Glacial till
Also called: Till, Boulder clay · Topic: Landforms and their Evolution: Rivers, Groundwater, Glaciers, Waves and Wind · NCERT: Class 11, Ch 6 "Landforms and their Evolution"
Meaning
Glacial till is the rock debris that melting glacier ice drops directly. Water does not move it first. It is an unsorted mix of large boulders, pebbles, sand and fine clay all jumbled together, which is why it is also called boulder clay. The pieces are mostly angular to sub-angular, meaning sharp-edged, because no water has rolled and rounded them. Moraines and drumlins are built of till. Its jumbled, angular character tells you ice laid it down.
Example
Below the Gangotri glacier's snout at Gaumukh in Uttarakhand, where the Bhagirathi rises, heaps of till cover the valley floor. Huge angular boulders lie mixed with sand and grey mud.
Don't confuse with
- Outwash deposit: meltwater streams lay down outwash, so it is roughly sorted, layered and somewhat rounded. Till is unsorted and angular.
- River alluvium: a river sorts and rounds its alluvium as it carries it. Till gets no such sorting.