The absence of storms off the north-west coast of Norway is shown by the forests which at this period covered all the outermost islands of Norway as far as Ingo Island, off North Cape. These islands are now barren, and their afforestation indicates a drier and especially a less stormy climate than the present, with a decreased frequency of winds from the sea. These conditions were well developed about 5000 B.C. This is the Early Neolithic period. Owing to the great development of forests, this period is sometimes called the Early Forest period.
The late-glacial history of North America was equally complicated. Consider first the region of the St. Lawrence Estuary and the Great Lakes. As the Wisconsin ice-sheet retreated across the present site of the Lakes, the latter underwent a remarkable series of fluctuations of area and outflow, which have been made the subject of brilliant studies by several American geologists. The opening stage began when the ice abandoned the high ground south of the lakes, leaving depressions bounded on the south by the hills and on the north by the ice. The earliest of these in the basins of Lakes Erie and Huron are known as the first and second Lake Maumee. These gradually grew in size and coalesced, forming several series of connected lakes, to which various names have been given; thus Lake Warren extended well outside the present limits of Lake Erie and southern Huron, and was held up by ice over Lake Ontario and northern Huron. At a later stage an enormous Lake Algonquin extended beyond the combined limits of Lakes Superior, Michigan and Huron, and communicated by broad channels with an enlarged Lake Ontario known as Lake Iroquois, and with Lake Erie. But even before this time the northern shores of the lakes, relieved of the major portion of their ice-load, had begun to rise rapidly, and ultimately reduced the lakes to their present size.
These great areas of ice-cold water, bathing the southern edges of the ice-sheet, must have had an unfavourable influence on the climate, keeping it cold and damp, and preventing dry continental conditions from becoming established. They probably retarded the ice-retreat in these regions quite considerably, so that a lobe of ice was left here long after the edge had retreated northwards on either side. At the same time the climate further south was dry, with æolian deposits; but as the anticyclonic winds blew off the Atlantic the evidence of drought is not so marked as in Europe.
After this slow retreat had been in progress for a considerable time a submergence, known as the “Champlain stage” set in, reaching a depth of at least 600 feet and opening the St. Lawrence regions wide to the Atlantic, which penetrated into Lake Ontario. The ice now retreated rapidly under the influence of a maritime climate little colder than the present. In phase this period corresponds to the second Yoldia Sea stage of Scandinavia; in point of time it was probably somewhat earlier. This was followed by elevation, the first result of which was to cut off the warm water and cause a sharp fall of temperature exactly analogous to that of the Ragunda moraines, but a few thousand years earlier and probably more marked. The continuance of elevation brought on a long continental period of extreme aridity, when trees grew on the peat-bogs of the eastern States, while the lakes of the Great Basin further west were almost or wholly dried up. At the maximum of the continental conditions the summers at least were warmer than at present, as indicated by the northward extension of various species of plants and fresh-water mollusca. The winters were probably more severe. Possibly the great aridity of this period was partly due to a sub-glacial continental anticyclone obstructing the path of depressions across America from west to east. The drainage area of the Great Basin received hardly any rainfall and was a hopeless desert, but the Atlantic States were able to grow trees on the old peat-bogs, probably with rainfall derived from the Atlantic. By reference to the cutting of Niagara gorge, we can infer that the warm dry period began about 6000 B.C., so that it corresponds exactly with the continental phase (Ancylus stage) of Europe. This period of aridity was finally ended by a fresh submergence, the “Micmac,” which carried the land about twenty feet below its present level.
In Yukon and Alaska, where the glaciation was not nearly so severe as further to the south-east, the depression of the land by the ice-load and consequently the subsequent rise on its removal were not great. There were no complicated geographical changes, and correspondingly there appear to have been no fluctuations of climate, but only a gradual passage to present conditions.
Even in Iceland there are indications of a dry period following the last glacial maximum, for tree-trunks, buried in the peat-bogs, show that the birch formerly had a much greater extension. It is also quite possible that there was an accentuation of desert conditions in Asia during the retreat of the glaciers in Europe and North America, which may have played a part in the wave of Neolithic migration that appears to have overwhelmed the artistic Palæolithic races of western Europe; but of this we have as yet no direct evidence. The Neolithic invasion of Europe took place along two main routes, the Nordics passing from the centre of Asia north of the Caspian, across Russia to the Baltic shores, where they became the Kitchen-midden people; and the Alpine race passing from Transbaikalia, south of the Caspian and Black Sea, into southern Europe. The Nordics drove before them an older race, characterized by the transitional Maglemose culture, which passed from east of Russia to the shores of the Baltic and ultimately to England, where harpoons of Maglemose type have been found beneath the peat of Holderness.
In the southern hemisphere the continental phase does not appear to have been so well developed. The uppermost part of the Pampean loess is possibly post-glacial; more certainly so are the sand-dunes on the coast near Buenos Aires, in which human remains have been found in association with the bones of some extinct animals. In New South Wales, after the retreat of the glaciers, there was a period with land a little above its present level, so that the stools of Eucalyptus trees are now found ten feet below sea-level; but there is no evidence as to the climate of this stage. In New Zealand we have no definite post-glacial beds of continental type. The occurrence of xerophilous plants, such as Aciphylla, still living in a climate which is now decidedly moist, may be a remnant of a continental phase in New Zealand, or may date back to the steppe conditions of the loess. As to Antarctica, we have, of course, no evidence.
BIBLIOGRAPHY
Brooks, C. E. P. “The evolution of climate in north-west Europe.” London, Q. J. R. Meteor. Soc., 47, 1921, p. 173.
Wright, W. B. “The Quaternary Ice Age.” London, 1914. (Ch. 17, Late-glacial changes of level in North America.)