Mechanisms for Glacial Transport

Subglacial Transport
Subglacial Transport
Fig.1 Image showing entrainment of bedrock at the base of a glacier by plucking.

Plucking (Fig.1)
Glaciers may become frozen to the base of the bedrock upon which they lie. Glacial flow down slope leads to the plucking of material from the underlying bedrock, these plucked boulders become entrained in the glacier and subsequently flow down slope.

Fig.2 Image showing an esker- a subglacial stream channel which transports sediment down glacier.
Stream Transport (Fig.2)
Subglacial streams and channels may develop at the base of a glacier from frictional melting of ice. These streams carry sediment down glacier.

Characteristics of Material Transported Subglacially
Rocks transported at the base of a glacier by entrainment or transport by basal channels tend to be more rounded and less angular. This material is subjected to abrasion against bedrock and tumbling action of flowing water which cause the material to lose its sharp edges. Many subglacially transported rocks are striated from contact with bedrock at the base of the glacier.

Rounded Rocks                 Striations

Supraglacial Transport

Supraglacial Rock Transport
Fig.3 Boulders and sediment debris are transported on top of the glacier down slope to the terminus of the glacier.

Rockfall and Sediment Deposition (Fig.3)
Landslides and rockfall onto glaciers are transported down the glacier and deposited on top of moraines or subsequently calved off as iceberg rafted debris.

Characteristics of Material Transported Supraglacially
Rockfall and sediment carried atop glaciers does not change in shape or roundedness as it travels down glacier. This material is sitting on the glacier surface and not being abraded by the rough bedrock below. These rocks tend to be more angular and are not striated.
Angular boulders

Iceberg Rafting
ice berg rafting
Fig.4 Boulders being transported by an iceberg calved off a glaicer. These boulders egan as supraglacial rock debris.

Characteristics of Material Transported by Iceberg Rafting (Fig.4)
Iceberg rafted debris tends to be angular and is not striated, this debris and sediment began as supraglacial rockfall and was not subjected to abrasion at the base of the glacier.

angular boulders