Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/541

1869.] MACKINTOSH—LANCASHIRE AND CUMBERLAND DRIFTS. 417 rounded limestone blocks, and bounded by cliffs. I had not time to ascertain if the protected sides of these blocks were striated; but they looked as if they had only accidentally escaped being transported by the floating ice, which, for all that we can tell, may have carried away many of their fellow boulders and dropped them into the slowly accumulating gravels and clays of the then adjacent sea-bottom.

4. Denudation of Drift Deposits.

The drift-areas of N.W. Lancashire present a succession of smoothly-rounded heights and hollows—the vertical extent of the undulations reaching 200 feet. Sections show that this varied surface is mainly the result of denudation, and that the denudation has proceeded irrespectively of the structure of the underlying deposits. The lower and upper part of a knoll may consist of distinct kinds of drift; one side of a knoll may be made up of one kind of drift, and the opposite side of another kind; and all the phenomena would seem to point to a denudation of a broader and more sweeping nature than any form of atmospheric action.

a. Origin of Lake-and Swamp-basins in Drift.—Perhaps the most prevalent form of hollow presented by the surface of drift-deposits (at least in some districts) is the shallow basin. It is merely a continuation of the general undulating surface; and there is no reason for supposing that its lowest side has been left by deposition. The basin becomes a swamp, marsh, mere, or temporary (sometimes permanent) lake. In many places these wet depressions remain; in most places they have been artificially drained. Scores of them may be seen between Blackpool and Carnforth, and not a few in the Furness peninsula. Were these basins scooped out by land-ice? The fact of their often occurring on the surface of Upper Boulder-clay shows that they could not have been subjected to land-glacial action (they generally occupy positions remote from upland valleys, in which glaciers may have lingered till after the glacial submergence), unless we agree with the Rev. O. Fisher in believing in a supplementary glacial period occurring between 100,000 and 200,000 years ago, during which a great sheet of land-ice gave the latest finish to the configuration of the ground. It might be out of place in a paper of this kind to discuss the question of the excavation of these drift-basins beyond expressing an opinion that the idea of their formation by submarine currents can be better included than any other in a consistent scheme of the succession of glacial and post- glacial events.

b. Subaerial Denudation of Drift Deposits.—Where the surface of the drift does not exhibit a succession of knolls and basin-shaped depressions, it spreads out in the shape of uniformly flat plains, as between Preston and Longridge, and other parts of South Lancashire. Shallow -__-shaped passes, at greater or less intervals, cross these plains often nearly at right angles, and without any connexion with the direction of the drainage*. Freshwater streams have taken advantage of

once tidal channels, run across from sea to sea.
 * Between the estuaries of the Mersey and the Dee two depressions, evidently