Man and the Glacial Period

by G. Frederick Wright

“It has been supposed that the drift-deposits were marine accumulations; but it is inconceivable that the cave could ever have been subjected to wave-action without the complete scouring out of its contents.

“To resume the delineation of the limits of the great Irish Sea Glacier: From the Vale of Clwyd the boundary runs along the range of hills parallel to the estuary of the Dee at an altitude of about nine hundred feet. As it is traced to the southeast it gradually rises, until at Frondeg, a few miles to the northward of the embouchure of the Yale of Llangollen, it is at a height of 1,450 feet above sea-level. Thence it falls to 1,150 feet at Gloppa, three miles to the westward of Oswestry, and this is the most southerly point to which it has been definitely traced on the Welsh border, though scattered boulders of northern rocks are known to occur at Church Stretton.

“Along the line from the Vale of Clwyd to Oswestry the boundary is marked by a very striking series of moraine-mounds. They occur on the extreme summits of lofty hills in a country generally almost driftless, and their appearance is so unusual that one—Moel-y-crio—at least has been mistaken for an artificial tumulus. The limitation of the dispersal of northern erratics by these mounds is very clear and sharp; and Mackintosh, in describing those at Frondeg, remarked that, while no northern rocks extended to the westward of them, so no Welsh erratics could be found to cross the line to the eastward. There are Welsh erratics in the low grounds of Cheshire and Shropshire, but their distribution is sporadic, and will be explained in a subsequent section.

“Having thus followed around the edges of this glacier, it remains to describe its termination. It is clear that the ice must have forced its way over the low water-shed between the respective basins of the Dee and the Severn. So soon as this ridge (less than 500 feet above the sea) is crossed, we find the deposits laid down by the glacier change their character, and sands and gravels attain a great predominance.[BO] Near Bridgenorth, and, at other places, hills composed of such materials attain an altitude of 200 feet. From Shrewsbury via Burton, and thence, in a semicircular sweep, through Bridgenorth and Enville, there is an immense concentration of boulders and pebbles, such as to justify the designation of a terminal moraine. To the southward, down the valley of the Severn, existing information points to the occurrence merely of such scattered pebbles as might have been carried down by floods. In the district lying outside this moraine there is a most interesting series of glacial deposits and of boulders of an entirely different character. ([See map].)

[BO] Mackintosh, Q. J. G. S.

“From the neighbourhood of Lichfield, through some of the suburbs of Birmingham, and over Frankley Hill and the Lickey Hills to Bromsgrove, there is a great accumulation of Welsh erratics, from the neighbourhood, probably, of Arenig Mawr.

“The late Professor Carvill Lewis suggested that these Arenig rocks might have been derived from some adjacent outcrop of Palæozoic rocks—a suggestion having its justification in the discoveries that had been made of Cumbrian rocks in the Midlands. To test the matter, an excavation was made at a point selected on Frankley Hill, and a genuine boulder-clay was found, containing erratics of the same type as those found upon the surface.

“The explanation has since been offered that this boulder-clay was a marine deposit laid down during a period of submerge nee.[BP] Apart from the difficulty that the boulder-clay displays none of the ordinary characteristics of a marine deposition, but possesses a structure, or rather absence of structure, in many respects quite inconsistent with such an origin, and contains no shells or other remains of marine creatures, it must be pointed out that no theory of marine notation will explain the distribution of the erratics, and especially their concentration in such numbers at a station sixty or seventy miles from their source.

[BP] Proceedings of the Birmingham Philosophical Society, vol. vi, Part I, p. 181.

“Upon the land-ice hypothesis this difficulty disappears. During the early stages of the Glacial period the Welsh ice had the whole of the Severn Valley at its mercy, and a great glacier was thrust down from Arenig, or some other point in central Wales, having an initial direction, broadly speaking, from west to east. This glacier extended across the valley of the Severn, sweeping past the Wrekin, whence it carried blocks of the very characteristic rocks to be lodged as boulders near Lichfield; and it probably formed its terminal moraine along the line indicated. (See lozenge-shaped marks on the map.) As the ice in the north gathered volume it produced the great Irish Sea Glacier, which pressed inland and down the Vale of Severn in the manner I have described, and brushed the relatively small Welsh stream out of its path, and laid down its own terminal moraine in the space between the Welsh border and the Lickey Hills. It seems probable that the Welsh stream came mainly down the Vale of Llangollen, and thence to the Lickey Hills. Boulders of Welsh rocks occur in the intervening tract by ones and twos, with occasional large clusters, the preservation of any more connected trail being rendered impossible by the great discharge of water from the front of the Irish Sea Glacier, and the distributing action of the glacier itself.