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Glacial Transportation & Deposition

Transportation

Rock material eroded by a glacier, or weathered debris that has fallen onto a glacier, is transported in one of three different positions.

  1. At the base or subglacial zone of the glacier: subglacial debris
  2. Within the glacier or englacial zone: englacial debris
  3. At the surface or supraglacial zone: supraglacial debris

 

Deposition

Glacial deposition occurs when rock debris, previously frozen into, or lying on a glacier, is laid down on a land surface by the melting of ice. This process occurs at the front of a glacier during the warm summer months and by pressure melting at the base of the glacier. All glacial deposits are collectively referred to as drift despite the fact that this term owes its origin to the early 19th century belief that these deposits were primarily derived from the melting of floating icebergs during the biblical flood. Drift can be subdivided into three types of glacial sediment.

Photo 51: Click to enlarge   Till:  an unsorted mixture of clay, sand, gravel and boulders that is deposited directly from melting ice.
Photo 52: Click to enlarge  

Ice contact stratified drift: stratified sand and gravel that is sorted by the action of meltwater streams and deposited next to the glacier.

Photo 53: Click to enlarge   Outwash deposits: well-sorted sand and gravel deposited over a wide area of the proglacial area (in front of the glacier) by meltwater streams emanating from the glacier.

 

Note: both ice contact stratified drift and outwash deposits are also called glaciofluvial deposits.

 

Till

The word ‘till’ was first used by A Gelkin in 1863 to describe stony soil in Scotland, and is now universally accepted as the name for unsorted debris deposited directly from a glacier. In Britain, the term boulder clay was often used to describe unsorted glacial deposits, but is now considered misleading since till may contain neither, clay or boulders. Till deposits show considerable variation in sediment structure, degree of compaction, orientation of large stones or clasts and shape of clasts (Photo). These variations reflect different processes involved in the formation of till and are used as the basis for till classification.

There are two basic types of till, subglacial and supraglacial and these can be further subdivided according to how the debris was released from the glacier and whether the till is modified after initial deposition.

 

Subglacial Till

Subglacial tills are deposited at the base of glaciers (Photo) and consist of compact, clay-rich debris with subrounded and striated clasts. (Photo) Compaction is caused by the compressive weight of the overlying ice, and the high proportion of clay and modified clasts results from glacial crushing and abrasion at the base of the glacier. These tills often display pronounced orientation of the long axes of large clasts parallel to the direction of glacier flow, which is caused by the alignment of stones in response to the forces promoted by ice movement.

The sub-varities of subglacial tills include the following. (Fig)

  • Subglacial meltout till: formed by the direct release of debris from the base of a melting glacier. Meltout can involve the release of a thick debris layer or single stones.
  • Lodgement till: formed by the accretion of subglacial debris against rough bedrock. These tills are highly compact and directly overlie rock surfaces.
  • Deformation till: forms by the deformation of weak rock or pre-existing till by the pressures exerted by moving ice. Not suprisingly, these tills exhibit various deformation structures such as folds and shears.

 

Supraglacial Till

These tills form at the surface and around the margins of glaciers. (Photo) They are composed of loosely compact, coarse blocky debris with little or no clay and silt. The majority of clasts are angular in shape since this surface debris (which is mainly frost weathered rock that has fallen onto a glacier surface) is not modified by abrasion or crushing during transport (Photo). However, subrounded glacial debris can be present because basal material may be moved to the surface along compressive thrusts.

The sub-varieties of supraglacial tills include the following. (Fig)


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