Basics of Environmental Science

Earth Sciences / 31

In winter the water expanded as it froze, widening the crevices, and in summer the water shrank as
it melted, releasing flakes of rock and also large boulders. For those few weeks in summer when the
weather was warm enough to thaw the surface layers of the permafrost, turning soil locked solid by
ice into wet mud, the mud, together with large boulders embedded in it, slid downhill, only to be
brought to a halt when the temperature dropped and the mud froze again. Today, although there is no
permafrost, the scattering of boulders around the tors remains as a record of the climate more than
10000 years ago. Similar periglacial processes acting on the weak, jointed chalk of southern England
caused slopes to retreat through the loss of material from their faces and produced large deposits of
the angular debris comprising fragments of varying sizes called ‘coombe rock’ or sometimes ‘head’
(other definitions confine ‘head’ to deposits other than chalk). There are similar periglacial relics in
North America and elsewhere in Europe.

The Lynmouth flood

On Exmoor, in south-west England, in the summer of 1952, almost 230 mm of

rain fell in 24 hours on to land that was already waterlogged. The water drained

northward, carried in two rivers, the East and West Lyn, which enter the sea

together at the small village of Lynmouth, falling some 300 m in rather more

than 1 km. Unable to carry the volume of water, during the night of 15 and 16

August both rivers flooded and the overflow from the West Lyn cut a new

channel that took it through Lynmouth, rejoining the original course at the mouth.

Houses, roads, and bridges were destroyed, an estimated 40000 tons of trees,

soil, boulders, and rubble and masonry from collapsed structures piled up in

the village, and 31 people were killed. The disaster was caused by nothing

more than rain. Lynmouth was subsequently restored and is now a popular

and attractive holiday resort.

Present permafrost regions occur in latitudes much higher than Britain. In Canada and Alaska within
the Arctic Circle in places the permafrost is 400 m thick and in parts of Siberia it is 700 m thick. In
Resolute Bay, in the Canadian Arctic, it extends to a depth of about 1000 m. Overall, nearly 20 per
cent of the land area within the Arctic Circle is permafrost, and has remained in this state since the
retreat of the ice sheets that once covered it.

Ice sheets are major sculptors of landscapes. As they move, they scour away all soil and other loose
material, pushing it ahead and to the sides of them, where it may form moraines. They smooth
angular rocks and the weight of the ice depresses the ground beneath. During a major glaciation ice
sheets may grow to a thickness of more than 2500 m and depress the underlying surface by 600 m,
which may take it to below sea level. As the ice retreats, the surface rises again, but it is a slow
process, at least as measured on a human scale. Northern Canada, where shore-lines rose several
tens of metres in less than 1000 years, and Scandinavia are still rising to compensate for the loss of
their ice sheets around 10000 years ago; in Scandinavia the surface was depressed by about 1000 m
and has subsequently risen by 520 m. This ‘glacioisostasy’ demonstrates the slight flexibility of the
Earth’s crust.

Because there is a lag between the disappearance of the ice sheets and the recovery of the original
surface elevation, bowls may remain where the ice was thickest. Depending on their location, these
may be flooded by the sea or fill with fresh water. The North American Great Lakes and the Baltic
Sea were made in this way. On a much smaller scale, so were the lakes of the English Lake District.