwash plain (sandur) are braided. This happens as the channels become choked with coarse material because of marked seasonal variations in discharge. On the outwash plain there is often a series of small depressions filled with lakes or marshes. These are known as kettle holes. It is believed that they are formed when blocks of ice, washed onto the plain, melt and leave a gap in the sediments. Such holes then fill with water to form small lakes. Aquatic plants become established in the lakes and this leads over time to the development of a marshy area and then peat.

Question 8

Explain what Varves are and their formation.

Answer 8

Lakes on the fringe of the ice are filled with deposits that show a distinct layering. A layer of silt lying on top of a layer of sand represents 1 year’s deposition in the lake and is known as a varve. The coarser, lgihter-coloured layer is the spring and summer deposition when meltwater is at its peak and the meltwater streams are carrying maxium load. The thinner, darker-coloured and finer silt settles during autumn and through the winter as stream discharge decreases and the very fine sediment in the lake settles to the bottom. Varves are a good indicator of the age of lake sediments and of past climates as the thickness of each varve indicates warmer and colder periods.