CHAPTER XXVII

POST-TERTIARY OR QUATERNARY PERIODS-PLEISTOCENE OR

POST-PLIOCENE-RECENT

We have now arrived at the last main division of the Geological Record, that which is named POST-TERTIARY or QUATERNARY, and which includes all the formations accumulated from the close of the Tertiary periods down to the present day. But no sharp line can be drawn at the top of the Tertiary groups of strata. On the contrary, it is often difficult, or indeed impossible, satisfactorily to decide whether a particular deposit should be classed among the younger Tertiary or among the Post-tertiary groups. In the latter,

all the molluscs are believed to belong to still living species, and the mammals, although also mostly still of existing species, include some which have become extinct. These extinct forms are numerous in proportion to the antiquity of the deposits in which they have been preserved. Accordingly, a classification of the Quaternary strata has been adopted, in which the older portions, containing a good many extinct mammals, have been formed into what is termed the Pleistocene, Post-pliocene, or Glacial group, while the younger deposits, containing few or no extinct mammals, are termed Recent.

The gradual refrigeration of climate which is revealed to us by the shells of the crag was prolonged and intensified in Post-tertiary time. Ultimately the northern part of the northern hemisphere was covered with snow and ice, which extended into the heart of Europe and descended far southward in North America. The previous denizens of land and sea were in large measure driven out or even in many cases wholly extirpated by the cold, while northern forms advanced southward to take their places. The reindeer, for instance, roamed in great numbers across Southern France, and Arctic vegetation spread all over Northern and

Central Europe, even as far as the Pyrenees. After the cold had reached its climax, the ice-fields began to retreat, and the northern flora and fauna to retire before the advance of the plants and animals which had been banished by the increasingly severe temperature. And at last the present conditions of climate were reached. The story of this Ice Age is told by the Pleistocene or Post-pliocene formations, while that of the changes which immediately led to the establishment of the present order of things is made known in the Recent deposits.

PLEISTOCENE, POST-PLIOCENE, OR GLACIAL.

The evidence from which geologists have unravelled the history of the Ice Age or cold episode which came after the Tertiary periods in the northern hemisphere may here be briefly given. All over Northern Europe and the northern part of North America the solid rocks, where of hardness sufficient to retain it, are found to present a characteristic smoothed, polished, and striated surface. Even on crags and rocky bosses that have remained for long periods exposed to the action of the weather, this peculiar worn surface may be traced; but where they have been protected by a covering of clay, these markings are often as fresh as when they were first made. The groovings and fine striæ do not occur at random, but in every district run in one or more determinate directions. The faces of rock that look one way are rounded off, smoothed, and polished; those that face to the opposite quarter are more or less rough and angular. The quarter to which the worn faces are directed corresponds with that to which the striæ and grooves on the rock-surfaces point. There can be no doubt that all this smoothing, polishing, grooving, and striation has been done by land-ice; that the trend of the striæ marks the direction in which the ice moved, those faces of rock which looked towards the ice being ground away, while those that looked away from it escaped. By following out the directions of the rock-striæ we can still trace the march of the ice across the land (see Chapter VI).

As the ice travelled, it carried with it more more or less detritus, as a glacier does at the present day. Some of this material may have lain on the surface, but probably most of it was pushed along at the bottom of the ice. Accordingly, above the ice-worn surfaces of rock, there lies a great deposit of clay and boulders, evidently the debris that accumulated under the ice-sheet and was left on the surface of the ground when the ice

retired. This deposit, called Boulder-Clay or Till, bears distinct corroborative testimony to the movement of the ice. It is always more or less local in origin, but contains a variable proportion of stones which have travelled for a greater or less distance, sometimes for several hundred miles. When these stones are traced to their places of origin, which are often not hard to seek, they are found to have come from the same quarter as that indicated by the striation of the rocks. If, for example, the ice-worn bosses of rock show the ice to have crept from north to south, the boulders will be found to have a northern origin. The height to which striated rock-surfaces and scattered erratic blocks can be traced affords some measure of the depth of the ice-sheet.

From this kind of evidence it has been ascertained that the whole of Northern Europe, amounting in all to probably not less than 770,000 square miles, was buried under one vast expanse of snow and ice. The ice-sheet was thickest in the north and west, whence it thinned away southward and eastward. Upon Scandinavia it was not improbably between 6000 and 7000 feet thick. It has left its mark at heights of more than 3000 feet in the Scottish Highlands, and over North-Western Scotland it was probably not less than 5000 feet thick. Where it abutted upon the range of the Harz Mountains, it appears to have been still not far short of 1500 feet in thickness.

