in the rigorous conditions of glaciation, when the ice-sheets dwindled almost entirely away, while plants and animals re-established themselves on the newly exposed soil. Glacialists may be ranged in two schools: those who believe that one or more phases of milder climatic conditions broke up the whole Glacial period into alternating epochs of glaciation and “deglaciation”; and those who believe that the intercalated deposits represent rather the localized recessional movements of the ice-sheets within one single period of glaciation. In addition to the stratified deposits and their contents, important evidence in favour of interglacial epochs occurs in the presence of weathered surfaces on the top of older boulder clays, which are themselves covered by younger glacial deposits.
The cause of the interglacial hypothesis has been most ardently championed in England by Professor James Geikie; who has endeavoured to show that there were in Europe six distinct glacial epochs within the Glacial period, separated by five epochs of more moderate temperature. These are enumerated below:
6th Glacial epoch, Upper Turbarian, indicated by the deposits of peat which underlie the lower raised beaches.
5th Interglacial epoch, Upper Forestian.
5th Glacial epoch, Lower Turbarian, indicated by peat deposits overlying the lower forest-bed, by the raised beaches and carse-clays of Scotland, and in part by the Littorina-clays of Scandinavia.
4th Interglacial epoch, Lower Forestian, the lower forests under peat beds, the Ancylus-beds of the great freshwater Baltic lake and the Littorina-clays of Scandinavia.
4th Glacial epoch, Mecklenburgian, represented by the moraines of the last great Baltic glacier, which reach their southern limit in Mecklenburg; the 100-ft. terrace of Scotland and the Yoldia-beds of Scandinavia.
3rd Interglacial epoch, Neudeckian, intercalations of marine and freshwater deposits in the boulder clays of the southern Baltic coasts.
3rd Glacial epoch, Polandian, glacial and fluvio-glacial formations of the minor Scandinavian ice-sheet; and the “upper boulder clay” of northern and western Europe.
2nd Interglacial epoch, Helvetian, interglacial beds of Britain and lignites of Switzerland.
2nd Glacial epoch, Saxonian, deposits of the period of maximum glaciation when the northern ice-sheet reached the low ground of Saxony, and the Alpine glaciers formed the outermost moraines.
1st Interglacial epoch, Norfolkian, the forest-bed series of Norfolk.
1st Glacial epoch, Scanian, represented only in the south of Sweden, which was overridden by a large Baltic glacier. The Chillesford clay and Weybourne crag of Norfolk and the oldest moraines and fluvio-glacial gravels of the Arctic lands may belong to this epoch.
In a similar manner Professor Chamberlin and other American geologists have recognized the following stages in the glaciation of North America:
The Champlain, marine substage.
The Glacio-lacustrine substage.
The later Wlisconsin (6th glacial).
The fifth interglacial.
The earlier Wisconsin (5th glacial).
The Peorian (4th interglacial).
The Iowan (4th glacial).
The Sangamon (3rd interglacial).
The Illinoian (3rd glacial).
The Yarmouth or Buchanan (2nd interglacial).
The Kansan (2nd glacial).
The Aftonian (1st interglacial).
The sub-Aftonian or Jerseyan (1st glacial).
Although it is admitted that no strict correlation of the European and North American stages is possible, it has been suggested that the Aftonian may be the equivalent of the Helvetian; the Kansan may represent the Saxonian; the Iowan, the Polandian; the Jerseyan, the Scanian; the early Wisconsin, the Mecklenburgian. But considering how fragmentary is much of the evidence in favour of these stages both in Europe and America, the value of such attempts at correlation must be infinitesimal. This is the more evident when it is observed that there are other geologists of equal eminence who are unable to accept so large a number of epochs after a close study of the local circumstances; thus, in the subjoined scheme for north Germany, after H. W. Munthe, there are three glacial and two interglacial epochs.
| Post-Glacial epoch | The Mya time = beech-time. The Littorina time = oak-time. The Ancylus time = pine- and birch-time. | |
| 3rd Glacial " | Including the upper boulder clay, "younger Baltic moraine" with the Yoldia or Dryas phase in the retrogressive stage. | |
| 2nd Interglacial epoch including the Cyprina-clay. 2nd Glacial epoch, the maximum glaciation. 1st Interglacial epoch. 1st Glacial epoch, "older boulder clay." | ||
Again, in the Alps four interglacial epochs have been recognized; while in England there are many who are willing to concede one such epoch, though even for this the evidence is not enough to satisfy all glacialists (G. W. Lamplugh, Address, Section C, Brit. Assoc., York, 1906).
