Origin of the Parallel Roads of Lochaher. 325 



the case of the Holmfirth, Oman, and other floods. Nor is evidence 

 wanting of similar catastrophes in connexion with glacier lakes. In 

 the notable case of the Gietroz Glacier barring the valley of the 

 Drance, a lake was formed which attained a length of nearly two 

 miles, and a depth at the barred end of 200 feet. So rapid was the 

 discharge when the barrier yielded, that the lake was drained in 

 twenty minutes. The still greater flood recorded by Vigne, which 

 descended a branch of the Indus in one day for a distance of 125 miles, 

 has since been shown by 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 one 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 Eoy to that of 

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

 the escape of the waters was stopped. 



Now, 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 Eoy 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 owing 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 slip 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 slip, 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 subsequent subaerial action and weathering, and by the 

 dressing of its slope on the occasion of the next fall of the lake, 

 constitutes the " road." 



Although in the case of the other " roads " there is not the same 

 evidence of a minor col-barrier, as the results are alike in all, the 

 causes which led to them must have been the same ; and it is shown 



