Popular Science Monthly/Volume 37/May 1890/Recent Glacial Work in Europe
|RECENT GLACIAL WORK IN EUROPE.|
AT the recent meeting of the British Association at Newcastle, Prof. James Geikie opened the Section of Geology with a summary of the results obtained during the last few years by continental glacialists. Sketching the steps by which the iceberg theory has been abandoned by German and Swiss geologists, he dwelt on certain features of the drifts of the peripheral areas, which for some time were hard to account for by land-ice. Of these, the bedded deposits occurring so frequently in the bowlder-clays of the peripheral regions, and the occasional silty and uncompressed character of the clays themselves, remained unexplained until a clew was found to their origin in the geographical distribution of the clays in which they occur. These stony clays, of inconsiderable thickness in Norway, the higher parts of Sweden, and in Finland, reach a thickness of about forty-three metres in southern Sweden, and eighty metres in the northern parts of Prussia; and in Holstein attain a depth of one hundred and twenty to one hundred and forty metres, and still greater depths in Hanover, Mark Brandenburg, and Saxony. The aqueous deposits associated with the stony clays also gradually acquire more importance as they are followed from the mountainous and high-lying tracts to the low ground, until, along the southern margin of the drift area, the "diluvium" appears to consist of aqueous accumulations alone. The explanations of these facts by German geologists have been summed up recently (1884) by Dr. Jentzsch, from whom Prof. Geikie quoted enough to show that they are quite in accordance with the views long held by glacialists elsewhere.
The general conclusions reached by continental glacialists, and summarized by Prof. Geikie, are:
1. Before the invasion of northern Germany by the inland ice, the low grounds bordering on the Baltic were overflowed by a sea which contained a boreal and arctic fauna.
2. The next geological horizon in ascending order is that which is marked by the glacial and fluvio-glacial detritus of the great ice-sheet which flowed to the foot of the Harz Mountains, and has been traced by the occasional presence of rock-striæ and roches-moutonnées, of bowlder-clay and northern erratics, rather than by recognizable terminal moraines.
3. A well-marked temperate fauna and flora marks the inter-glacial beds which follow, and which, in their geographical distribution and the presence in them of such forms as Elephas antiquus, Cervus elephas, and C. megaceros, and a flora comparable to that now existing in northern Germany, justify geologists in concluding that this era was one of long duration, and characterized in Germany by climatic conditions apparently not less temperate than those that now obtain.
4. To this well-marked interglacial era succeeded a second overflow of Scandinavian inland ice, confined to a region much narrower than that covered by the first. Its boundaries are shown not only by the geographical distribution of the youngest bowlder-clay, but by the direction of rock-striae, the trend of erratics, and the position of well-marked moraines.
Concerning the ground-moraines of the Alpine lands of central Europe, the only question that has recently given rise to much discussion is the origin of the materials themselves. The observations of able investigators appear to Prof. Geikie to have demonstrated that these materials have been derived, in chief measure, from the underlying rocks by the erosive action of the ice that overflowed them. German geologists are not agreed upon this much-debated question of glacier-erosion—a few still maintaining that glaciers have little or no eroding power. But where the evidences of erosion have been studied over a wide region, from which the ice has completely disappeared, rather than at the lower ends of existing glaciers, some of the strongest opponents of glacier-erosion have been compelled to go over to the other camp. As an example, Prof. Geikie quoted Dr. Blaas, who, through his observations on the glacial formations of the Inn Valley, has recanted his former views and become a formidable opponent of the very theory which he once upheld. To his books and to memoirs by Penck, Brückner, and Bohn, and especially to the chapter on glacier-erosion by the lastnamed author, Prof. Geikie refers those who may be anxious to know the last word on this question.
