James Gelhie — Denudation in Scotland, 23 



the Boulder-clay had usually suffered much erosion, while in regions 

 where the sand and gravel did not appear, it had commonly a much 

 less denuded aspect, often forming well-marked terraces in upland 

 valleys, and in straths frequently showing parallel mounds or broad 

 ridges, which were probably, in the main, original inequalities 

 acquired while the till was in course of formation as a moraine pro- 

 fonde. 



The character and mode of accumulation of the upper or marine 

 drifts were next considered. It was shown that these dej)osits con- 

 sisted chiefly of sand and gravel, at other times of a coarse Boulder 

 earth, and in some places fossiliferous clay also occurred. These 

 beds had no doubt been derived chiefly from the waste of the Lower 

 Boulder-clay, and also in some measure from the droppings of 

 icebergs. In a district were the sands and gravels were typically 

 developed it was often found that, as we left the central parts of the 

 broad valleys and straths and approached the contiguous higher 

 grounds, these drifts appeared to thin away from heaps of well- 

 stratified materials to meagre, irregular sprinklings of earth and 

 stony rubbish. The fine sands and gravels with diagonal bedding 

 occupied chiefly the bottoms of the valleys, the kaims and mounds 

 of coarse and angular debris lay for the most part along the hill-sides. 



These facts appeared to point to a passage from a deeper to a 

 shallower sea-bed. The capricious distribution of the sand and 

 gravel was then touched upon, and it was shown how this might 

 give us an index to the probable maximum of depression attained 

 during the marine period. The evidence furnished by the angular 

 gravels, which seemed to mark out the upper limits reached by the 

 Glacial sea, agreed with the independent testimony derived from the 

 occurrence of the kaims in certain valleys and their absence from 

 others. If we supposed the land to be submerged to the highest 

 levels reached by the coarse angular gravels it would be observed 

 that kaims only occurred in such valleys, as on a depression to this 

 extent would form straits, channels, or comparatively open seas — that 

 it was precisely in the same localities where the lower till was exces- 

 sively eroded — while in those valleys which, under like conditions, 

 must become long narrow fiords, marine drift did not appear, and 

 the Boulder-clay had not been subjected to the same degree of denu- 

 dation. In the straits marine currents would have free scope to 

 plough up the deposits of the earlier period, while in the narrow 

 firths no such action could be carried on, as we learned should be the 

 case from a study of our own Highland sea lochs or the fiords of 

 Norway. The later drifts, consisting of terminal moraines and that 

 surface-wash which seems to have gathered under the snow or neve, 

 showed us that, after the re-elevation of the land, glaciers continued 

 to reach the sea for a time, and in their downward progress from 

 the snowfields, scooped out the accumulations which had formed 

 during the previous ages. 



Having thus given a resume of facts connected with the mode of 

 occurrence of the drift deposits, and the kind oferosion to which they 

 were subjected during the Glacier period itself, the lecturer then pro- 



