Geological processes in the British Isles
Geological processes in the British Isles

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Geological processes in the British Isles

1 Landscape formation

This course is an introduction to the geological history of the British Isles, a remarkable part of the world upon which many of the great events in Earth history have left their mark. In this course we use the term ‘British Isles’ in its geographical sense, referring to the islands of Great Britain and Ireland and the adjacent lesser isles. The British Isles did not exist as such until comparatively recent times, and the surface environment of the continental crust that now forms this region has undergone dramatic changes during the geological history of the Earth.

You will gain the most from this course, and enhance your understanding of the geology of the British Isles, if you purchase the British Geological Survey's (BGS) 1:625 000 bedrock geology maps of the UK. These are available direct from the BGS at:

http://shop.bgs.ac.uk/Bookshop/product.cfm?id=BGUKN

http://shop.bgs.ac.uk/Bookshop/product.cfm?id=BGUKS

(accessed 26 March 2008).

They are referred to as Ten Mile Maps (N) or (S) throughout the course.

Figure 1, below, is an artist's impression of what part of northern England looked like during different periods of the region's geological history. You can see that over a relatively short period of time (~100 million years), the area experienced significant changes in:

  • climate (varying from subtropical to arid and semi-arid);

  • sea-level (with two transgressions onto the land by shallow seas); and

  • landmass relief (a progressive decrease in the height of the highlands).

Figure 1
Figure 1 A reconstruction of northern England looking westwards towards the Lake District at different geological times from the Early Carboniferous (a) to the Late Permian (d). The progressive northwards drift of the British Isles resulted in rocks from a wide range of environments being formed and later preserved. Much of the Lake District highlands consists of Ordovician volcanic rocks. During the Early Carboniferous (a), the region lay close to the equator, with the low ground covered by a shallow sea in which corals and other marine fauna and flora prospered. As this sea receded, this was replaced by the Late Carboniferous coal swamps (b) and later by the deserts of the Early Permian (c), as the British Isles continued to drift northwards towards the present latitude of the Sahara. By the Late Permian (d), a shallow sea had encroached on this area once again

Students who attend the Open University residential school week in northern Britain associated with this course will begin to evaluate the range of environments that existed there in the geological past. In the Lake District, a deep ocean has left a thick sequence of mudstones, shales and siltstones overlying and juxtaposed to a series of volcanic rocks. In contrast, the somewhat younger rocks outcropping in the Pennines (running down the centre of northern England) were laid down in a warm shallow subtropical sea conducive to the formation of limestones containing corals and other marine organisms (Figure 1a). At several times across the region, marine conditions were interrupted by sands and silts deposited in river deltas, with organic material, which eventually formed coal, deposited in coastal swamps (Figure 1b). Later still in the Vale of Eden (south of Carlisle), alluvial fans and sand dunes indicate a desert environment (Figure 1c). This was in turn superseded by a shallow sea, now represented by limestones and evaporite deposits (Figure 1d), which are found throughout the counties of Cleveland and Durham.

In addition to this sedimentary succession, a number of igneous extrusive and intrusive episodes have left their mark. Periods of uplift, deformation and erosion have also influenced the geological history of northern England and indeed the British Isles as a whole.

Geologists find out about these and similar changes by examining the different types of rock formed throughout the geological record, and comparing them with their modern-day equivalents. A number of climatic, sea-level and topographical changes can be recognised throughout the 3.8 billion years of the geological history of the British Isles. The majority of these relate to the ever-changing position of the Earth's tectonic plates.

Throughout this course you will find that geological Periods may be prefixed by the terms, ‘Early’, ‘Mid-’ and ‘Late’, or ‘Lower’, ‘Middle’ and ‘Upper’. The first three are used in the context of time (e.g. a specific age period), whereas the last three refer to successions of rocks. So, for example, Lower Jurassic rocks were formed during the Early Jurassic.

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