Glacial Systems

Glacial systems operate much like viscous fluvial systems. Sediment is abraded and plucked from beneath in a source area and transported to a depositional basin. Unless in a coastal setting, the preservation potential for glacial deposits is low.

Conversion of Snow to Ice

If snow accumulates more rapidly than it melts, over time it will undergo a conversion to a dense ice that resembles a metamorphic rock and is capable of flow.

Plastic Deformation

To be considered a glacier, ice must be thick enough that it can flow under its own weight.

Sediment Transport

Because of its high viscosity, ice is a highly competent agent of erosion and sediment transport. Glaciers act as conveyor belts, moving extremely large quantities of sediment from the source area to the basin.

Controlling Factors

Cold temperatures and significant precipitation are both required to produce ice of sufficient thickness to flow as glaciers. Therefore, glaciers are restricted to areas of high latitude and/or high altitude.

Mass Balance

Whether a glacier front advances or retreats is controlled by the relative balance between precipitation in the zone of accumulation and melting in the zone of ablation. These are separated by the snow line, which identifies the point where these two factors are equal.

Frozen Drainage System

Glaciers are, in essence, a frozen drainage system. Water (snow/ice) and sediment are collected primarily near the head and transported to a depositional basin at the terminous.

Erosion

Weathering(Frost Wedging)

Deposition

Transportation

Dominant Processes

Internal shear

Erosional Features

Plucking extracts angular boulders leaving irregular depressions. Plucked boulders and other materials become frozen into the glacier base and are drug over the surface causing abrasion, which leaves striations and grooves.

Advancing Glacier Front

Actively advancing glaciers occur when snow accumulates more rapidly than it melts and are characterized by steep, well-defined fronts of relatively clean ice.

Retreating Glacier Front

Glacial retreat occurs when melting occurs more rapidly than snow accumulates and is characterized by gentle, poorly-defined fronts , potentially covered by debris, lakes, and vegetation.

1957 1980

Simplified Model

As glaciers advance and retreat, they create a wide and complex variety of depositional facies.

Continental Glacial Deposits

Till

Because of the high viscosity of ice, glaciers transport all sizes of sediment from clay to very large boulders. This sediment is all deposited together in morraines as till.

Ground Morraine

If more material accumulates beneath the glacier than can be transported, it is deposited as a ground morraine, consisting of irregular mounds of sediment. Upon melting numerous small, nonconnected lakes tend to form.

Eratics

Materials can be transported long distances by ice, depositing sediment that does not match the surrounding rock units.

Eskers

Water flows in tunnels beneath the ice as subglacial streams, carrying large quantities of sediment. Upon melting, the sediment is deposited upon the surface.

Outwash Delta

Esker

Outwash

As the ice melts, water and sediment are transported away from the glacier front. Because of the high volume and large size of sediment, outwash streams tend to be braided. Also referred to as glaciofluvial or fluvioglacial deposits.

Glacial Map of North America