This great diversity of opinion is eloquent of the difficulties of the subject; it is impossible not to see that the discovery of interglacial epochs bears a close relationship to the origin of certain hypotheses of the cause of glaciation; while it is significant that those who have had to do the actual mapping of glacial deposits have usually greater difficulty in finding good evidence of such definite ameliorations of climate, than those who have founded their views upon the examination of numerous but isolated areas.
Extent of Glacial Deposits.—From evidence of the kind cited above, it appears that during the glacial period a series of great ice-sheets covered enormous areas in North America and north-west Europe. The area covered during the maximum extension of the ice has been reckoned at 20 million square kilometres (nearly 8 million sq. m.) in North America and 612 million square kilometres (about 212 million sq. m.) in Europe.
In Europe three great centres existed from which the ice-streams radiated; foremost in importance was the region of Fennoscandia (the name for Scandinavia with Finland as a single geological region); from this centre the ice spread out far into Germany and Russia and westward, across the North Sea, to the shores of Britain. The southern boundary of the ice extended from the estuary of the Rhine in an irregular series of lobes along the Schiefergebirge, Harz, Thüringerwald, Erzgebirge and Riesengebirge, and the northern flanks of the Carpathians towards Cracow. Down the valley of the Dnieper a lobe of the ice-sheet projected as far as 40° 50′ N.; another lobe extended down the Don valley as far as 48° N.; thence the boundary runs north-easterly towards the Urals and the Kara Sea. The British Islands constituted the centre second in importance; Scotland, Ireland and all but the southern part of England were covered by a moving ice-cap. On the west the ice-sheets reached out to sea; on the east they were conterminous with those from Scandinavia. The third European centre was the Alpine region; it is abundantly clear from the masses of morainic detritus and perched blocks that here, in the time of maximum glaciation, the ice-covered area was enormously in excess of the shrivelled remnants, which still remain in the existing glaciers. All the valleys were filled with moving ice; thus the Rhone glacier at its maximum filled Lake Geneva and the plain between the Bernese Oberland and the Jura; it even overrode the latter and advanced towards Besançon. Extensive glaciation was not limited to the aforesaid regions, for all the areas of high ground had their independent glaciers strongly developed; the Pyrenees, the central highlands of France, the Vosges, Black Forest, Apennines and Caucasus were centres of minor but still important glaciation.
The greatest expansion of ice-sheets was located on the North American continent; here, too, there were three principal centres of outflow: the “Cordilleran” ice-sheet in the N.W., the “Keewatin” sheet, radiating from the central Canadian plains, and the eastern “Labrador” or “Laurentide” sheet. From each of these centres the ice poured outwards in every direction, but the principal flow in each case was towards the south-west. The southern boundary of the glaciated area runs as an irregular line along the 49° parallel in the western part of the continent, thence it follows the Mississippi valley down to its junction with the Ohio (southern limit 37° 30′ N.), eastward it follows the direction of that river and turns north-eastward in the direction of New Jersey. As in Europe, the mountainous regions of North America produced their own local glaciers; in the Rockies, the Olympics and Sierras, the Bighorn Mountains of Wyoming, the Uinta Mountains of Utah, &c. Although it was in the northern hemisphere that the most extensive glaciation took place, the effects of a general lowering of temperature seem to have been felt in the mountainous regions of all parts; thus in South America, New Zealand, Australia and Tasmania glaciers reached down the valleys far below the existing limits, and even where none are now to be found. In Asia the evidences of a former extension of glaciation are traceable in the Himalayas, and northward in the high ranges of China and Eastern Siberia. The same is true of parts of Turkestan and Lebanon. In Africa also, in British East Africa moraines are discovered 5400 ft. below their modern limit. In Iceland and Greenland, and even in the Antarctic, there appears to be evidence of a former greater extension of the ice. It is of interest to note that Alaska seems to be free from excessive glaciation, and that a remarkable “driftless” area lies in Wisconsin. The maximum glaciation of the Glacial period was clearly centred around the North Atlantic.
Glacial Epochs in the Older Geological Periods.—Since Ramsay drew attention to the subject in 1855 (“On the occurrence of angular, subangular, polished and striated fragments and boulders in the Permian Breccia of Shropshire, Worcestershire, &c., and on the probable existence of glaciers and icebergs in the Permian epoch,” Q.J.G.S., 1855, pp. 185-205), a good deal of attention has been paid to such formations. It is now generally acknowledged that the Permo-carboniferous conglomerates with striated boulders and polished rock surfaces, such as are found in the Karoo formation of South Africa, the Talkir conglomerate of the Salt Range in India, and the corresponding formations in Australia, represent undeniable
