Vol. 70.] ANNIVERSARY ADDRESS OP THE PRESIDENT. lxXXV 



Problems of Post-Glacial Denudation. 



The problem of denudation, in all but the latest geological period, 

 is one of infinite difficulty, for it involves consideration of varying 

 conditions which can never be known. The distribution of 

 different types of rock in the land-surface of past ages can only be 

 ascertained locally and imperfectly ; yet their power of resistance 

 is a dominant factor in the problem. The vagaries of climate are 

 not calculable now, and are past guessing in former geological 

 periods. The tropical climate, with extremes of temperature and 

 spasmodic rainfall, the wet climate, the dry climate with persistent 

 wind, are a few of a long list of types, each intensely active. That 

 they existed in past times we know, but of their distribution and 

 duration we know next to nothing. Nor is it possible to estimate 

 the protective or destructive effects of the organic life of land- 

 surfaces of geological times. 



An estimation of the effects of denudation during the latest 

 geological period, though beset with difficulties, is less open to 

 these objections. Indeed, it seems evident that geologists should 

 prove the possibility of forming an estimate with some degree of 

 precision for that period during which conditions not widely dif- 

 ferent from those now existing prevailed, before aspiring to furnish 

 statistics for primaeval times. I propose, therefore, in dealing with 

 the question of denudation, as evidenced in this country, to confine 

 my remarks to post- Glacial time, to consider what methods are 

 possible of estimating the amount of denudation which has been 

 effected during that period, and of measuring the rapidity with 

 which the denuding agents are now carrying on their work. 

 Without suggesting that those agents have been uniformly effective 

 during the changing conditions which have prevailed in post-Glacial 

 time, we may claim that they have operated under conditions so 

 comparable with those of the present day, as to bring their effects 

 within the range of legitimate speculation. 



The problem of post-Glacial denudation is open to attack from 

 more than one direction. We have no such sensational example in 

 the British Isles as the gorge of Niagara, but we can show a well- 

 developed series of Glacial deposits, which have been under close and 

 prolonged observation. In proportion as we have learnt to distin- 

 guish these from the various gravels that have been formed out 

 of their waste, we have become the better able to recognize topo- 

 graphical features of post-Glacial origin. River-gorges, re-excavated 