This vast mantle of ice was in continual motion, creeping outward and downward from the high grounds to the sea. The direction taken by its principal currents can still be followed. In Scandinavia, as shown by the rock-striæ and the transport of boulders, it swept westward into the Atlantic, eastward into the Gulf of Bothnia, which it completely filled up, and southward across Denmark and the low grounds of Northern Germany. The basin of the Baltic was completely choked up with ice; so also was that of the North Sea as far south as the neighbourhood of London. From the same evidence we know that the ice which streamed off the British Islands moved eastward from the slopes of Scotland into the hollow of the North Sea, part of it turning to the left to join the south-western margin of the Scandinavian sheet, and move with it northwards and westwards across the Orkney and Shetland Islands into the Atlantic, and another branch bending southwards and moving with the southerly expansion of the Scandinavian ice along the floor of the North Sea and the low grounds of the east of England; and that on the west side of Scotland the ice filled up and crept down all the fjords, burying

the Western Islands under its mantle and marching out into the Atlantic. The western margin of the ice-fields, from the southwest of Ireland to the North Cape of Norway, must have presented a vast wall of ice some 2000 miles long, and probably several hundred feet high, breaking off into icebergs which floated away with the prevailing currents and winds. The Irish Sea was

likewise filled with ice, moving in a general southerly direction.

Northern Europe must thus have presented the aspect of North Greenland at the present time. The evidence of rock-striæ and ice-borne blocks enabled us to determine approximately the southern limit to which the great ice-cap reached. As even the southern coast of Ireland is intensely ice-worn, the edge of the ice must have extended some distance beyond Cape Clear, rising out of the sea with a precipitous front that faced to the south. Thence the ice-cliff swung eastwards, passing probably along the line of the Bristol Channel and keeping to the north of the valley of the Thames.

That the northern ice moved down the bed of the North Sea is shown by the boulder-clays and transported stones of the eastern counties of England, among which fragments of well-known Norwegian rocks are recognisable. Its southern margin ran across what is now Holland, and skirted the high grounds of Westphalia, Hanover, and the Harz, which probably there arrested its southward extension. There is evidence that the ice swept round into the Lowlands of Saxony up to the chain of the Erz, Riesen, and Sudeten Mountains, whence its southern limit turned eastward across Silesia, Poland, and Gallicia, and then swung round to the north, passing across Russia by way of Kieff and Nijni Novgorod to the Arctic Ocean.

In Europe no distinct topographical feature appears to mark the southern limit reached by the ice-sheet; this limit can only be approximately fixed by the most southerly localities where striated rocks and transported blocks have been observed. In North America, however, the margin of the great ice-cap is prominently defined by a mound or series of mounds of detritus which seem to have been pushed in front of the ice. These mounds, beginning on the coast of Massachusetts, run across the Continent with a wonderful persistence for more than 3000 miles. They form what American geologists call the "terminal moraine."

The detritus left by the ice-sheet consists of earthy, sandy, or clayey material (Boulder-Clay, Till) more or less charged with stones of all sizes up to blocks weighing many tons. For the

most part it is unstratified, and bears witness to the irregular way in which it was tumbled down by the ice. In some districts, it has been more or less arranged in water, and then assumes a stratified character. The stones in the detritus, more especially where they are hard and are imbedded in a clayey matrix, present smooth striated surfaces, the striæ usually running along the length of the stone, but not infrequently crossing each other, the older being partially effaced by a newer set (Fig. 24). This characteristic striation points unmistakably to the slow creeping motion of land-ice.

But the boulder-clays, earths, and gravels left by the great icesheet are not simply one continuous deposit. On the contrary, they contain intercalations of stratified sand, clay, and even peat. In these included strata organic remains occur, for the most part those of terrestrial plants and animals, showing that the ice again and again retreated, leaving the country to be covered with vegetation, and to be tenanted by land animals; but that after longer or shorter periods of diminution it once more advanced southward over its former area. These intervals of retreat are known as "interglacial periods." Probably they were of prolonged duration, the climate becoming comparatively mild and equable while they lasted. The occurrence of boulder-clays above the interglacial deposits shows a subsequent lowering of the temperature, with a consequent renewal of glacial conditions.

The Pleistocene deposits thus reveal to us a prolonged period of cold broken up by shorter intervals of milder climate. The fossils which they contain throw curious and interesting light on these oscillations of temperature. Among the plants, leaves of Arctic species of birch and willow are found far to the south of their present limits; on the other hand, remains of plants now confined to temperate latitudes are found fossil in Siberia, and others, now living in more genial climates than those of Central Europe, are associated in interglacial deposits, with the remains of the still indigenous vegetation.

To the same effect, but still more striking, is the testimony of the Pleistocene fauna, with its strange mingling of northern and southern forms. The marine shells imbedded in the glacial clays of Scotland, though chiefly belonging to species that still live in the adjoining seas, include a few that are now restricted to more northern latitudes (Pecten islandicus, Leda lanceolata, Tellina lata, etc., Fig. 204). Turning to the terrestrial mammals, we find among the Pleistocene deposits the remains of the last of the