1879.] 



of the Parallel Roads of Lochaber. 



19 



escape of the waters was consequently more prolonged, and the fall 

 slower. 



Various phenomena in connexion with the great ice-sheet in 

 Lochaber and their connexion with the general question, are next 

 considered. The author objects to the term of moraine profonde to 

 describe the drift of sub-glacial origin, as apt to lead to misunder- 

 standing, although Hogard and other geologists have used it in the 

 wider sense. It is evident that the old ice-sheet acted under very 

 different conditions to an ordinary glacier, and it is better to use such 

 terms as moraine detritus or sub-glacial detritus for the sum total of 

 sub-glacial products of the former, than employ a term which was 

 originally and still is generally restricted to a single and comparatively 

 small product of the former. 



Besides this sub-glacial debris, there is the large quantity of debris 

 that must have been scattered over the surface of the ice during its 

 melting by streams and rivulets, or spread out in the temporary lakes 

 which were formed at all levels, and may have given rise in many 

 instances to sand and gravel terraces of variable extent. But though 

 true beaches are formed in certain glacial lakes, for ledges or 

 shelves, such as constitute the Parallel Roads to be formed, a number 

 of conditions must have concurred — such as sufficient slopes, a detrital 

 covering, barriers at the mouth of the glens, and cols of escape at their 

 upper end. 



As the barrier ridges on the old ice-sheet melted, or burst, the 

 waters escaped to lower levels, carrying with them on or beneath 

 the ice, a large portion of the surface detritus. Formed at all levels 

 up to 2,000 feet or more, these glacial lake waters in descending to 

 lower levels met with other large bodies of water, and the transporting 

 forces increased in power till the last stage was reached and open 

 channels formed in the distant plains, leaving as marks of their 

 passage down the valleys — here great banks of gravel — there deep 

 beds of sand, according to the distance from the point of outburst. To 

 these floods, combined with river inundations and the modifications 

 since brought about by fluviatile action, are due the various forms of 

 escars, terraces, and other less defined detrital accumulations. 



Conclusions. 



The general conclusions drawn by the author from the phenomena 

 in Lochaber and surrounding district, are : — 



1st. That at the period of the first great glaciation of Scotland, 

 the ice-sheet in Lochaber attained a thickness of not less than 2,000 

 to 2,500 feet, but that in consequence of the peculiar physiographical 

 conditions of the district, the large ice-currents from Ben Xevis so 

 clashed with others in the Spean and Lochy Valleys, that a block 



C 2 



