Popular Science Monthly/Volume 42/December 1892/Recent Glacial Discoveries in England
Contour and glacial map of the British Isles
THE accompanying map, prepared for Prof. Wright's new work on Man and the Glacial Period from data furnished by the latest investigations in Great Britain, embodies a vast amount of information, and for the most part tells its own story. It is largely the outcome of the work of the late Prof. Carvill Lewis, whose untimely death left his large collection of English notes still unpublished. But, in response to the interest aroused by him, a society embracing the most active geologists of northern England was formed to follow out and complete his work. The president of this society is Prof. Percy F. Kendall, now of Leeds, who prepared the chapter on the glacial geology of Great Britain for Prof. Wright's book, and who has furnished the principal data for the construction of this map. We are glad to be informed also that the field notes of Prof. Lewis, under the joint editorship of Rev. Dr. Crosskey, of Birmingham, and Prof. Kendall, are soon to be published by Mrs. Lewis in England.
Prof. Lewis was the first one to attempt a careful delineation of the boundary of glacial action in England and Ireland, as he was one of the first to do this work in the United States. Soon after Profs. Cook and Smock, of New Jersey, had published their map of the terminal moraine in New Jersey, Profs. Lewis and Wright took up the task of following it out through Pennsylvania. The results of their work there are embodied in Volume Z of the Geological Report of that State. Upon completing this work the two professors, by previous arrangement, divided the work of exploration—Prof. Wright carefully surveying the line westward to the Mississippi River, and with more or less care to Alaska, while Prof. Lewis went to England to do the work of which we have spoken there. Last year Prof. Wright also went to England, at the request of those who were following up Prof. Lewis's work there, and went over a large part of the most important ground under their lead; hence an unusual degree of confidence can be placed in the results which have been for the first time systematically presented in this map and the accompanying description. Space will permit us to give but the very briefest summary of the conclusions respecting English glacial geology, some of which are really revolutionary.
In the first place the investigations demonstrate beyond controversy that the glacial phenomena in the British Isles are the product of land ice, and not of floating ice. This may not seem very important to American geologists, who are all of one mind, but in England it means a good deal, where there are many who still cling to the old idea that icebergs and not glaciers 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 from the mountains of Wales, reaching eastward to Birmingham. But the two movements did not quite join. An unglaciated area was left between.
During all this time the Irish Sea was slowly filling up with the ice which was shed from the mountains of northern England, southwestern Scotland, and Ireland. This finally reached the obstruction presented by the mountains of Wales and divided—one branch of the ice-current going southwestward along the channel of the Irish Sea, and the other southeastward through the vale of Chester into the upper part of the Severn Valley. The bowlders transported by this movement are distributed down to a very definite line as marked in the map, and they overlie those from the Welsh mountains. It is in this area, containing bowlders from the lake district and southwestern Scotland, and in that covered with Scandinavian ice, that shells are found in the glacial deposits. Over the uncolored portion of the map and outside the limits of these two movements there is nothing to suggest a glacial or interglacial submergence. For a popular but full and comprehensive statement of the facts in the case the reader must consult Prof. Kendall's chapter in the volume from which we are permitted to copy this extremely interesting map.