Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/538



f. Origin of the Blackpool Drifts.— Leaving the question of the origin of the Postglacial deposits to those who have devoted more attention to their character as exhibited on different parts of the Lancashire coast, I may remark that while few would doubt the marine-glacial origin of the Upper Boulder-clay, and while no one can dispute the purely marine origin (with the exception of boulders drifted into it by floating ice) of the middle sand and gravel formation, the precise mode of accumulation of the Lower Boulder-clay appears involved in mystery. At first sight any one might hastily conclude that it is a land-glacial deposit ; but a reconsideration of its varied phenomena, and a correlation of this clay with what we have reason to believe are equivalent deposits on the other side of Morecambe Bay (see sequel) and in South Lancashire, would seem to point to accumulation under mixed glacial and marine conditions. Admitting that the stones and a great part of the clay were furnished by land-ice (in the Carnforth area the stones would appear to have been transported chiefly by coast-ice ; see sequel), the sea at least must have kept up what may be called a running accompaniment to the accumulation of the drift-matter, by washing out and re-depositing its finer parts in the shape of laminated loam. In some parts of Northwest Lancashire the Lower Boulder-clay may be entirely a land accumulation; but it is difficult to resist the belief that where this formation spreads out continuously over large flat areas, it must have been deposited under the surface of the sea. It may likewise be remarked that at Blackpool the numerous granitic boulders and stones, which could not