348 GLACIAL EROSION IN NORTH WALES. [Aug. 1909, 



Mr. Crook raised a question), structure was of fundamental 

 importance : it was the first term in the systematic method for 

 the description of land-forms in terms of ' structure, process, and 

 stage', with which the Author was experimenting. But the 

 influence of structure decreased with the advance of the cycle of 

 erosion, and in late maturity, when the surface became covered 

 with creeping waste, structure had relatively small value, unless 

 strongly contrasted hard and soft rocks were present. The subdued 

 mountains of North Carolina, to which he (the Author) had 

 referred as types of normal late mature forms, were composed 

 chiefly of crystalline schists. He believed that a comparison 

 might fairly be made between these mountains and the subdued 

 pre-Glacial mountains of Wales. 



Prof. Watts and several other speakers enquired as to how an 

 eroding glacier could produce cwms, lake-basins, and valley-floor 

 steps. As to cwms, the detachment of rock-blocks by freezing 

 and thawing of water in the bergschrund around the head of 

 a glacier, as suggested and observed by Dr. Willard D. Johnson, 

 was certainly an effective process so far as it went ; but it did not 

 go deep enough. The deepening of the broad cwm-floor required 

 another process, and of all suggestions, the Author thought 

 frictional dragging or plucking was of most value. It might be 

 aided by freezing and thawing of water in joints, due to changes 

 of pressure instead of to changes of temperature, as had been 

 pointed out by several geologists. The production of valley-floor 

 steps and basins was a necessary corollary of glacial erosion. The 

 transformation of a mature pre-glacial valley into a mature glacial 

 trough must be gradual : during its progress, the deepening of the 

 trough-bed must be somewhat irregular ; steps and basins were a 

 most likely result, wherever any differences of structure occurred. 

 The Author would be glad to hear a fuller explanation of 

 Prof. Watts's statement that every earth-movement in a glaciated 

 district was ' registered as a lake ' : surely a general uplift along 

 the mountain-axis, whereby the slope of a glacier must be increased, 

 would not be so registered ; neither would a local up-faulting of 

 the upper end of the glacier-filled valley produce a lake ; nor 

 would a warping of the valley produce a lake, if the glacier wore 

 down the up-warped part of the valley. 



The Author differed from Prof. Watts as to the real difficulties 

 of the question. The explanation of steps and basins in cwms and 

 valleys offered no special difficulty, if it were once proved that 

 glaciers erode their troughs ; for, in that case, some inequality of 

 trough-erosion was as likely as in the erosion of a channel by a 

 river. Differences of structure sufficient to cause basins and steps 

 might be perceptible to an eroding glacier, even if not perceptible 

 to an observant geologist. The real difficulty therefore was not in 

 the explanation of minor features, such as basins and steps, but in 

 the process of general glacial erosion, of which basins, steps, and 

 various other features were necessarv and reasonable corollaries. 



