Interglacial period

“ The terraces often contain fossils, and this may cross-date the terrace materials with cave deposits. But scientists are often faced with the problem of picking the right glacial or interglacial period on scanty evidence, and the still more difficult problem of setting the period in the terms of our years. There is room here for much disagreement. Glaciologists do not agree as to the age or the length of the various glacials and interglacials (see illustration, page 55) . Some prehistorians accept and use the dates of one glacialist; some choose another’s. ”

“ But there are intercalated in the boulder clays those irregular stratified and partially stratified masses of sand, gravel and loam, frequently containing marine or freshwater shells and layers of peat with plant remains, which have given rise to the conception of “interglacial epochs”—pauses 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. ”

“ 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. ”

“ During the last twenty years the evidence of interglacial conditions both in Europe and America has so increased that geologists generally no longer doubt that the Pleistocene period was characterised by great changes of climate. The occurrence at many different localities on the Continent of beds of lignite and freshwater alluvia, containing remains of Pleistocene mammalia, intercalated between separate and — 242 — distinct boulder-clays has left us no other alternative. ”

“ It is improbable that the ice ever entirely vanished from the continent. We shall see in Chapter XIV that even the comparatively brief warm period known as the post-glacial climatic optimum extended to the Antarctic coast, and this is additional argument for extending the much greater interglacial oscillation southward beyond its known limits in Tierra del Fuego, Australia and New Zealand, but here the matter must be left. ”

“ Now the Pleistocene was characterized by one of those periods of glacial cold which have swept over the earth—by one of those great winters which have so chilled the continents that few forms of life could survive them—and man comes in at the close of this cold period, in what is called the Post-glacial age. Some geologists, it is true, hold to an interglacial warm period, in which man is supposed to have existed, but the evidence of this is extremely slender and doubtful, and it carries back in any case human antiquity but a very little way. ”

“ As these conditions have great significance in relation to the appearance of man, I have tabulated them for reference as they occur in Scandinavia, Great Britain, and North America. The so-called “Interglacial Periods” of some geologists are in reality local results of the stages of intermittent elevation in which were deposited beds which in some cases, as in Scotland, Sweden, and Eastern Canada, hold sea-shells, and in others, as in the central areas of North America, contain remains of plants of northern species. ”

“ In America, also, the opinion was gradually gaining strength that the Ice Age was marked by several prolonged intervals of warmer conditions; and in France, Germany, and Scandinavia many geologists were added to the ranks of those who maintained the importance of interglacial periods.
Hardly less convincing than the results of Penck and Brückner’s investigations into the repeated glaciation of the Alps were the advances which have been made by the study of palæolithic deposits, especially in France, Belgium, and Germany, during the last twenty years. ”

“ The local Permian glaciations of Europe and North America, some of which fell close to Wegener’s equator, are easily explicable as due to mountain glaciers similar to those of Ruwenzori and other tropical mountains during the Quaternary. There were interglacial periods in South Africa, Brazil and New South Wales, which increase the resemblance between the Permian and Quaternary Ice Ages.
In Upper Permian times there was a widespread development of arid climates, especially in the present temperate parts of North America and Europe. ”

“ It is natural that this problem should have found its solution in America, where the Pleistocene is admirably displayed, and where the State and Federal surveys were actively engaged in areal mapping. In 1883 Chamberlin [64] presented his views under the bold title, “Preliminary Paper on the Terminal Moraine of the Second Glacial Epoch,” and the existence of deposits of two or more ice sheets and the features of interglacial periods 142were substantially established by the interesting debate in the Journal led by Chamberlin, Wright, Upham and Dana. ”

“ An elevation of land in the northern part of North America and Europe could not produce glaciation in their southern parts. The deflection of the Gulf Stream by the sinking of the Panama, Professor Geikie argues, could not produce the conditions which prevailed during the glacial epoch. The Earth-Movement hypothesis, he believes, accounts neither for the widespread phenomena of the ice-age, nor for the remarkable interglacial climates. ”

“ There must always be great difficulty in the correlation of widely-separated interglacial deposits, and the time does not seem to me to have yet come when we can definitely assert that all those interglacial beds belong to one and the same geological horizon. ”

“ The possibilities of dispersion during the glacial, interglacial and post-glacial periods may, accordingly, be generalized as follows: Each time the Northern Hemisphere was glaciated, the migrations of men must have been confined for the most part within the Indo-Mediterranean-Malaysian belt and the southern peninsulas of the Old World. ”

“ Since most of the skeletal and cultural remains of man can now be definitely attributed to certain glacial, interglacial, or postglacial stages, vast interest attaches to the very difficult problem of the duration of the whole Ice Age and the relative duration of its various glacial and interglacial stages. ”

“ When to this uniform testimony of the palæontological evidence we add the equally uniform absence of any indication of those ice-borne rocks, boulders, and drift, which are the constant and necessary accompaniment of every period of glaciation, and which must inevitably pervade all the marine deposits formed over a wide area so long as the state of glaciation continues, we are driven to the conclusion that the last glacial epoch of the northern hemisphere was exceptional, and was not preceded by numerous similar glacial epochs throughout Tertiary and Secondary time. ”

“ May I be permitted to ask aid from some American contributor to Science who follows the lore of glacial geology? I learn that some American glacialists are satisfied that there have been two periods of glaciation, and I would inquire whether the interglacial deposits contain, like those of Switzerland, remains of mammalia, and if so, what they are. ”

