OP THE OUTER HEBRIDES. ' 831 



tain-slopes that extend along the whole northern shore of West 

 Loch Tarbert. Throughout that region roches moutonnees and striae 

 plainly indicate a movement from the south-east, the average direc- 

 tion being from E. 20° S. to W. 20° N. "We traced the glaciation 

 in this part of Harris up to a height of 1600 feet on the shoulders 

 of the Cliseam and the Langa, the two highest hills in the island. 

 These heights were obtained by aneroid measurement, corrected by 

 reference to Ordnance Survey data, which were kindly supplied to 

 me by the late Major-General Cameron, C.B., Director of the Survey. 

 Above the level of 1600 feet there is no trace of any glaciated sur- 

 face or outline ; on the contrary, the rocks are excessively scarred, 

 shattered, and weather-worn, some of the hill- tops being ridged and 

 peaked. The line of demarcation between the non-glaciated rock 

 above and the smoothed and moutonnee surface below is well marked, 

 but is of course most conspicuous when viewed from a distance, when 

 the eye can take in the whole mountain-mass from summit to base. 

 Nothing, indeed, can be more striking than the contrast between 

 the two rock-features when viewed under such conditions. Rounded, 

 dome-shaped rocks, bald, grey, and sometimes almost white, cluster 

 upon all the slopes, especially on spurs and prominences facing the 

 south-east, and sweep up and over hill-tops which are under 1600 

 feet in height. Above that height, however, they do not extend, 

 and the horizontal line that marks their upper limits seems, from a 

 distance, as straight as if drawn with a ruler. Beyond this line the 

 rocks are craggy, irregular, and broken ; they show no smooth faces 

 to reflect the light, and look much darker, therefore, than the grey 

 masses below. 



Roches moutonnees are frequently ruinous, of course, as well at low 

 as at high elevations ; for much depends on the character of the rock 

 and the form of the ground. In general, however, mamillated sur- 

 faces occur in best preservation at the lower levels ; and this is as 

 one would expect, for the rocks at the upper limits of the ice-sheet 

 would be earlier exposed to the action of frost, and might, in many 

 cases, become broken up before the glaciers had melted away from 

 the valleys. Nor can it be doubted that even now the action of 

 frost is, as a rule, more destructive at the higher than the lower levels 

 of the land. Here and there the signs of former glaciation are 

 passing more or less rapidly away. This is conspicuously the case 

 upon such cliff-faces as those that occur in Gleann Sgaodail, Bealach 

 na Ciste, and Bealach Miabhag, where, owing to the steepness of 

 the cliffs and the abundant jointing of the gneiss, frost is enabled to 

 rupture and shatter the rocks, and to shower their debris upon the 

 lesser slopes below. 



I have said that the average direction of the glacial striae along 

 the northern shores of West Loch Tarbert is from E. 20° S. to 

 W. 20° N. This direction holds good throughout all the grey ma- 

 millated hills and hill-slopes that undulate upwards to the shoulders 

 of the Langa and the Cliseam. Here and there, owing to the un- 

 even surface of the ground, there are slight deflections of the striae ; 

 but such are quite local, the general trend of striae and roches mo%i- 



