Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/382

368 1200 feet. In Scotland the highest level from which they have yet been obtained is 5134 feet; and in this instance they lie in one of the interstratifications in the boulder-clay. The coast of Scotland is fringed by a succession of raised beaches which, up to that at 100 feet above the present mean tide-level, are often remarkably fresh. The l0O—feet terrace forms a wide plateau in the estuary of the Forth, an.l the .30-feet terrace is as conspicuous on the Clyde. The elevation of the land has brought up within tide--marks some of the clays deposited over the sea-floor during the time of the submergence. In the Clyde basin and some of the western fjords these clays (Clyde beds) are full of shells. Comparing the species with those of the adjacent seas, we find them to be more boreal in character; nearly the whole of the species still live in Scottish seas, though a few are extremely rare. Some of the more characteristic northern shells in these deposits are Pecten Islamlicus, Telliua rfalcareu, Leda tr-zmcata, L. lanccoluta, Sa.ricaz'a rugosa, Ihuzopcea .l'orvqr/ica, Trap/ion clat/zratzmz, and .l'ut'z'ca rlctztsu. That ice continued to tloat about in these w'ater.s is shown by the striated stones contained in the fine clays, and by the remarkably contorted structure which these clays occa- sionally display. Sections may be seen where, upon per- fectly undisturbed horizontal strata of clay and sand, other similar strata have been violently crumpled, while horizontal beds lie directly upon them. These eontortions must have been produced by the horizontal pressure of some heavy body moving upon the originally flat beds. No doubt the agent was ice in the form of large stranding masses which were driven aground in the fjords or shallow waters where the clays accumulated. Another indication of the presence of floating ice is furnished by large boulders scattered over the country, an-I lying sometimes on the stratified sands and gravels, though no doubt ma.ny of the so—called erratics belong to the time of the chief glaciation. One of the most puzzling members of the Drift is the series of sands and gravels which cover the low grounds in nuny places, and rise up to heights of 1000 feet and more. These deposits are sometimes spread out in undulating sheets, which, in the lower districts, seem to merge insen- sibly into the marine terraces and raised beaches. But they are also very commonly disposed in remarkable mounds and in ridges which run across valleys, along hill-sides, and even over watersheds. These ridges are known in Scotland as kames, in Ireland as eskers, and in Scandinavia as osar. They consist sometimes of coarse gravel or earthy detritus, but more usually of clean, well-stratified sand and gravel, the stratification towards the surface corresponding with the external slopes of the ground, in such a manner as to prove that the ridges are original forms of deposit, and not the result of the irregular erosion of a general bed of sand and gravel. Some Writers have compared these features to the submarine banks formed in the pathway of tidal currents near the shore. Others have supposed them rather to be of terrestrial origin, due to the melting of the great snow-fields and glaciers, and the consequent discharge of large quantities of water over the country. But no very satisfactory explan- ation has yet been given. Ile-emer_r/ence.——Lrzter Glaciers.—When the land began to rise again, the temperature allover central and northern Europe was still severe. Vast sheets of ice still held sway over the mountains, and continued to descend into the lower tracts and to go out to sea. To this period are ascribed certain terraces or “parallel-roads” which run along the sides of valleys in the Scottish Highlands. It is believed that the mass of ice descending from some of the loftier snow- fields of this time was so great as to accumulate in front of lateral valleys, and to so choke them up as to cause the water to accumulate in them and flow out in an opposite GEOLOGY [v1. STIMTIGIIAPHICAL. direction by the col at the head. In these natural reservoirs the level at which the water stood for a time was marked by a horizontal ledge or platform due partly to erosion of the hill-side and partly to the arrest of the descending debris when it entered the water. Every group of mountains nourished its own glaciers; even small islands, such as Arr-an in Scotland, had their snowtields, whence glaciers crept down into the valleys and shed their moraines. It would appear indeed that some of the northern glaciers of Scotland continued to reach the sea—lcvel even when the land had risen to within 50 feet or less of its present elevation. On the east side of Sutherlandshire the moraines dcscenl to the 50- feet raised beach ; on the west side of the same county they come down still lower. The higher mountains of Europe still show the descendants of these later glaciers, but the ice has retreated from the lower elevations. In the Vosges the glaciers have long disappeared, but their moraines remain still fresh. In Wales, Cumberland, and the southern uplands and Highlands of Scotland, moraines, perched blocks, and roches moutrnmrfes attest the abund- ance and persistence of the last glaciers. It is sometimes possible to trace the stages in the gradual retreat of the ice towards its parent snow-fields, for the crescent-shaped moraine mounds he one behind another until they finally die out about the head of the valley, near what must have been the edge of the snow-field. We know as yet very little of the fauna and flora of the land during the Glacial Period in Europe. The vegetation was doubtless in great measure the same as that of arctic Norway at the present day. The animals included the musk-ox, lemming, rein-deer, and other still living arctic forms, but included some which have become extinct, such as the hairy mammoth and woolly rhinoceros. During the milder inter-glacial periods denizens of warmer regions found their way northwards into Europe. Thus the hippopotamus haunted the rivers of the south of England. By degrees, as the climate ameliorated, the arctic vegetation was finally extirpated from the lower grounds of central and western Europe, and plants loving a milder temperature, which had doubtless been natives of Europe before the period of great cold, were enabled to reoccupy the sites whence they had been driven. On the higher mountains, where the climate is still not wholly uncongenial for them, colonies of this once general arctic flora still survive. The a.rctic animals have also been driven away to their northern homes, or have become wholly extinct. And thus, as imperceptibly as it began, the long and varied Ice Age came to an end as it merged into the next succeeding geological period. hscexr on IIL'1I_I' Pemoo. That long succession of ages to which the name of the Glacial Period has been given shaded without abrupt change of any kind into what is termed the Human or Recent Period. Indeed it may be said with truth that the Glacial Period still exists in Europe. The snow-fields and glaciers have disappeared from Britain, but they still linger among the Pyrenees, remain in larger mass among the Alps, and spread over wide areas in northern Scandinavia. This dove- tailing or overlapping of geological periods has been the rule from the beginning of time, the apparently abrupt transitions in the geological record being due to imperfec- tions in the chronicle. The question has often been asked whether man was coeval with the Ice Age. To give an answer, we must know within what limits the term Ice Age is used, and to what particular country or district the question refers. For it is evident that even to-day man is contemporary with the

Ice Age in the Alpine valleys and in Fi11rr1a1'k. There can