“ It is essentially a theory of alternation; and it is certainly remarkable in how many cases geologists have independently deduced some alternations of climate as probable. Such are the interglacial deposits indicating a mild climate, both in Europe and America; an early phase of very severe glaciation when the "till" was deposited, with later less extensive glaciation when moraines were left in the valleys; several successive periods of submergence and elevation, the later ones becoming less and less in amount, as indicated by the raised beaches slightly elevated above our present coast line ”

“ He advocated also the existence of man in glacial times, and recapitulating views already enumerated in some papers which he had published in the Geological Magazine, he argued that the palæolithic gravels of England were to be regarded as glacial and preglacial, and not, as was widely believed, of interglacial and post-glacial age. ”

“ Outside the area of the European continental ice sheet, facts are adduced in striking confirmation of the multiple ice epoch theory. These facts are found in Switzerland, where evidences of multiple glaciation have been recognized, and in the Pyrenees where evidences of three separate ice epochs have been found. In France, evidences of an interglacial interval have been found in the region of the Puy de Dôme of such duration as to allow the excavation of valleys to a depth of 900 feet. ”

“ Let us now see what the fauna and flora, as far as we know it, tell us of the climate of the Glacial period. At the very outset of our inquiry we are confronted with one very serious difficulty in the problem, and that is the supposed occurrence of inter-glacial mild phases alternating with colder ones during the Ice Age. At first, when traces of a temperate flora and fauna were discovered intercalated between two layers of boulder clay, their presence was explained by the supposition of a mild inter-glacial period. ”

“ Throughout earlier Tertiary and Secondary times an equally high excentricity often occurred, but it never produced a glacial epoch, because the north temperate and polar areas had less high land, and were more freely open to the influx of warm oceanic currents. But wherever great plateaux with lofty mountains occurred in the temperate zone a considerable local glaciation might be produced, which would be specially intense during periods of high excentricity; and it is to such causes we must impute the indications of ice-action in the vicinity of the Alps during the Tertiary period. ”

“ The cause of the Ice Ages remains at present an unsolved problem. More than one Ice Age has occurred during the long geological history. The marks of such a period are found in Archæan rocks, in the Cambrian, when glaciers flowed down to the sea level in China and South Australia within a few degrees of the tropics, and above all in early Permian times. The Dwyka conglomerate of the Karroo formation of South Africa (deposits of Permo-Carboniferous age) show evidence of extensive glaciation ”

“ This is the period of preglacial land elevation in North America, Europe, and Asia. The land was greatly altered in topography. Mountain ranges were born, streams changed their courses, and isolated volcanoes broke out all over the world.
10,000,000 years ago began an age of widespread local land deposits on the lowlands of the continents, but most of these sedimentations were later removed. Much of Europe, at this time, was still under water, including parts of England, Belgium, and France, and the Mediterranean Sea covered much of northern Africa. ”

“ Greenland, for instance, which is a typical case, shows us what happens when a continental mass of land stands at a high elevation in high latitudes with no Gulf Stream, but instead of it cold currents from a Polar ocean, and seas around it frozen or covered with icebergs for nine months out of the year. We have a dome of solid ice piled up to the height of 9000 feet or upwards, and sending millions upon millions of tons of glaciers down to the sea to be floated off as icebergs. The only trace we can see here of the old great glacial period is that these conditions were formerly more intense. ”

“ And on the scarred and grooved rocks was the final evidence; for these deep scratches had surely been ground by the advancing glacier as it slowly and silently moved onward. “The idea that such phenomena were not restricted to regions where glaciers now are found, but that traces of glacial action could be seen over enormous tracts of the earth’s surface, perhaps including regions in the tropics, and that in countries now temperate there might be discovered, not only the remains of tropical fauna and flora, but also distinct indications of a period of arctic cold—this was as new as startling.” ”

“ The surface of the ocean will therefore rise toward the north pole, and fall toward the south. The land will not sink under the sea, but, what amounts to the same thing, the sea will rise upon the land. The extent of submergence will be in proportion to the weight of the ice.
It is easy to see that glaciation would not be contemporaneous on both hemispheres. One hemisphere would be covered with ice and snow, while the other would be enjoying a perpetual spring. A glacial epoch resulting from the eccentricity of the earth's orbit would extend over a period of a hundred thousand years. ”

“ But in all the regions visited by the last great incursion of the mer de glace, such relics are comparatively rare; it is only when we get beyond the districts that were overwhelmed that the ancient interglacial remains are well preserved. Beyond the southern extremity reached by the latest general ice-sheet—that is to say, in — 119 — the regions south of the Humber, we find the country often sprinkled with tumultuous heaps and wide-spread sheets of gravel and brick-earth, which seem to owe their origin to the floods and torrents that escaped from the melting ice. ”

“ But directly we go back a very short period, we find the superficial evidences of glaciation to an enormous extent over three-fourths of the globe. In the Alps and Pyrenees, in the British Isles and Scandinavia, in Spain and the Atlas, in the Caucasus and the Himalayas, in Eastern North America and west of the Rocky Mountains, in the Andes of South Temperate America, in South Africa, and in New Zealand, huge moraines and other unmistakable ice-marks attest the universal descent of the snow-line for several thousand feet below its present level. ”