[BV] For a full statement of the evidence see Prehistoric Europe, chaps. xvi., xvii.
We speak of Pleistocene or Glacial and of Post-glacial periods as if the one were more or less sharply marked off from the other. Of course, that is not the case, and in point of fact it would be for many reasons preferable to include them under some general term. Taken together they form one tolerably well-defined cycle of time, characterised above all by its remarkable climatic changes—by alternations of cold and genial conditions, which were most strongly contrasted in the earlier stages of the period. It is further worthy of note that various oscillations of the sea-level appear to have taken place again and again both in the earlier and later stages of the cycle.
We may now proceed to inquire whether the phenomena we have been considering can be accounted for by movements of the earth’s crust—a view which has recently received considerable support, more especially in America. I need hardly say that the view in question is no novelty. Many years ago, while our knowledge of the Pleistocene phenomena was somewhat rudimentary, it was usual to infer that glaciation had been induced by elevation of the land. This did not seem an unreasonable conclusion, for above our heads, at a less or greater elevation, according to latitude, an Arctic climate prevails. One could not doubt, therefore, that if a land-surface were only sufficiently uplifted it would reach the snow-line, and become more or less extensively glaciated. But with the increase of our knowledge of Pleistocene and post-glacial conditions, such a ready interpretation failed to satisfy, although not a few geologists have continued to defend the “earth-movement hypothesis,” as accounting fairly well for the phenomena of the Glacial period. By these staunch believers in the adequacy of that view, it has been pointed out that elevation might not only lift lands into the region of eternal snow, but, by converting large areas of the sea-bed into land, would greatly modify the direction of ocean-currents, and thus influence the climate. What might not be expected to happen were the Gulf Stream to be excluded from northern regions? What would be the fate of the temperate latitudes of North America and Europe were that genial ocean-river to be deflected into the Pacific across a submerged Isthmus of Panama? The possibility of such changes having supervened in Pleistocene times has often been present to my mind, but I long ago came to the conclusion that they could not account for the facts. Moreover, I have never been able to meet with any evidence in favour of the postulated “earth-movements.” Having carefully studied all that has been advanced of late years in support of the hypothesis in question I find myself more than ever constrained to oppose it, not only because it is grounded on no basis of fact, but because it altogether fails to explain the conditions that obtained in Pleistocene and post-glacial times.
There are various forms in which the hypothesis has appeared, and these I shall now consider seriatim, and with such brevity as may be. It has been maintained, for example, that at the advent of the Glacial period vast areas of northern and north-western Europe, together with enormous regions in the corresponding latitudes of North America, stood several thousand feet higher than at present. But when we ask what evidence can be adduced to prove this we get no satisfactory reply. We are simply informed that a glacial climate must have resulted from great elevation, and that the latter, therefore, must have taken place at the beginning of the Glacial period. Some writers, however, have ventured to give reasons for their faith. Thus Mr. W. Upham, pointing to the evidence of the fiords of North America, and to the fact that drowned river-valleys have been traced outwards across the 100-fathoms line of the marginal plateau to depths of over 3500 feet, maintains that the whole continent north of the Gulf of Mexico stood at the commencement of the Glacial period some 3000 feet at least higher than now. Of course he cites the fiords of Europe as evidence of a similar great upheaval for the northern and north-western regions of our Continent. Mr. Upham even favours the notion that during glacial times a land-connection probably existed between North America and Europe, by way of the British Islands, Iceland, and Greenland. When “this uplifting attained its maximum, and brought on the Glacial period,” he says, “North America and north-western Europe stood 2500 to 3000 feet above their present height.”[BW]
[BW] American Geologist, vi., p. 327.
That fiords are simply submerged land-valleys has long been recognised: that they have been formed mainly by the action of running water—just in the same way as the mountain-valleys of Norway and Scotland—has been the belief for many years of most students of physical geology. But it is hard to understand why they should have been cited by Mr. Upham in support of his contention, seeing that their evidence seems to militate strongly against the very hypothesis he strives to maintain. No one acquainted with the physical features and geological structure of Scotland and Norway can doubt that the valleys which terminate in fiords are of great geological antiquity. Their excavation by fluviatile action certainly dates back to a period long anterior to the advent of the Ice Age. And a like tale is told by the fiords and drowned valley-troughs of North America, which cannot be referred to so recent a period as post-Tertiary times. Those who are convinced that our continental areas have persisted throughout long æons of geological time, and that rivers frequently have survived great geological revolutions—cutting their way across mountain-elevations as fast as these were uplifted—will readily believe that some of the submarine river-troughs of North America, such as that of the Hudson, may belong even to Secondary times.[BX] It would be hard to say at what particular date the excavation of the Scottish Highland valleys commenced—but it was probably during the later part of the Palæozoic era. The process has doubtless been retarded and accelerated frequently enough, during successive movements of depression and elevation, but it was practically completed before the beginning of Pleistocene times, and that is all that we may trouble about here. Precisely the same conclusion holds good for Norway: and such being the case it is obvious that the question of the origin and age of the fiords has no bearing on the problem of the glacial climate and its cause. In point of fact the evidence, as already remarked, tells against the “earth-movement hypothesis,” for it shows us that, during a period when Europe and North America stood several thousand feet higher, and extended much further seawards, rivers, and not glaciers, were the occupants of our mountain-valleys. It was not until all those valleys had come to assume much the appearance they now present that general glaciation supervened.
