Bedrock Geology UK North: The surface layer
|This topic provides descriptions of the rock types appearing on the British Geological Survey 1:625 000 scale map of the UK North and gives a brief explanation of their origins.
Author: P Stone (BGS); Contributor: A A Jackson (BGS)
The great variety of rocks that underlie Britain and Ireland can only rarely be clearly seen at the surface. Sea cliffs and mountain crags provide the exception rather than the rule, and for the most part the geological features are concealed beneath a cover of loose detritus, soil and vegetation. Some of this has been formed in situ, from the weathering of the underlying rocks, but across much of northern Britain and Ireland the superficial cover was introduced, quite recently in geological terms, by ice sheets and meltwater during an ‘ice age’ that began to affect north-west Europe from about 2.6 million years ago with the last glaciers disappearing from the Scottish Highlands only about 10 500 years ago. During this period, bitterly cold intervals alternated with relatively warmer conditions, and the glaciers and ice sheets advanced and retreated accordingly. As they did so, they carried in and left behind vast quantities of sand, rubble and huge erratic boulders (Plate P002861). Sheets of till — a heterogeneous mixture of rock debris — were plastered over the landscape, and sculpted by the passage of the ice into streamlined, elongate mounds. The till deposits commonly range up to tens of metres in thickness. Once the ice began to melt, copious meltwater washed out sand and gravel and redeposited it in enormous fluvial spreads, whilst piles of debris built up as moraines around the margins of the residual glaciers and now create distinctive landforms (Plates P219672 and P064460).
Although in rather less dramatic fashion, the cycle of erosion and deposition continues today. Eroded detritus from our hills and mountains is carried down by rivers and either redeposited in flood plains and estuaries, or carried farther out to sea into the subsiding marine basins that surround our coasts. So are formed the rocks of tomorrow.
A large erratic boulder of Devonian conglomerate dumped in glacial outwash sand and gravel near Callander, Perthshire. P002861.