Norber Erratics

The boulders on the hillside at Norber are erratics from the last Ice Age. They are of national interest to geographers and geologists; being a very good example of the displacement of older rocks onto the top of younger rocks by the movement of glaciers and ice sheets.

During an Ice Age weather conditions are much colder, with snow lying throughout the year. As it accumulates the snow becomes so deep that the sheer weight of it compresses the lower layers into ice. This normally forms on high ground where it is colder and the snowfall is greatest. The ice behaves rather like water and flows downhill following the existing river valleys, being fed continuously by further snowfall on high ground. However it moves extremely slowly, advancing at a rate of perhaps 2 - 3 metres per year. As it progresses the ice grinds and plucks away at the rock underneath, deepening the valleys and rounding off any projections, and often carrying the rock debris some distance over a period of time. These 'rivers of ice' are known as glaciers. Modern examples are found in all the high mountain regions of the world, including the Alpine regions of France and Switzerland. As the glaciers build up they can eventually join to cover all low-lying land as vast sheets of ice, with the hill tops projecting above the ice.

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A warming of the climate heralds the end of an Ice Age and the ice sheets and glaciers eventually melt, often depositing rocks some distance from their original position. These 'out of place' rocks are known as 'erratics'. At Norber the erratic boulders were carried by ice flowing out of Crummackdale and over the brow of the hill. They have probably been carried about a kilometre during a journey lasting thousands of years.

The basic geology of Crummackdale is fairly straightforward. There are two major rock types present; Carboniferous limestone which was formed some 330 million years ago on the floor of a shallow tropical sea, and the heavily folded Silurian greywacke (a tough muddy-coloured sandstone) and slate (mudstone and shale compressed under pressure) which were formed about 100 million years earlier. As you would expect, the younger limestone lies on top of the older Silurian rocks. The junction between the two rock types can be seen at the foot of Studrigg Scar.

The glacial ice picked up Silurian rocks from the floor of Crummackdale and carried them along onto the limestone at Norber. When the ice melted these rocks were abandoned on top of the limestone as perched boulders. These are the Norber Erratics.

Since the end of the Ice Age 13,000 years ago they have acted as natural umbrellas and sheltered the soluble limestone immediately underneath them from rain erosion. Whilst the general surface of the bedrock has been lowered by about 0.5 metres, the erratics have been left standing on small plinths of protected limestone.

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