[BX] Professor Dana inclines to date the erosion of the Hudson trough so far back as the Jura-Trias period.—American Journ. Science, xl., p. 435.
We are not without direct evidence, however, as to the geographical conditions that obtained in the ages that immediately preceded the Pleistocene period. The distribution of the Pliocene marine beds of Britain entitles us to assume that at the time of their accumulation our lands did not extend quite so far to the south and east as now. The absence of similar deposits from the coast-lands of North America is supposed to support the view of great continental elevation in pre-glacial times. All it seems to prove, however, is that in Pliocene times the North American continent was not less extensive than it is at present. It is even quite possible that in glacial times pre-existing Pliocene beds may have been ploughed out by the ice, just as seems to have been the case in the north-east of Scotland. But without going so far back as Pliocene times, we meet with evidence almost everywhere throughout the maritime regions of the glaciated areas of Europe and North America, to show that immediately before those tracts became swathed in ice the geographical conditions were much the same as at present. The shelly boulder-clays in various parts of our islands, and the similar occurrence of marine and brackish-water shells in and underneath the Diluvium of north Germany, etc., prove clearly enough that just before the coming-on of glacial conditions neither Britain nor the present maritime lands of the Continent were far removed from the sea. It is true that the buried river-channels of Scotland indicate a pre-glacial elevation of some 200 or 300 feet above the existing sea-level, but it is quite certain that the Minch, St. George’s Channel, the Irish Sea, the North Sea, and the Baltic were all in existence at the commencement of the Glacial period. And we are led to similar conclusions with regard to the geographical conditions of North America at that time, from the occurrence of marine shells in the boulder-clays of Canada and New England. We note indeed that there is abundant evidence of land-submergence during glacial times. Indeed, we may say that the Pleistocene marine deposits of northern latitudes are almost invariably indicative of colder conditions than now obtain.
If it be true that cold climatic conditions were contemporaneous in our latitude with submergence, it is equally true that an extensive land-surface in north-west Europe has, sometimes at least, co-existed with markedly genial conditions. In Tertiary times, for example, as the Oligocene deposits of Scotland, the Faröe Islands, Iceland, and Greenland testify, a land-connection existed between Europe and the North American continent. Again, it has been shown that during the interglacial phase of the Pleistocene period Britain was continental, and enjoyed at the time a peculiarly genial climate. And somewhat similar geographical and climatic conditions again supervened in post-glacial times. In other words, when the land was more elevated and extensive than now, it enjoyed a warmer climate. Nor can we escape the conclusion that the excavation of the fiord-valleys of northern latitudes, which is a very old story (far older than the Pleistocene), was the work not of glaciers but of running water, at a time when north-western Europe and the corresponding regions of America were much more elevated than they are now.
Thus there appears to be no evidence either direct or indirect in favour of the view that glacial conditions were superinduced by great continental elevation. But it may be argued that even although no evidence can be cited in proof of such elevation, still, if the glacial phenomena can be well explained by its means, we may be justified in admitting it as a working hypothesis. Movements of elevation and depression have frequently taken place—the Pleistocene marine deposits themselves testify to oscillations of the sea-level—and there can be no objection, therefore, to such postulations as are made by the hypothesis under review. All this is readily granted, but I deny that the conditions that obtained in Pleistocene times can be accounted for by elevation and depression. Let us see how the desiderated elevation of northern lands would work. Were north-western Europe and the corresponding latitudes of North America to be upheaved for 3000 feet, and a land-passage to obtain between the two continents by way of the Faröe Islands, Iceland, and Greenland, how would the climate be affected? It is obvious that under such changed conditions the elevated lands in higher latitudes might well be subjected to more or less extensive glaciation. Norway would become uninhabitable and glaciers might well appear in the mountain-valleys of Scotland. But it may be doubted whether the climate of France and Spain, or the corresponding latitudes of North America, would be much affected. For were a land-passage to appear between Britain and Greenland no Arctic current would flow into the North Atlantic, while no portion of the Gulf Stream would be lost in Arctic seas. The North Atlantic would then form a great gulf round which a warm ocean-current would circulate. The temperature of that sea, therefore, would be raised and the prevailing westerly and south-westerly winds of Europe would be warmer than now. However much such warm moist winds might increase the snow-fall in North Britain and Scandinavia, we cannot suppose they could have much influence in central and southern Europe, and in North Africa; and still less could they affect the climate of Asia Minor and the mountainous regions of the far east, in most of which evidence of extensive glaciation occurs. And how, we may ask, could the postulated geographical changes bring about the glaciation of the mountainous tracts on the Pacific sea-board? In fine, we may conclude that however much the geographical changes referred to might affect north-western Europe and north-eastern America, they are wholly insufficient to account for the glacial phenomena of other regions. The continuous research of recent years has shown that the lowering of temperature of glacial times was not limited to the lands which would be affected by any such elevation as that we are considering. A marked and general displacement of climatic zones took place over the whole continent of Europe; and similar changes supervened in North America and Asia. Are we then to suppose that all the lands within the Northern Hemisphere were extensively and contemporaneously upheaved?