Glacial Deposition
When does glacial deposition occur?
Deposition of material carried by the glacier occurs either as partial deposition as a result of a reduction in the velocity of the glacier or complete deposition as the glacier retreats and melts.
Glacial sediment is collectively known as drift. Material that is deposited directly by ice is known as till (or boulder clay), however, material may also be deposited by glacial meltwater, these deposits are known as outwash deposits. There is an important distinction between the characteristics of glacial and fluvio-glacial deposits as fluvio-glacial are well sorted due to the action of water (as water looses its energy its drops the largest material first), whereas glacial deposits are both unsorted and unstratified (not layered) (see table below). There is a third group of deposits known as ice-contact stratified drift, these are deposited close to melting ice and they are partly sorted by meltwater and roughly stratified.
Differences in characteristics between glacial and fluvio-glacial deposits
| Glacial Deposits | Fluvio-glacial Deposits (includes deposits once deposited by ice and re-deposited by meltwater) |
| Unstratified (difficult to identify layers) | Stratified (vertical layering due to seasonal / annual layers of sediment accumulation) |
| Material is angular, from physical weathering and erosion (unaffected by water) and various shapes and sizes (boulders - rock flour) | Material is smooth and rounded (due to attrition), it is sorted and graded. |
| Unsorted (random sorting as ice melts and deposits material regardless of size) | Sorted - larger rocks and boulders are deposited first as the meltwater looses energy. |
1. GLACIAL DEPOSITION
Glacial Till deposited directly by a glacier, can be classified as either lodgement till or ablation till.
 | Lodgement till is where material beneath the glacier has become lodged in the bed as the glacier is advancing or retreating, for example where a glacier is so overloaded with debris that it smears it onto the valley floor. An increase in ice thickness can also increase friction beneath the ice, causing lodgement. |
| Ablation till, this is where material is deposited as the ice around it melts away. This may because of solar radiation causing melting along the margins of a glaciers, or it may even be due to the melting of basal ice due to geothermal heating. As a glacier retreats due to ablation at the snout, material is left behind. |
A number of landforms (e.g. drumlins, moraines etc.) are created by glacial deposition (see diagram below), it is important that for each feature you are able to describe its characteristics and explain its formation, making use of labelled diagrams and named examples.
2. FLUVIOGLACIAL DEPOSITION
Meltwater is abundant in warm-based glaciers and the term 'fluvioglacial' is used to describe the processes and effects of this meltwater.
If deposition by meltwater occurs within subglacial, englacial or supraglacial streams, this is known as ice-contact stratified drift (see above). Where meltwater streams flowing beyond the margins of the glacier carry and deposit material, it is known as outwash.
Where does the meltwater come from?
| surface melting (this is the most important source) - e.g. effects of solar radiation |
| precipitation - entering the glacial system as runoff from the surrounding slopes |
| geothermal heating - causing basal melting as temperatures rise |
| mechanical heating - e.g. as a result of increased pressure (reaching pressure melting point) as a glacier passes over / round an obstacle. |
There are two main drainage systems in glaciers:
i. supraglacial streams / englacial water near the surface of a glacier, these may feed subglacial streams where they flow down crevasses;
ii. subglacial water at the base (this is often important as it may be flowing at high pressures under the glacier, cutting its own channel in the bedrock of the valley floor). It is this water that helps to act as a lubricant facilitating the movement of a glacier.
Streams known as proglacial streams are also created in front of a glacier carrying meltwater away. Meltwater discharge has a diurnal pattern, highest in the day when solar radiation increases melting and low at night. At times the sudden release of meltwater stored within/on a glacier causes a glacial outburst. These are frequent in places such as Iceland where geothermal heat creates large amounts of meltwater. The resulting large outbursts are known as jökulhlaups.
Landforms of fluvio-glacial deposition may be formed in ice-contact situations under or within the glacier or as a result of outwash ahead of the glacier snout.
A number of landforms (e.g. kames, eskers etc.) are created by fluvioglacial deposition (see diagram below), it is important that for each feature you are able to describe its characteristics and explain its formation, making use of labelled diagrams and named examples.
GLACIAL & FLUVIOGLACIAL DEPOSITS AND THEIR RESULTING LANDFORMS
