Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/380

Rh 366 The effect of the movement of the ice was necessarily to remove the soils and superﬁcial deposits of the land surface. Hence in the areas of country so atfected, the ground having been scraped and smoothed, the glacial accumulations laid down upon it rest abruptly, and without any connexion, on whatever underlies them. Considerable local differences may be observed in the nature and succession of the differ- GEOLOGY cnt deposits of the glacial period, as they are traced from ' district to district. It is hardly possible to determine, in some cases, whether certain portions of the series are coeval or belong to different epochs. But the following are the I lealing facts which have been established for the N orth- ' European area. First, there was a gradual increase of the cold, though with warm intervals, until the conditions of modern North Greenland extended as far south as Middlesex, Wales, the south-west of Ireland, and 50° N. lat. in central Europe. This was the culmination of the Glacial period. Their followed a considerable depression of the land and the spread of cold arctic water over the submerged tracts, with abundant ﬂoating ice; next came a re-elevation, with renewed augmentation of the s11ow—fields and glaciers. Very gradually, and after intervals of increase and diminu- tion, the ice retired towards the north, and with it the arctic ﬂora and fauna that had peopled the European plains. The existing snow—ﬁelds and glaciers of the Pyrenees, Switzerland, and Norway are remnants of the great ice- sheets of the glacial period, while the arctic plants of the mountains are relics of the northern vegetation which was universal from Norway to Spain. Ice-worn Rec/cs.—Beginning at the base of the deposits from which this interesting history is compiled, we ﬁnd the solid rocks over the whole of northern Europe to present the characteristic smoothed ﬂowing outlines which can be pro- duced only by the grincling action of land-ice (ante, p. 292). Where they have been long exposed, this peculiar surface is apt to be eﬁaced by the disintegrating action of the weather, though it retains its hold with extraordinary pertinacity. Along the fjords of Norway and the sea—lochs of the west of Scotland, it may be seen slipping into the water, smooth, bare, polished, and grooved as if the ice had only recently retreated. But where the protecting cover of clay or other s iperfieial deposits has been newly removed, the peculiar ice- worn surface is as fresh as that by the side of a modern glacier. Observations of the directions of the striae have shown that on the whole these markings diverge from the main masses of high ground. In Scandinavia they run west- wards and south—westwards on the N orwegian coasts, and eastwards or south-eastwards across the lower grounds of Sweden. When the ice descended into the basin of the Baltic and the plains of northern Germany, it moved southwards and south-westwards, but seems to have slightly changed its direction in different areas and at different times. Its move- ments can be made out partly from the strize on the solid rock, but more generally from the glacial drift which it has left behind. Thus it can be shown to have moved down the Baltic into the North Sea. At Berlin its movement must have been from east to west. But at Leipsie, as recently ascertained by Credner, it came from N .N .W. to S.S.E., being doubtless shed off in that direction by the high grounds of the Harz mountains. Its southern limit can be traced with tolerable clearness from J evennaar in Holland eastwards across the Ithine valley, along the base of the Westphalian hills, round the projecting promontory of the Harz, and then southwards through Saxony to the roots of the Erzgebirge. Passing next south-eastwards along the ﬂanks of the Biesen and Sudeten chain, it sweeps across Poland into Itussia, circling round by Kiefi", and northwards by Nijni Novgorod towards the Urals. It has been estimated that, excluding Finland, Scandi- navia, and the British Isles, the ice must have covered not ['I. STRATIGRAPHICAL. less than 1,700,000 square kilometres of the present low- lands of Europe. Its influence on the solid rocks over which it passed has not been everywhere equal. Over much of the north German plain, indeed, the rocks are con- cealed under drift. But in the more undulating hilly ground, particularly in the north and north—west, the ice has effected the most extraordinary abrasion. It is hardly possible, indeed, to describe adequately in words these regions of most intense glaciation. The old gneiss of Norway and Sutherlandshire, for example, has been so eroded, smoothed, and polished, that it stands up in endless rounded hum- mocks, many of them still smooth and flowing like dolphins’ backs, with little pools, tarns, and larger lakes lying between them. Seen from a height the ground appears like a billowy sea of cold grey stone. The lakes, every one of them lying in a hollow of erosion, seem scattered broadcast over the laudscape. So enduring is the rock, that even after th lapse of so long an interval, it retains its ice-worn aspect almost as unimpaired as if the work of the glacier had been done only a few generations since. Some idea of the massivencss of the iec-sheet is obtain- able from a consideration of the way in which the striac run across important hill ranges, and athwart what might seem to be their natural direction. Whilst there was a general southward movement from the great snow—ﬁelds of Scandi- navia, the high grounds of Britain were important enough to have their own independent ice, which, as the Strize show, radiated outward, some of it passing westwards into the Atlantic and some of it eastward into the North Sea. So thick must it have been as it moved off the Scottish High- lands that it went across the broad plains of l’erthshirc, filling them up to a depth of at least 2000 feet, and pa.<.-in-g across the range of the Ochil Hills, which at a distance of l2 miles runs parallel with the Highland niount-.1in.<, and reaches a height of 2352 feet. In such cases it has been observed that the strize along the lower slopes of the hill barrier run either parallel with the trend of the ground or slant up obliquely, while tl1ose on the summits may cross the ridge at right angles to its course. This shows that there nmst have been a differential movement in the great ice- sheet, the lower parts, as in a river, becoming embayed, and being forced to move in a direction sometimes even at a right angle to that of the general advance. On the lower grounds, also, the strize, converging from diﬁ'crent sides, unite at last in one general trend as the various icc—shu-ts must have done, as they descended from the high grounds on either side and coalesced into one connnon inass. This is well seen in the great central valley of Scotland. Still more marked is the deﬂexion of the strive in C1lllh- ness and the Orkney and Shetland Islands. In these districts the general direction of the striation is from which, in Caithness, is nearly at right angles to what might have been anticipated. This deﬂexion has been attributed to the coalescence of the ice from Norway and from the northern Highlands in the basin of the North , Sea, and its subsequent progress along the resultant line into the Atlantic. But it may have been due to the fan- shaped sprea_ding out of the vast mass of ice descending into the .Ioray Firth; for the strize on the south side of that inlet run E. by S., and at last S.E., on the north—east of Aberdeenshire, showing that the ice, on the one hand, turned southwards into the North Sea, until it met the N. E. stream from Kincardincshire and the valleys of the Dec and Don, while, on the other, it moved northward so as no doubt to join the Scandinavian sheet, and march with it into the Atlantic. The basin of the North Sea must have been choked up with ice in its northern parts, if not entirely. At that time England and the north—west of France were united, so that any portion of the North Sea basin not invaded by land-ice must have formed a lake;