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7. What happens when the ice starts to melt?
When the ice sheet begins to melt and retreat this is the start of deglaciation. During this time huge amounts of meltwater are released, hence it is no surprise that the majority of landforms deposited during deglaciation are associated with glacial meltwater deposition. Most of the landforms are deposited at the edge of the ice sheet, or the ice margin. Others are deposited beneath or within the ice sheet in cavities, or originate on top of the melting glacier.Moraines are accumulations of till, gravels, sands, silts or clays which have been transported by ice at its margin. Terminal moraines are ridges which define the maximum extent of the ice sheet during that glaciation. As the ice advances during glaciation it pushes material in front of it and when it begins to melt and retreat having reached its maximum extent this material is left as the upstanding end moraine ridge. Ridges left at the margin while the ice retreats towards its source are termed recessional moraines. Therefore recessional moraines are always situated up-ice from terminal moraines. Moraines in lowland Ireland are often hummocky and composed of sands and gravels. They are usually discontinuous ridges which are traceable over a long distance. They are typically characterised by steep ice proximal slopes and less steep ice distal slopes.
Outwash river issuing from a glacier snout in Iceland.
Note the braided nature of the river.
As the ice melts rivers flow from the melting glacier towards the sea. Often these rivers are very wide and braided and carry huge volumes of sediment from the glacier. The rivers sort the material and deposit it in an outwash plain (see photo above). These plains are generally composed of well sorted sands and gravels. Many of Ireland's rivers were much wider during deglacial times and hence are today flanked by sands and gravels which were deposited when the river was much wider. Examples are the Barrow, the Liffey, the Boyne and the Lee.
Eskers are long narrow sinuous ridge of sand and gravel deposited by a stream which flowed under the ice and left behind after the ice melted. They are composed of alternate layers of sorted sands and gravels. They may be up to 30 m high and are often several kilometres long. They are very common in Ireland and occur extensively in the Midlands. Examples are the Clara and Clonmacnoise eskers in County Offaly, the Trim Esker system in County Meath and the Athlone esker system in Westmeath/Roscommon.
Kames are formed in depressions in the glacier surface and other cavities within the ice. They usually contain rounded glaciofluvial sands and gravels. They appear as rounded or irregular hills. As the ice melts, the sands and gravels which accumulate in cavities on and in the ice are let down onto the ground surface in the form of irregular mounds. They are commonly interspersed with kettle holes, which are rounded depressions left when blocks of dead ice from the ice sheet melted. Hummocky, pitted and irregular landscapes are thus said to have 'kame and kettle' topography. Kame and kettle topography is common in the Irish Midlands, in Westmeath, south Roscommon and Offaly.
