The subterranean forces which throughout geological time have been so potent in the area of the British Isles, appear to have on the whole remained nearly quiescent during the centuries of authentic history. Volcanic energy, which played so notable a part throughout most of the geological past of our region, has never awakened here since the remote Tertiary ages, but has shifted its site northwards to Iceland, where it continued its activity even during the Ice-Age and has remained vigorous down to the present time. But two forms of underground movement have affected Britain since man appeared here. In the first place, there have been important changes in the relative levels of sea and land, and in the second place, the islands have frequently been shaken more or less perceptibly by earthquakes.
In southern Scotland, the north of England and of Ireland, the land has been upraised some twenty feet or more. As a consequence of this elevation a selvage of flat alluvial land has been added to a number of the estuaries, particularly to those of the Forth, Tay and Clyde. Canoes and other remains of human workmanship, found imbedded in the sands and silts of this marine terrace, show that the upheaval has taken place since some part of Neolithic time. The addition of these strips of level ground to the margin of the land has had an important influence on the human population of these districts, for it has provided many hundreds of square miles of arable ground, comprising admirable sites for dozens of villages and towns as well as for excellent coast-roads.
On the other hand, a long tract of land, extending from Cork across the Bristol Channel and the southern counties to the coast of Yorkshire, has sunk fifty feet or more since the Neolithic period. The result of this depression has been to allow the sea to ascend far up many of the estuaries which had previously been river-valleys, and thus to create, or at least to deepen and widen, the numerous natural harbours which indent the south coasts of England, Wales and Ireland. Not improbably the downward movement, by helping to lower the narrow isthmus which in the earlier ages of the human period connected Kent with the north-west of France, contributed not a little to the insulation of England from the continent.