Glacial Environments

At least three ice ages when artic climates were more extensive - the late Precambrian, the Ordovician and the Permo-Carboniferous.Glacial Processes and ProductsVast amounts of sediment were eroded and transported by the actions of ice, water and wind during the Quaternary. Material deposited from beneath glaciers and ice sheets is known as till or boulder clay and covers much of the the surface in northern England and the Midlands. Following the melting of the ice sheets and glaciers, large volummes of sand and gravel were transported along river valleys or deposited along the edges of the ice sheets. In some areas, large ice-damned lakes formed in which clays and sands were laid down.Boulder ClayWhere rocks obstruct the movement of a glacier, the pressure of the ice on the upstream side of the rock causes local melting of the ice. The water flows to the downstream side of the rock where it freezes in any fractures that exist in the rock. This allows the ice to pluck up large fragments away as it refreezes and the ice moves on due to gravity.Fragments of rock at the base of a glacier scratch striations on underlying rock surfaces. Abrasion during transport produces fine grained, crushed rock fragments.When ice melts, it deposits poorly sorted material in the form of boulder clay (till), which may become lithified to form tillite. Boulder clay or till is composed of the unsorted materials of glacial erosion. The main characteristics are: poor sorting angular fragments large clasts randomly orientated and scattered throughout the clay clasts may be striated may contain erratics Boulder clay deposited from melting ice sheets covers very large areas.Fluvio-glacial DepositsThese are sediments that have been transported by glacial meltwater streams before being deposited on an outwash plain some distance away from the melting ice. They are mainly composed  of gravels and sands but can contain coarser pebbles. They are sorted by fast flowing braided streams and may show cross bedding and sometimes graded bedding. Most of the finer mud and silt is carried further away, often to lakes, where it deposited as varves.VarvesWhen glacier ice melts, lakes are formed. During the spring thaw, streams transported fine sand, silt and clay. The sand and silt size grains settle to the floor of the lake first. The very fine grains remain in suspension, settling when the lakes freezes in winter. This produces fine grained lamination sediments called varves, in which the silty layer represents summer deposition and the clay layer represents winter deposition. Each pair of varves represents one year's deposition. They have been counted and used to date events at the end of the last ice age.Evidence for Glacial Deposits from FossilsGlacial sequences indicate cold climates, i.e. deposition in polar regions or during ice ages. Glacial deposits contain few fossils because the climate was generally too harsh for most forms of life.Oxygen isotope ratios can be measured using shells of marine microfossils. The ratios show ocean water temperature. Pollen grains are preserved in lake sediments. Some plants such as pine tolerate cold more than others such as oak and this affects their abundance in pollen samples from lake sediments.

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