1869.] MACKINTOSH LAlffCASHIRE AND CUMEEELAND DEIPTS. 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 feUow boulders and dropped them into the 

 slowly accumulating gravels and clays of the then adjacent sea- 

 bottom. 



4. Denudation of Dkift Deposits. 



The drift-areas of IST.W. Lancashire present a succession of 

 smoothly-rounded heights and hoUows — the vertical extent of the 

 undulations reaching 200 feet. Sections show that this varied sur- 

 face 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 Sivamj)-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 con- 

 tinuation 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 per- 

 manent) 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 tiU after the glacial submer- 

 gence), unless we agree with the Eev. 0. Pisher 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. 



h. Suhaerial 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 direc- 

 tion of the drainage*. Freshwater streams have taken advantage of 



* Between the estuaries of the Mersey and the Dee two depressions, evidently 

 once tidal channels, run across from sea to sea. 



