till

The unsorted sediment deposited directly below a glacier, which exhibits a wide range of particle sizes, from fine clay to rock fragments and boulders. The rate of till flow beneath a glacier is thought to depend on basal shear stress (as for bedload) and on the effective pressure N, defined as the difference between overburden pressure and the interstitial pore water pressure within the sediments.

The lithological character of a till depends on the geology of the region the glacier has travelled over. Till is usually responsible for monotonous relief, sometimes diversified by the presence of kettle holes, and sometimes overlain by ablation moraine. Where sheets of till are old, they may form till plateaux, such as the smooth surface lying between Cambridge and St Neots, filling up stream valleys beneath.

Till results from melting at the surface and at the base of the glacier; the latter probably being of more importance. Basal tills are likely to be formed when the lower, debris-rich layers of a glacier are slowed down, perhaps by an obstacle. The material is then compressed, and water squeezed from it, by the weight of the ice above. A till plain blankets the ground, with only a few mounds and ridges poking through. Other tills are classified by means of their origin: ablation, or meltwater till, lodgement till, and sublimation till. Flow till is created when saturated debris, found at the top of the ice, flows into depressions within the ice and is then deposited.

Till fabric analysis is the study of the fabric of till to determine the movements of the glacier. The direction of ice advance is indicated by the orientation of the long axes of the pebbles incorporated in till, or by tracing the origin of pebbles. Where tills are interbedded with other sediments, such as meltwater or periglacial deposits, then a sequence of events—a glacial stratigraphy—can be built up.