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Historical geology reveals fascinating facts about the British Isles

Posted in Geography, Geology, Historical articles on Friday, 10 August 2012

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This edited article about geology originally appeared in Look and Learn issue number 761 published on 14th August 1975.

Geologists, picture, image, illustration

Victorian geologists advanced this novel scientific study of the British Isles by Peter Jackson

Many Britons travel abroad to see the scenic wonders of other lands. Yet they have one of the world’s geological marvels right here under their feet. For the geologist, the British Isles are almost a complete “text-book” in themselves.

In the thousands of millions of years that it has taken the Earth’s surface to reach its present state, there have been many stages. Great subterranean upheavals have forced the Earth’s crust upwards, or caused rifts or faults; volcanic eruptions have poured out lava to form new layers of rock; seas have deposited their sediments to form yet more strata. In between, the erosive action of wind and water, frost and ice, has played its ceaseless part in shaping hill and plain.

In the British Isles, almost every stage of this complex geological process can be found recorded.

Great Britain and Ireland, and the thousands of smaller islands, are really part of Europe. They rest on a part of the continent which is submerged – the Continental Shelf. Some 500 to 600 million years ago, the whole of the region, including much of what is now the continent, is believed to have been under water. During the long period that this continued, deposits built up on the sea-floor to build layers on top of the even older rocks.

In time, disturbance of the Earth’s crust caused uplifting of the sea-floor, volcanic eruptions playing their part in the changes that followed. Land began to appear above the waters. Earliest to appear were parts of Scotland and Wales, and England’s Lake District and south-western region. At one point Scotland was joined to Norway as part of a “Scandinavian continent”.

All over the British Isles region, first one area and then another became dry land, only to sink again beneath the waters later. These successive processes have left their mark all over our islands – in the volcanic rocks of many of the mountain regions, and the various sandstones and limestones formed from the sediment in long-vanished seas.

At one time, there were large areas of swampy forest land, flourishing in a warm climate. The decayed remains of these forests became the abundant coal measures of today.

The continental area was also undergoing changes. There came a time when a wide stretch of sea covered much of southern Britain and the neighbouring area of northern France. The tiny marine creatures in this sea left their shells and skeletons on the sea-bed, and this deposit became a thick layer of chalk.

When uplifting of the earth occurred, and the sea receded, great plains of chalk were exposed. Though it is not certain that the whole of the English Channel area was uncovered, certainly a wide land-bridge existed between Britain and the mainland. And the rock systems of our south coast are now duplicated on both sides of the Channel, from the chalk of cliffs of Dover and Calais to the granite and other rocks farther west.

By about a million years ago, our islands had probably acquired something like their present shape, though the Dover Strait did not yet exist. The North Sea was a great estuary for the Rhine, with the Thames flowing in as a tributary.

Soon after this period, there began the most recent of the major geological events – the Great Ice Age, during which polar ice invaded the temperate zone on several occasions. Glacier-borne rock deposits have been identified as far south as the Thames Valley. The Highlands of Scotland, previously a high plateau, were carved by the ice into their present shape.

At some point during or soon after the Ice Age, our island finally became severed from the continent. Today Great Britain can be divided into two distinct zones by drawing a line from the mouth of the Tees in north-east England to that of the Exe in the south-west. To the north and west of this line, the land is largely mountainous, to the south and east lies lowland Britain.

The flat or undulating country of the lowlands may seem to vary little. But both surface soils and the rocks lying hidden, or appearing in outcrops, vary greatly.

To travel from London to Birmingham is to go back in time about 200 years. From the comparatively young clays and gravels of the London Basin, you cross successive lines and escarpments of different geological composition, starting with the chalk of the Chilterns, and ending with the red sandstone of the north and west Midlands – the remains of what was once an extensive desert.

This is called “new” red sandstone, to distinguish it from the even more ancient “old red sandstone” of Devon and Cornwall, Wales and south-west Ireland.

Ireland’s geological pattern is fairly simple, with a central plain resting on a limestone bed, and highlands distributed round the rim. There is a marked resemblance between the rocks of northern Ireland and those of Scotland.

These include volcanic rocks of great antiquity, like the basalt columns of Antrim’s Giant’s Causeway – matched on the Scottish side in Fingal’s Cave – and the gneisses of Donegal and the North-west Highlands. The Snowdonia peaks in Wales, and those of the Lake Districts, are also the result of volcanic activity.

The endless variety of structure can be observed almost everywhere. The clay in a London garden, the rich alluvial soil of the Fenlands, the marl of the Trent Valley – all have a geological story to tell.

More obviously interesting is the evidence of bare rocks – strata revealed on a cliff face, for example, or the differing character of pebbles on our beaches. In the south of England, the North and South Downs are surviving edges of a great dome of chalk which once covered the Weald country. The tough limestone of the Border country contrasts with the softer limestone, or oolite, of the Cotswolds.

In some places, isolated outcrops of volcanic rock, the residue of primeval mountains, protrude through the younger sedimentary strata – as in Devon, and parts of Wales, Ireland and the Scottish Lowlands.

For anyone who keeps his eyes open, there are many such examples to be seen of the endless variety that makes up the jigsaw we call the British Isles.

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