Page:Popular Science Monthly Volume 42.djvu/184

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THE POPULAR SCIENCE MONTHLY.

were the agency which scored the rocks and distributed the bowlders over the island. In the second place, these investigations have explained away, in a very complete and satisfactory manner, the evidence which had been supposed to prove that there was a submergence of the northern part of England and Wales during an interglacial period amounting to fourteen hundred or two thousand feet.

This evidence consisted of shell-beds inclosed in true glacial deposits eleven hundred feet above the sea at Macclesfield, near Manchester, and fourteen hundred feet above the sea at Moel Tryfaen, on the northern flanks of Snowdon, in Wales. Prof. Lewis, and those who have followed out the clews which he started, have proved that these shell-beds were not direct deposits during a submergence of the country, but rather beds washed out of true glacial deposits which had been shoved along by the ice in its passage over the bottom of the Irish Sea. The shells were pushed up with the mud from the sea-bottom, as pebbles are known to have been in so many instances. The melting of the ice furnished the water necessary for partially working over the original deposit and sorting out and stratifying the inclosed gravel and shells.

The demonstration of this theory of Prof. Lewis consists in showing that the deposits of shells are limited to those portions of the glaciated area which can be proved, by the transported bowlders, to have been overrun by ice which passed over the sea-bottom. Over this area shells are more or less mingled with the till, or bowlder clay, just as pebbles are, and limited beds of gravel and shells are of frequent occurrence, though the shells for the most part are very much broken up. An additional point of evidence of great weight is found in the fact that the shells are not such as would collect in the same place under water. In these beds rock-haunting and mud-loving species, and shallow-water and deep-water species are indiscriminately mingled together.

The course of ice movement is clearly shown on the map by the lines indicated in the transportation of bowlders. Briefly stated, the movements were as follows: Scandinavian ice flowed westward over the shallow basin of the German Ocean until it reached the coast of England from Flamborough Head to the latitude of London. It was warded off from Scotland and the northern coast of England by the glaciers which had preoccupied that region. Scandinavian bowlders are found scattered over the eastern counties of England, and there is evidence that the ice from that direction penetrated to the vicinity of London and up nearly to the head-waters of the Ouse and of the South Branch of the Humber. Meanwhile a glacial movement had been in progress