1879.] 



of the Parallel Roads of Locliaher. 



15 



Mr. Drew to have been caused by the bursting of a detrital barrier 

 formed by a landslip. In consequence of this a lake had been formed, 

 which he estimates to have been 35 miles long by 1 mile broad and 

 300 feet deep at its lower end. The whole was drained in a day. 



In the same way, it is to be assumed that the Glen Glaster barrier, 

 which was probably formed by a remnant of the glaciers descending 

 from the mountain ranges (2, 994 feet) at the head of the glen, at last 

 gave way with great suddenness, and caused the rapid fall of the 

 waters from the level of the higher "road " in Glen Roy to that of 

 that glen's second "road," at the height of the Glen Glaster Col, when 

 the escape of the waters was stopped. 



ISTow, it must be borne in mind that, at this time, the great mantle 

 of snow and ice which had so long covered the country, was passing 

 away, leaving the surface of the hills in Glen Roy covered with 

 a thick coating of angular local debris mixed with sand and clay, the 

 result of the intense cold and of the decomposition of the underlying 

 schistose and granitic rocks. This and the glacial debris must have 

 long remained bare and unprotected by vegetation ; at all events, that 

 below the water-line was so. Now, the angle of repose of purely 

 angular and subangular debris varies within the limits of from 35° to 

 48°, but that of clayey sands, which when dry is from 21° to 37°, 

 becomes, when saturated with water, as low as 14° to 22°. The angle 

 of repose of the hill-side debris would, therefore, depend on the relative 

 proportion of the angular materials and their matrix, and on the extent 

 of saturation. The slopes of the hills being on the whole greater than 

 that of the angle of repose of the saturated under- water rubble, this 

 latter, easily set in motion on to the settlement of its constituent 

 parts as the water drained from it, would as the level of the lake 

 water fell, tend to slip or slide down with the falling water, and this 

 slide would continue until the disturbing cause ceased, and the 

 momentum of the mass was checked by the inertia of the water* 

 gradually coming to rest on reaching the level of the col of escape. 

 The effect of the arrested slide, combined with the state of maximum 

 saturation of the mass, would be to project it more horizontally 

 forward, and form a ledge. This ledge, modified slightly by subse- 

 quent subaerial action and weathering, and by the dressing of its slope 

 on the occasion of the next fall of the lake, constitutes the " road." 



As the results are alike in the case of the other "roads," although 

 the causes which led to them must have been the same, there is 

 not the same evidence of a minor col-barrier ; and it is shown 

 that there is nothing incompatible in the features of the ground 

 with the existence of such barriers, or rather that there is some 

 evidence in each glen, however slight, of water lines at levels higher 

 than the "roads." There are difficulties in the way of the lower 

 * Even now after heavy rains large slides take place on the steeper slopes. 



