Glacial Till :: miscellaneous

Glacial Till The characteristics of glacial till reflect the erosional, transportational and depositional history of the constituent sediment.’ Glacial till is a heterogeneous mixture of rock fragments ranging in size from clay to boulders and is deposited directly from glacial ice without water transport. (Strahler A. H. & Strahler A. N. 1976). Ice sheets deposited in the Pleistocene period may be more than 30 m deep and inevitably the constituents of glacial till will reflect the retreats, advances and stationary phases of the ice which have taken place during the history of a glacier since its formation. I shall outline the various forms of glacial till, giving consideration to the type of prevailing climatic conditions which give rise to their formation and deposition to illustrate how the constituent sediment does provide evidence for glacial activity over time. As glaciers are so effective at erosion and transport, large quantities of debris is also associated with them. According to it’s location with respect to the glacier, such debris transported as ice mass may be divided into three main categories. There are three main positions that a glacier can transport debris, englacial debris which occurs within the glacier, supraglacial debris which occurs on the glacier surface and subglacial debris which occurs on the base of the glacier, (please look at figure 15.12 below). While debris is being transported it may remain in any one of these positions until it is deposited by the ice directly or it may end up being reworked by melt water. Deposition of the transported material is a complex process, but the fraction deposited directly from the ice is called till. It consists of a wide range of grain sizes, so it is often referred to as boulder clay. It also posses very little stratification and frequently contains far travelled erratic material, which tends to have clast with edges and corners blunted by abrasion. (Briggs D. Et. Al. 1997) There are two main different types of till which have been recognised, lodgement till and ablation till. Lodgement till is laid down subglacially when debris is released directly from the sole of the ice. Ablation till accumulates initially in a supraglacial position and is later lowered to the ground surface by undermelting. Ablation till can be further subdivided into meltout till and flow till. Meltout till is a direct product of ablation continuing beneath a cover of detritus and flow till consists of debris that has built up on ice and after saturation with melt water becomes so unstable that it flows or slumps into near by hollows.