Fisher — Glacial Origin of Denudation. 487 



a small glacier bed. In the same down there are also some very 

 singular combs just beneath Oliver's Camp, which have all the ap- 

 pearance of glacier beds. At their lower end, a dry bed of a former 

 torrent sudd^ly commences, as if it were the seat of the old glacial 

 river. 



About Tollard Eoyal, in the Chalk downs north of Tisbury, 

 some of the combs are very deep, and their sides steeper than will 

 allow the Chalk, as it disintegrates, to lie upon them. It forms a 

 talus at the bottom. This points to an agency which has left nearly 

 perpendicular walls upon the sides of the combs, an effect which 

 glaciers would produce. ■ 



Agreeing, as I do, with Professor Eamsay in his theory of the 

 erosion of lake basins by ice, I look to that agent for the formation 

 of such basins as the Broads, of Norfolk ; ^ and I think that many of 

 the basins in other districts now occupied by lacustrine deposits may 

 have been formed in a similar manner. Indeed, on any other theory, 

 it is hard to account for peat bogs and lacustrine deposits in certain 

 situations. 



It is obvious that the view I have taken of the mode of denuda- 

 tion is not in any way inconsistent with those phenomena which have 

 been attributed to the effects of solution, such as pipes in Chalk and 

 Limestone strata, or with the formation of caverns by solution of 

 the rock. A constant supply of water to the surface would be 

 afforded by the liquefaction of the lower surface of the ice. 



After the disappearance of the ice, the surface of the country 

 would become gradually covered with vegetation, while the work 

 of rain and rivers and frost would commence to modify the form and 

 condition of the surface. But the grand general features, which, to 

 my apprehension, are not attributable to the last-named agents, 

 would have been already impressed upon it. 



We have geological evidence of constantly alternating conditions 

 of an arctic and a temperate climate in these latitudes, at least, since 

 the period of the Norwich Crag. 



But I am much mistaken if, in the eastern plain of England, any 

 part of the configuration of the surface dates further back than the 

 great submergence which deposited the Boulder-clay containing 

 so much Chalk and Oolitic debris — the "Upper-drift" of Mr. S. V. 

 Wood, Jun. Since that period the surface has been under constantly 

 varying conditions ; the glacial periods having in all probability 

 become less and less severe. The present contour of the surface is 

 the accumulated result of these varied conditions throughout long ages. 

 My object in this article has been to suggest that the glacial periods 

 have been those to which the form of surface is chiefly due. 



^ Possibly Norfolk, from its situation, may have been colder than other parts of 

 England, being in the neighbourhood of an arctic cui'rent ; and its yalleys may have 

 been filled by ice while, in other places, alluvial beds ■were in course of formation. 



