Alfred Ilarliev — Suhaeriiil Erosion of SI:i/e. 4f)l 



the bed of the river, granite largely predorainnting on the right ami 

 being almost waiuiiig on the left. The distance l)etvveen the two 

 lines thus represents the total downstream movement during post- 

 Grlacial times. It is between eighty and ninety yards as measured 

 along midstream, being less along the right bank and more along 

 the left. No distinction has been made between smtU pebbles an 

 inch in diameter and boulders a foot or more, so that the result 

 obtained expresses the average amount of displacement of the 

 whole collectively. Doubtless the small stones have travelled fartlier 

 with the stream than the large, and indeed this a[)pears clearly 

 when we examine other places at which the same river crosses the 

 dividing-line in the drift. There are six such places above Sligachan 

 bridge, and at such of them as afford suitahle conditions for observa- 

 tion a variable amount of downstream transport can be verified. It 

 reaches nearly 250 yards in the extreme case, this being for fine 

 gravel. 



One remark is necessary to guard against misunderstanding. It 

 is not asserted that a stream like tlie Sligachan River is not competent 

 in times of flood to move even large boulders with ease ; only that 

 it does not do so to any important extent at the present time and 

 under natural conditions. A state of equilibrium has been attained, 

 in which the stones are so packed as to support one another and 

 offer the greatest possible resistance to the stream. The displace- 

 ment by any means of a keystone may cause movement among 

 others adjacent to it, just as stepping-stones artificially placed may 

 be swept away by a heavier spate than usual. The following 

 observation is instructive in this connection. The positions of 

 a number of boulders in the Sligachan River were noted by stretching 

 a string from bank to bank and marking where the string fell upon 

 each boulder. After a j^ear and a half, examination showed tiiat 

 only one of the train had suffered any appreciable displacement, and 

 this boulder, which had been moved (by sliding, not rolling) two 

 feet or more downstream, was one of the largest in the neighbour- 

 hood, weighing probably not less than three-quarters of a ton. 

 The circumstances left no reasonable doubt that its movement had 

 been rendered possible by the artificial displacement of smaller 

 stones which had served to protect and support it. Indeed, it may 

 fairly be conjectured that the downstream displacement of eighty 

 or ninety yards recorded near Sligachan bridge is mostly of 

 comparatively late date, and is due in great part to disturbances 

 consequent on the movements of anglers. A similar remark applies 

 to loose screes among the mountains. I have observed no sliding 

 of scree-material that was not directly due to sheep or to shepherds 

 and other pedestrians, and it is probable that before the introduction 

 of sheep during the present century there was no such sliding. To 

 whatever class of phenomena we turn, we are led to the same 

 conclusion, that in the district discussed the agents of atmospheric 

 degradation, erosion, and transportation are at the present time 

 almost wholly inoperative, and have accomplished only insignificant 

 results during the whole of post-Glacial time. 



