The unstratified, unsorted debris dropped more or less randomly by the glaciers is called till. Tills can be of two types –
- Lodgement Till
It is laid down subglacially when debris is released directly from the sole of the ice. This involves pressure melting of basal so that debris in it is released and then lodges or plastered on to the glacier bed. It may occur beneath stagnant ice, but it certainly occur beneath actively moving ice and hence, some of the till has the water squeezed out of it . During this process, alignment of the long axes of particles occurs. Lodgment tills can be built up to a thickness of many meters by progressive melting out from the glacier bed and by shearing of one layer of till over another.
Lodgment till may be shaped into a variety of landforms Where the ice is relatively thick and actively flowing, extensive, more or less flat till-plains may form. Differential stresses within the ice moulds the till into streamlines features such as flutes and drumlins. Flutes vary in height from a few centimeters to several meters and may be between a few meters and many kilometers in length.
- Ablation Till
It accumulates initially in a supraglacial position and is later lowered to the ground surface by melting. Ablation till consists of –
Meltout till is the direct product of ablation continuing beneath a cover of detritus.
Flow till consists of debris that has built up on the ice and after saturation with meltwater. It becomes so unstable that it flows or slumps into nearby hollows. Till is deposited as series of distinctive landforms.
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