Collins New Naturalist Library. L. Matthews Harrison

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      In all the glacial stages there were at least two maxima of cold separated by less cold interstadial intervals, and similarly in the interglacial stages the climate fluctuated between cold, temperate, and warm. The beginnings and ends of the glacial stages were gradual, so that as the ice retreated after a glaciation the land was at first polar desert becoming steppe or tundra as the temperature rose; it was then invaded by open boreal forest with birch and pine dominant, which in turn was replaced by dense deciduous forest with alder, oak, ash and other broad-leaved trees. As a glacial stage approached the succession was reversed.

      Fig. 4. Stages of the Pleistocene in the British Isles.

      Apart from the climatic changes correlated with the glaciations and producing their advances and retreats, there were during the Pleistocene great changes in the level of the sea in relation to the land. The enormous masses of water withdrawn from circulation and locked up in the form of ice caused a fall in sea level of many hundreds of feet – indeed, it is reckoned that if all the ice even now in the form of glaciers and ice-caps were to melt the level of the sea would rise about three hundred feet.¹⁵⁵ On the other hand the land is depressed towards sea level during glaciation by the sheer weight of ice resting upon it. At the same time there has been throughout the Pleistocene from time to time a slow upraising or lowering of the land, the eustatic movements of the tectonic plates.

      An important consequence of these changes in sea level, whether caused by withdrawal of liquid water or by movement of the land, was that the British Isles were periodically part of the continent of Europe so that they shared its fauna and flora. Thus the bed of the southern part of the North Sea has for long periods been dry land, and the final opening of the Straits of Dover did not come about until some seven thousand years ago. One cannot help wondering whether this was a sudden dramatic happening in some furious equinoctial gale when low atmospheric pressure and a high spring tide combined with a surge such as those that have brought disastrous floods to East Anglia in recent times, broke the crumbling barrier and sent the waters of the North Sea pouring over into the English Channel – or whether an unusually high tide crept over a low dune between the salt marshes on each side so that the waters met and mixed with so little fuss that no one would have noticed.

      The connection with the continent facilitated the return of the flora and fauna after it had been exterminated by each glaciation. At the time of the greatest glaciation some 450,000 years ago an unbroken sheet of ice covered the whole of northern Europe, including the British Isles, except southern England south of a line joining the Thames to the mouth of the Severn.⁶¹ The part left free of ice was deeply covered with winter snow, and the sea was full of floating ice. It is doubtful if any of the flora or fauna was able to live there; certainly no mammals could survive, and consequently our present fauna must have arrived after the ice of the great glaciation retreated.

      Subsequent glaciations were less extensive so that the midlands as far north as York and the southern part of Ireland were free of ice and provided a possible habitat for those species that could withstand the arctic or subarctic conditions. The changes in flora and fauna are sometimes spoken of as retreats to more congenial climates in the south during the glaciations – the distribution of the plants and animals retreated, but there was no physical movement of individuals, they were merely killed. The return during interglacials was different; the flora gradually spread in by the usual manner of seed dispersal, but the animals and especially the mammals did move in ‘on the hoof’, not as mass migrations but in the course of populations extending their ranges under pressure of numbers as new habitats became available.

      The amount of extermination among the mammalian species even in the last glaciation, which did not blanket the whole of the British Isles and ended some twelve to ten thousand years ago, is shown by comparing the 167 species of land mammal now living in western Europe with the 41 of Great Britain and the 21 of Ireland.¹⁵¹ Our fauna is not so much ‘impoverished’ as incomplete; there was not a long enough period of time before the breaching of the Straits of Dover for more species to extend their range into the islands. As H. W. Bates, the naturalist of the Amazon and later for many years secretary of the Royal Geographical Society, СКАЧАТЬ