lxXXYl PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 1914. 



pre-Glacial valleys, and dissected plateaus are some among the 

 many forms in which the effects of post- Glacial denudation are 

 presented for consideration. 



In this connexion, I leave out of account the weathering of the 

 rock-surface itself, as being too indefinite a factor to furnish a cal- 

 culation for the present purpose. In some districts the effects of 

 glaciation are displayed in such freshness as to suggest that the 

 Glacial Period is an episode of yesterday ; elsewhere the surface 

 has so perished as to leave merely a reminiscence of the forms 

 characteristic of glaciation. Much has depended upon the character 

 of the rock and of the climate, but no less on the nature of the 

 material bv which the rock was covered and the length of time 

 during which the surface has been exposed to the air. Attempts 

 have been made also to determine the relative ages of Glacial 

 deposit by the amount of weathering, as distinct from erosion, 

 which they have undergone. But I put these effects aside, as 

 yielding no data suitable for my present purpose. 



Such objections apply in a less degree to the removal of Glacial 

 deposits by the ordinary agents of denudation. There are regions 

 where it is not only possible to trace the evolution of the existing- 

 landscape, but where there would seem to be a possibility of esti- 

 mating the bulk of the material removed in the process of evolution. 

 The work has been done by streams and other agencies now in 

 operation. The next step, therefore, would be to ascertain the rate 

 at which those agencies are working, in order to calculate the time 

 required for the removal of the material under existing conditions. 

 How far these conditions have varied would still remain for 

 consideration. 



I can best illustrate my meaning by mentioning examples where 

 measurements of post-Glacial work may be within reach. One such 

 case occurs on the Welsh Border in Denbighshire. There a plateau 

 of Glacial sand and gravel extends eastwards from the Welsh hills 

 for 2 or 3 miles. It ranges from 250 to 300 feet above the sea, and 

 terminates eastwards in a somewhat steep declivity, from the foot 

 of which extends a broad plain of Boulder Clay with a height of 

 about 50 feet above the sea. 



The River Alyn, leaving the Welsh hills, crosses the plateau and 

 the Boulder-Clay plain. At first it descended the declivity, as we 

 may suppose, by a series of rapids, but for a short time only — for in 

 the loose gravel cutting-back must have commenced at once. At 

 the present day, the river has established a course of fairly steady 



