Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/381

Rh GLACIAL DErosrrs.] GEOLOGY 367 with its outlet by the hollow through which the Strait of pieces of the Highland rocks, which must have come from Dover has since been opened. during such a condition of things the widespread fresh- It has been suggested that ; water deposit termed Loess was formed, which covers so ' large a space in the lower plains of the Rhine and the north of Belgium, and appears in the valleys of the south— ' east of England. The ice is computed to have been at least between 6000 and 7000 feet thick in Norway, measured from the present sea-level. From the height at which its transported debris has been observed on the Harz, it is believed to have been at least 1470 feet thick there, and to have gradually risen ' some other height. in elevation as one vast plateau, like that which at the ‘ present time covers the interior of Greenland. Among the Alps it attained almost incredible dimensions. The present snowfields and glaciers of these mountains, large though they are, form no more than the mere shrunken remnants , and surface. They are usually oblong, have one or more ﬂat of the great mantle of snow and ice which then over- .<pl‘C:'.1(l Switzzrland. In the Bernese Oberland, for example, the valleys were ﬁlled to the brim with ice, which, moving northwards, crossed the great plain, and actually overrode at part of the J ura mountains. and other rocks from the central chain of the Alps are found high on the slopes of that range of heights. ]}uI([tl(’7'-Cl(l_{/ or Till.-—Under this name is included the debris which accumulated a11d was pushed onward under Huge fragments of granite the ice-sheet, the “ grund—n1oriine ” or “ moraine profo11de” I (rmte, p. 282). Allover the low grounds of North Germany, Denmark, and Holland lies a stony or earthy clay passing into sand or gravel, in which, together with locally derived debris, there is a greater or less proportion of fragments from the north. Some of the rocks of Scandinavia, Finland, and the Upper Baltic are of so distinctive a kind that they can be recognized in small pieces in the boulder-clay. Thus the peculiar syenite of Laurwig in the south of Norway has been recognized abundantly in the drift of Denmark; it occurs in that of Hamburg, and in the boulder-cla.y of the Holderness cliffs in Yorkshire. The well-known Bhomben- porphyr of southern Norway has likewise been recognized at Holdcrness. Fragments of the Silurian rocks from Gothland, or from the Russian islands Dago or Oesel, have been met with as far as the north of Holland. These trans- ported fragments, so abundant within the li11e of demarca- tion just traced, are an impressive testimony to the move- ments of the northern ice-sheet and ﬂoating ice. The boulder-clay is not spread as a uniform sheet over the ground, but varies greatly in thickness a11d in irregu- larity of surface. Round the mountainous centres of dis- persion it is apt to occur in long ridges or “ drums” which run in the general direction of the rock-striation. In those areas which served as independent centres of dispersion for the ice-sheet, the boulder-clay partakes largely of the local character of the rocks of each district where it occurs. Thus in Scotland the clay varies in colour and composition as it is traced from district to district. Over the Carboniferous rocks it is dark, over the red sandstones it is red, over the Silurian rocks it is fawn-coloured. The great majority of the stones also are of local origin, not always from the immediately adjacent rocks, but from points within a distance of a few miles. Evidence of trans- port, however, can be gathered from the stones, for they are found in almost every case to include a proportion of fragments which have come from beyond the district. The direction indicated by the percentage of travelled stones is always the same for each region. Thus, in the lower pa.rt of the valley of the Firth of Forth, while most of the frag- ments are from the surrounding Carboniferous formations, from 5 to 20 per cent. is found to have come eastward from the Old Red Sandstone range of the Ochil Hills—a distance of 25 or 30 miles, while 2 to 5 per cent. are the high grounds at least 50 miles to the north-west. The evidence of dispersion revealed by the stones in the boulder- clay harmonizes with that of the striae on the rocks. Every main mass of elevated ground in Britain seems to have caused the ice to move outward from it for a certain dis- tance, until the stream coalesced with that descending from In Scotland the ice was massive enough to move out into the basin of the North Sea (then doubtless in great part usurped by the glaciers) until it met that which was streaming down fron1 Scandinavia. Hence no Scandinavian blocks have ever been found in Scotland. But the Norwegian ice which crept southwards across Denmark, may once have extended across the North Sea to the Yorkshire coast, if the Scandinavian stones of Holder'- ness were not carried on ﬂoating ice. The stones in the boulder-clay have a characteristic form sides or “soles,” are smoothed or polished, and have their edges worn round. Where they consist of a ﬁne—grained enduring rock, they are almost invariably found to be striated, the strize running with the long axis of the stone. These markings are precisely similar to those on the solid rocks underneath the boulder clay, and like them have manifestly been produced by the friction of stones a11d grains of sand as the whole mass of debris was being steadily pushed on in one given direction by the resistless advance of the ice (mate, 1). 282). Inte7'glacial1)’eds. The boulder-clay is not one uniform mass of material. In a limited section, indeed, it usually appears as an unstratiﬁed mass of stiff stony clay. But it is found on further examination to be split up with various inconstant and local interstratiﬁcations. Beds of sand, gravel, ﬁne clay, and peaty layers occur in this way in different platforms in the boulder clay. In Scotland and elsewhere these interpolated beds bear witness to inter- vals when the ice retired from the area, and the land, so far as uncovered, was clothed with vegetation and traversed by herds of the hairy mammoth, reindeer, and musk- ox. Hence the long glacial period must have been inter- rupted by episodes probably of considerable duration when a milder climate prevailed. Such an alternation of con- ditions is explained on the hypothesis discussed in previous pages (mete, pp. 218-220). Evirlences of Sz¢lmze7'ge2zcc.—After the ice had attained its greatest development, the land, which perhaps in north- western Europe stood at a higher level above the sea than it has done since, began to subside. The ice-ﬁelds were carried down below the sea-level, where they broke up and cumbered the sea with ﬂoating bergs. The heaps of loose debris which had gathered under the ice, being new exposed to waves, ground—swell, and marine currents, were thereby more or less washed down and reassorted. Coast—ice, no doubt, still formed along the shores, and was broken up into moving ﬂoes, as happens every year now in northern Greenland. The proofs of this phase of the long glacial period are contained in the sands, gravels, erratic blocks, and stratiﬁed clays which overlie the coarse older till. It is difficult to determine the extent of the submergence, for when the land rose the more elevated portions continued to be the seats of glaciers, which, moving over the surface, destroyed the deposits which would otherwise have remained as witnesses of the presence of the sea. The most satis- factory evidence is undoubtedly that supplied by beds of marine shells. These have been observed on Moel Tryfan, in North Wales, at a height of no less than 1350 feet, but as the same kind of deposits in which they occur extend to a much greater height, the submergence probably consider- ably exceeded the limit at which the shells occur. In Cheshire beds of shells have been met with at a height of