Observations by Drs. Brückner and Penck have led to the opinion that the loess is of interglacial age. Examining a wider range of evidence, Prof. Geikie has little doubt that the loess belongs to no particular horizon, though it must be considered strictly a Pleistocene accumulation. Concerning its mode of formation he discussed the various theories advanced, and gave it as his opinion—an opinion formed from what he has himself seen of the loess in various parts of Germany, from reading, and from conversation with those who have worked over loess-covered regions—that it is for the most part of aqueous origin, formed in the slack waters of the great rivers, and in the innumerable temporary lakes which occupied or partly occupied many of the valleys and depressions of the land. Probably some may have been derived from the denudation of bowlder-clay, some from "rainwash" while much of the so-called Bergloess with its abundant land-shells, and its generally unstratified character, owes its origin to rain, frost, and wind. Admitting that some of the loess of the lower grounds may have been reworked by the same agents, Prof. Geikie found no evidence in the facts adduced by German geologists of a "dry-as-dust" epoch having obtained in Europe during any stage of the Pleistocene period.
Within recent years the fossils of the loess have received close attention, and through them so much knowledge has been gained of the various modifications experienced by Pleistocene organisms that, taken with other evidence of interglacial conditions, there is little room to doubt that this period was characterized by great changes of climate. How often arctic, steppe, prairie, and forest faunas and floras have replaced each other is yet a matter of dispute. The occurrence of fossiliferous deposits intercalated among glacial accumulations throughout all the glaciated tracts of Europe show that however many advances and retreats of the ice there may have been, they were on a gigantic scale characterizing all the glaciated areas.
The bearing of the establishment of at least two eras of glaciation on the position of Palaeolithic man was pointed out by Prof. Geikie. The mere occurrence of glacial deposits underneath implement-bearing beds no longer proves these latter to be post-glacial. The horizon of glacial accumulations underlying Palaeolithic gravels must now be determined by ascertaining their relative position; and it is a remarkable fact that the bowlder-clays which occur beneath such old alluvia belong, without exception, to the earlier stages of the Glacial period. In 1871-, 72 Prof. Geikie published a series of papers in the Geological Magazine, maintaining that the alluvial and cave deposits must be assigned to preglacial and. interglacial times, and in chief to the latter. Evidence was adduced to show that during the last stage of the Glacial period man lived contemporaneously with a northern and Alpine fauna, in such regions as southern France; and that Palæolithic man and the southern mammalia never revisited northwestern Europe after extreme glacial conditions had disappeared. Prof. Geikie at the same time colored a map to show at once the areas covered by the glacial and fluvio-glacial deposits of the last Glacial era, and the districts in which the implement-bearing and ossiferous alluvia had been found; and this clearly brought out that the latter never occurred at the surface within the regions occupied by the former. Similar evidence has been recently obtained by continental geologists; and a map published by Dr. Penck in 1884, showing the areas covered by the earlier and later glacial deposits in northern Europe and the Alpine lands, and indicating at the same time the various localities where Palæolithic finds have occurred, does not give a single locality within the regions covered by the accumulations of the last Glacial era. So greatly are students of the Pleistocene ossiferous beds influenced by what is known of the interglacial deposits and their organic remains, that many do not now hesitate to correlate with those beds the old ossiferous and implement-bearing alluvia which lie altogether outside of glaciated regions. In France, where the relation of Pleistocene alluvia has been especially canvassed, these alluvia have been also included among interglacial deposits. M. Boule also, in the Revue d'Anthropologie, 1889, correlates the Palæolithic cave and river deposits of France with those of other countries, and shows that they must be of interglacial age. He is satisfied that in France there is evidence of three glacial and two well-marked interglacial eras. The oldest of the Palæolithic stages of Mortillet culminated during the last interglacial era, while the more recent Palæolithic stages coincided with the last great development of glacier ice. The Palæolithic age, so far as Europe is concerned, came to a close during this last cold phase of the Glacial period.
Interesting as is the development of the climatic and geographical changes of which our Palæolithic predecessors were the witnesses, the clearing up of the history of Pleistocene times is not the only end that workers in this field have in view. Prof. Geikie, therefore, closed his address with a hope that the definite knowledge of the conditions of the Pleistocene period and of the causes which gave rise to them would lead to the better understanding of the climatic conditions of still earlier ages; the success with which other problems have been attacked by geologists forbidding him to doubt that ere long we shall have done much to dispel some of the mystery still enveloping the question of geological climates.