14 



Prof, J. Prestwich. On the Origin 



[May 1, 



or cliff line such as would be due to tlie wearing back of a shore line, 

 and of any projecting ledge such as would result from the throwing 

 forward of the shore debris. The slope of the hills above and below 

 the " roads " varies from 25° to 40°, and the inclination with the 

 horizon of the " roads " themselves, which are from 50 to 70 feet 

 wide, varies within the limits of from 5° to 30°. 



Of the internal structure of the "roads" very little is known. 

 The only published section is the one given by the Rev. Thomas Brown 

 in his paper on the " Parallel Roads." This shows a thick substratum 

 of about 20 feet of sandy clay with angular debris and boulders, covered 

 by a thick seam of stratified clay, and then by 2 to 3 feet of clay with 

 stones. There is no appearance in this section nor in another un- 

 published section of Mr. Brown's, of any such structure as would 

 result from successive additions to the ledge by the tipping over of 

 debris removed from the shore. 



Although therefore the "roads" indicate a line of water-level, 

 there is nothing in their form or structure to show that they have 

 been formed by the long- continued action of lake waters on a shore 

 line. To what then are they to be ascribed ? 



The first or highest "road " is confined to Glen Gluoy, the second 

 and third to Glen Roy, and the fourth or lowest to Glen Roy and 

 Glen Spean. What the conditions were immediately antecedent to the 

 formation of the first, second, and fourth road, is not shown, but in the 

 case of the third road, the conditions preceding its formation are to be 

 traced uninterruptedly from the conclusion of No. 2 " Road." When 

 the lake stood at the level of "Road " No. 2, its waters escaped by the 

 col leading to Glen Spey, while when they stood at the level of No. 3 

 " Road," they escaped by the Glen Glaster Col. Now as there is a 

 difference of 76 feet between the height of the two cols, it is evident 

 that a barrier must have existed on the latter col during the time the 

 lake stood at the higher level. Whether the barrier was detrital or 

 ice-formed is immaterial for the argument. In both cases, either by 

 gradual weathering or melting, the time would come when the barrier 

 would be lowered in some place to the level of the lake. 



Now, it is well known to engineers that a breach once established in 

 a detrital barrier becomes so rapidly enlarged that, if not at once 

 stopped, nothing can stay the rapid destruction of the barrier, as, in 

 the case of the Holmfirth, Crinan, 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 2 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 



