Mammals in the British Isles. L. Harrison Matthews

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), cold, Antian (river Ant, Norfolk) warm, and Baventian (Easton Bavents, near Southwold, Suffolk) cold. At the end of the Baventian stage another gap in the record lasting about a million years is followed by the warm Pastonian stage (Paston, near Cromer, Norfolk), the first stage of the middle Pleistocene, about half a million years ago.

      The following Beestonian (Beeston, near Dereham, Norfolk) was the first cold stage of the middle Pleistocene and was succeeded by a warm stage, the Cromerian (Cromer, Norfolk), which lasted until the onset of the great glaciation over 450,000 years ago. This, the Anglian glacial stage (East Anglia), lasted between fifty and sixty thousand years and covered the whole of the British Isles as far south as the Thames with a sheet of ice that produced the greatest glaciation in the whole of the Pleistocene. When the Anglian stage came to an end the land was free of ice for about 185,000 years during the temperate Hoxnian stage (Hoxne, on the Suffolk–Norfolk border near Eye and Diss); in this stage the temperature was at times higher than that of the present day.

      The next glaciation, the Wolstonian (Wolston, near Coventry, Warwickshire) lasted some 60,000 years from about 240,000 to 180,000 B.P. The ice cover did not extend as far south as in the Anglian stage; the ice edge ran south from northern Norfolk and then west across the midlands to the mid Welsh border, thence turning south to reach and follow the north coasts of Somerset, Devon and Cornwall. The succeeding Ipswichian (Ipswich, Suffolk) temperate stage lasted about 60,000 years until about 120,000 B.P. when the cold returned with the onset of the last, Devensian, glacial stage in which the ice covered Scotland, northern England, Wales, and most of Ireland. A large area of the midlands and east Yorkshire was thus free from ice cover, though the ice covering the North Sea encroached on the east coast as far south as Norfolk. The ice of the Devensian stage melted comparatively quickly some twelve thousand years ago so that before 10,000 B.P. the post glacial or Flandrian temperate stage was established, which extends to the present day; it takes its name from the transgression of the North Sea over the former dry land bordered by England and Flanders, when the sea level rose as the water from the ice returned to the sea.

      Fig. 3. Limit of ice covering during (a) the Anglian, (b) the Wolstonian and (c) the Devensian glaciations.

      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.

Epoch	British Isles	Climate
Holocene	Flandrian	Temperate
Upper Pleistocene	Devensian Ipswichian Wolstonian	Glacial with permafrost Temperate Glacial with permafrost
Middle Pleistocene	Hoxnian Anglian Cromerian Cromerian Cromerian	Temperate Glacial with permafrost Temperate Cold with permafrost Warm
Lower Pleistocene	(Gap) (ca. 1 M. years)	Cold with permafrost Temperate Cold Temperate
	Baventian Antian Thurnian Ludhamian
	(Gap) (ca. 1/2 M. years)
Pliocene	Waltonian

      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