The term load is used for all drift that’s in transport by a glacier at a given time. As with fluvial sediment transport, keep in mind the distinction between the load and the transport rate. The transport rate of a glacier is the time rate of passage of sediment past some cross section through the glacier that’s stationary relative to the underlying bedrock.
The load is generally classified on the basis of where it’s transport in the glacier:
- supraglacial load: load transported on the surface of the glacier. It gets there by falling onto the glacier. It’s thus restricted mainly to valley glaciers. But near the terminus of the glacier, drift can reach the surface by ablation of the ice, and also by upthrusting.
- englacial load: load transported within the glacier. The quantities of englacial drift are always much smaller than of supraglacial drift and subglacial drift, because the load is obtained by the glacier at the bottom and the top of the glacier. Solids deposited on the surface of the glacier in the area of accumulation are buried and thus become englacial drift.
- subglacial load: load transported at the base of the glacier. This constitutes most of the load, just because most material is entrained at the base. Cold-based glaciers have little subglacial load. The dirtiest warm- based glaciers might have concentrations of up to tens of percent by volume, for thicknesses of a few meters above the base. The way this is known is by examination of now-dead ice at a stagnant terminus.