Glaciers in the Past

When a glacier emerges from its U-shaped valley it spreads out over the surrounding lowlands as an ice sheet. Much of the surface material eroded by the glacier and carried by it to the plains is depos­ited when the ice starts to melt. The pre-glacial lowland landscape is therefore often markedly modified by various deposits left behind by the ice.

Surface Deposits

When the great northern continental ice sheets reached their most southerly extent, they depos­ited a ridge-like terminal moraine that was con­tained at the ice front.

Similar ridges, known as recessional moraines, have resulted from pauses during the retreat of the ice sheet. The North German Plain is traversed by a series of parallel cres­cent-shaped (arcuate) moraines which were formed as the Scandinavian ice sheet advanced across the Baltic. The main line of low morainic hills can be traced southwards through the Jutland peninsula, and then eastwards through northern Germany and Poland.

The Baltic Heights repres­ent the most clearly defined moraine, reaching more than 360m in height near Gdansk. Similarly a series of moraines cross the plains to the south of the Great Lakes, marking the various halts in the recession of the North American ice sheet.

Behind each terminal moraine groups of low, hummocky hills known as drumlins often occur. These hills were formed as the ice sheet retreated and most are elliptical mounds of sand and clay, sometimes up to 60m high, and elongated in the direction of the ice movement. How they were formed is not known but it is thought that they were caused by the overriding of previous ground moraine.

Drumlins are arranged in an echelon, or belt, and form a distinctive drumlin topography. A drumlin field may contain as many as 10,000 drumlins - one of the largest known is on the north-western plains of Canada. Around Strangford Lough in County Down, Ireland, drumlins form islands within the lough itself.

Winding across glaciated lowlands, there are often long sinuous gravel ridges called eskers. They are thought to be deposits formed by subglacial streams at the mouths of the tunnels through which they flowed beneath the ice. Eskers are common in Finland and Sweden, where they run across the country between lakes and marshes.

When a delta is formed by meltwater seeping out from beneath the ice front, it develops into a mound of bedded sand and gravel known as a kame. In some areas kames are separated by water-filled depressions called kettle holes, formed orig­inally as sediment piled up around patches of stranded ice which melted after the recession of the ice sheet.

The chief product of glacial deposition is boulder clay, which is the ground moraine of an ice sheet. It comprises an unstratified mixture of sand and clay particles of various sizes and origins. For example, deposits in south-eastern England contain both chalk boulders of local derivation and igneous rock from Scandinavia.

Blocks of rock that are transported far from their parent outcrop are known as erratics. The largest blocks are commonly seen resting on the boulder clay surface or even perched on exposed rock plat­forms.

The unsorted ground moraine behind the ice front contrasts strongly with the stratified drift of the outwash plain beyond. Meltwater streams deposit sand and gravel on the outwash plains to form the undulating topography so typical of the Luneburg Heath of Germany or the Geest of the Netherlands.

Proglacial Lakes

At the end of the Ice Age, many rivers were dammed by ice and their waters formed proglacial lakes. During the retreat of the North American ice sheet, for example, a large lake - Lake Agassiz - was dammed up between the ice to the north and the continental watershed to the south. The remnants of this damming can be seen in Lake Winnipeg, which is now surrounded by lacustrine silts that were deposited on the floor of the ancient Lake Agassiz.

Beach strand lines are sometimes visible, which indicate the water levels at various stages in the draining of a lake. This probably occurred when the proglacial lake overflowed through spillways at successively lower levels as the ice began to recede.

In north-eastern England there is striking evidence of the diversion of drainage by ice. Pre-glacial rivers flowed eastwards into the North Sea, but were blocked by the Scandinavian ice front as it approached the base of the North York Moors. The Eskdale valley in the moors was turned into a lake which overflowed southwards via a spill­way into Lake Pickering, about 25km distant. This lake in turn drained through the Kirkham Abbey Gorge about 10km away, and today the River Derwent still follows the southward route to the River Humber, having been diverted by ice from its pre-glacial eastwards course.

Pre-glacial Features

Beyond the ice sheet margin lies the periglacial zone of permafrost, in which repeated freeze and thaw cycles result in the breaking of the soil sur­face and the differential sorting of loose fragments of rock so that a pattern is produced. On flat sur­faces polygonal arrangements of stones occur, whereas on sloping surfaces, parallel lines are formed. Another periglacial landform is the pingo, or ice mound, created when a body of water freezes below ground and produces an ice core which raises the surface into a low hillock.

Evidence of solifluction is also found in peri­glacial areas - a special type of soil flow in sloping ground in which a highly saturated soil layer over­lies the permafrost. Solifluction debris, of frost-shattered fragments in a clay matrix, is referred to as head or coombe rock and often occurs at the foot of scarp slopes or in chalk valleys.