1879.] 



of the Parallel Roads of Lochaber. 



17 



or more, and attains at one point where the carriage road crosses it, a 

 height of 752 feet, that of the lower Parallel Road just above it being 

 860 feet. 



The great barrier needed at the entrance of Glen Spean presents 

 more difficulty on account of the width and depth of the valley. The 

 river is here 200 feet above the sea level, and the length of barrier 

 required, 4 miles in extent. It is, however, precisely on this ground 

 that the great glaciers of Ben Nevis met the ice stream coming down 

 Strathspean. The ground rises, also, rather rapidly on both sides of 

 the river, and from behind the line of barrier marked by the Ordnance 

 Survey the rise is continuous to the summit of Unachan Hill, 613 feet 

 high. Now, this hill and the rising ground on the flanks of the valley 

 consist of a thick substratum of till or boulder clay, with a covering of 

 gravel, the latter formed in greater part, if not entirely, from the 

 destruction of the former, so that there is little doubt that the detrital 

 barrier here also was at one time much more important. Still, although 

 the detrital matter formed a considerable element, the author believes 

 that the great mass of ice constituted the essential element in the 

 barriers. 



The Till, although accumulated in larger masses in the above-named 

 sites, is found in places all up the valley, generally in the form of 

 terraces covered by gravel, as at Inverroy, Murlaggan, Inverlaire, and 

 elsewhere. There is one feature common throughout, namely, the 

 levelling and terracing of this glacial debris by subsequent water- 

 action, which could not have been effected in the still waters of a lake 

 bed, but probably took place on the bursting of the main barriers and 

 during the rapid outflow of the waters. To this cause may, also, be 

 owing the terraces of the Turret, Grulban, and others, for it is to be 

 remembered that the same glens which subsequently furnished the 

 streams, had previously furnished glaciers. Though these happened 

 to be at the mouth of streams to which their origin has been ascribed, 

 similar terraces occur elsewhere where there are no streams, and where 

 the structure is the same. The levelling of the original glacial 

 mounds having been effected at the time of the drainage of the lakes, 

 and having been then covered and masked by gravel, the terminal slopes 

 were either formed at the same time by the outpouring waters as they 

 fell to a lower level, or subsequently by wearing back by the present 

 streams. Some of the terraces ascribed by Chambers to sea margins 

 may be due to these causes, at the same time it is possible that some 

 of his lower terraces outside the Spean Valley, at levels of 70 to 

 200 feet, may be due to later marine action, and correspond with the 

 raised beaches of Jura and the coast further north, at levels extending 

 from 40 to 160 feet or more. 



An objection may occur to the hypothesis here suggested for 

 the formation of the Parallel Roads, in that, with elements so 



VOL. xxix. c 



