Collins New Naturalist Library. L. Matthews Harrison

Чтение книги онлайн.

      Fig. 2. Position of the ice edge at maximum cover of Eurasian glaciation during the Anglian glaciation of the British Isles.

      As with all new theories before they become accepted as established truth, the glacial theory at first met with much opposition as well as ridicule – indeed Buckland himself at first strongly disagreed, and it was only during a tour of Scotland in company with Agassiz in 1840 that he was convinced. Thereafter he has strongly supported the theory and, through the evidence of glacial action on polished and scoriated rocks and the presence of morraines, most of the leading geologists of the day agreed with him. He was also the first to suggest that the famous parallel roads of Glenroy in Scotland were the former shore lines of a glacial lake formed by the damming of Glen Spean by two glaciers coming down the north and east sides of Ben Nevis.

      The knowledge that the country, and later that all countries on both sides of the north Atlantic, had once been in the grip of an Ice Age stimulated geologists to more detailed research, and it soon became apparent that there had been not one Ice Age but several. The difficulties of identifying and dating them were enormous, because younger glaciations are bound to disturb, distort, and confuse the traces of older ones, as are denudation, erosion, and changes of sea level in the often long intervals between them. Local variations in the extent and intensity of glaciation further complicate the problem.

      The basic pattern of the successive glaciations in Europe was appropriately worked out by investigating the glaciations of the Alps, where Agassiz had first discovered evidence of the ‘Ice Age’. About the beginning of this century Penck & Brückner¹²², after prolonged study of the gravel terraces laid down by rivers rushing forth from beneath the melting glaciers, concluded that there had been four main ice ages separated by long interglacial periods when the land was free from ice-cover and the climate was comparatively warm. They named the four glaciations after rivers flowing down from the Austrian alps to southern Germany, in the valleys of which they examined the fluvioglacial gravels and moraines; the oldest they named Günz, and the succeeding ones Mindel, Riss and Würm.

      The last glaciation, the Würm, reached its peak about 20,000 years ago, but it was not so severe or long-lasting as some preceeding ones. During the Mindel glaciation the ice sheets reached their greatest size and covered an enormous area of Europe, much more extensive than that covered in the later Riss and Würm stages. The interglacial stage between the decline of the Mindel and the onset of the Riss lasted nearly a quarter of a million years, during which a contemporary intelligence might have thought that ice ages had gone never to return. Although the Günz was designated the oldest or first glaciation, there are now known to be indications of numerous glaciations older still, hence the differences of opinion between authorities on the probable length of the Pleistocene epoch. There cannot, in any case, have been any sharply defined boundary between the Pliocene and the Pleistocene, for the whole of geological and biological evolution is a continuous process. The boundaries between all the geological epochs are arbitrary, and are used merely as a convenience with the tacit admission that they cannot represent any specific moment in time.

      The history of the Pleistocene is, however, by no means the simple and clear cut sequence as might appear from the basic pattern. During glaciations the edges of the ice sheets advance and retreat to different extents and in different places, and during interglacial periods they re-advance from place to place and retire again in an unending chain of fluctuations that bring variations in topography, climate, flora and fauna. Furthermore, the sequence worked out for the glaciations of the Alps may not correspond exactly with those found elsewhere.

      Geologists of many lands studying the glaciations and alternating interglacial periods of the Pleistocene in their own countries have gone deeply into the problems of correlating their local findings with the basic alpine pattern. A general measure of success has been achieved in this though much detail remains obscure, and the sequences in Scandinavia and northern Europe and in North America are found to correspond reasonably closely. They are, as well, found to correspond with the pluvial sequences found in land further south which, though never covered with ice sheets, experienced periods of high rainfall when the ice held more northern latitudes in its grip.

      Although the pattern of successive glaciations in the Alps corresponds roughly with that of other parts of Europe and elsewhere, it is in some ways a special case. Even at the maximum of glaciation when a continuous sheet of ice blanketed northern Europe and Asia and covered the British Isles and the site of the North Sea, the ice cap over the Alps was separate and not continuous with the great ice sheet. The causes of the glaciations were similar for both regions but the effects were subject to local variations; consequently the nomenclature for the Alpine glaciations is now applied less uniformly to those of regions further north, including the British Isles.

      The difficulty of making exact correlations between Pleistocene events in different places has been resolved by classifying them according to local stratigraphy. Pleistocene deposits, both those of glacial and interglacial stages, are not continuous, and the geologists have to put together the history of the epoch from the examination of scattered and limited samples from many different places. The glacial and interglacial stages are named after the places where well-known deposits of each stage have been studied, and consequently the nomenclature for north western Europe differs from that for the Alps, and from that for the British Isles. Thus the last or Würm glaciation of the Alps corresponds to the Weichselian glaciation of north-western Europe, and the Devensian of the British Isles.

      In the British Isles many of the typical pleistocene sites are found in East Anglia and take their names from nearby towns and villages of Norfolk, Suffolk and Essex. The last glaciation however takes its name from the Devenses, the ancient British tribe that lived over 50,000 years later in the area including Four Ashes in Staffordshire, the typical site.¹²⁸ The succession of deposits is not complete, so that information is lacking about the earliest Pleistocene, and for a period of about a million years in the middle Pleistocene. In spite of these gaps the deposits indicate alternating colder and warmer phases but give no unequivocal evidence of glaciation, with ice sheets covering much of the country, until comparatively late in the epoch when ice cover reached its maximum during the Anglian glaciation, corresponding with the Elster glaciation of northwest Europe and the Mindel of the Alps.

      Conditions immediately after the end of the Pliocene, some two to two-and-a-half million years ago, are imperfectly known but there appears to have been a cold stage at first, represented by the Nodule Bed at the base of the Red Crag deposits of East Anglia. A gap in the record of nearly half a million years is then followed by an alternation of two warm and two cold stages represented by pre-glacial deposits of the lower Pleistocene. These are the Ludhamian (Ludham, near Norwich) warm, Thurnian (river Thurn, Norfolk), СКАЧАТЬ