APPENDIX. 907 



On each side of this long, hollow, deep valley, which is bounded by 

 dark and lofty mountains, and at a great elevation, three strong lines 

 are seen, parallel to each other and to the horizon ; the levels of the 

 opposite ones coinciding precisely with each other : and so striking is 

 this symmetrical character, that the observer can with difficulty divest 

 himself of the idea that he is contemplating some cyclopean work of 

 the olden times. A slight examination of the nature of these parallel 

 terraces is, however, sufficient to convince the uninstructed observer, 

 that they are nothing more than the shores of an ancient lake, fed 

 from the neighbouring Alpine regions, which at distant periods be- 

 came shallower, and at length entirely disappeared, from the erosion 

 of the barrier which formerly confined its waters.* The following 

 explanation of the phenomenon is from a paper that has recently 

 appeared; and corroborates the opinions of Professor Playfair and 

 Dr. Macculloch : f — 



The parallel shelves or terraces of Lochaber consist generally of 

 bared rocks, forming sloping channels or water-courses ; and they bear 

 no accumulations of littoral deposits or detritus. They are perfectly 

 horizontal, and are all coincident with some summit level, so as to 

 admit of the water flowing over that level as over a lip. Thus the 

 uppermost shelf of Glen Gluoy is exactly coincident with the water- 

 shed ridge which divides that glen from Glen Roy ; so that the waters 

 which stood at that height must have flowed out at the head cf Glen 

 Gluoy into Glen Roy. In like manner the uppermost terrace in Glen 

 Roy is coincident with the water-shed ridge dividing Glen Roy from 

 the valley of the Spey : the waters which stood in Glen Roy, at the 

 second level, must therefore have flowed over the head of the glen 

 into Spey-Yalley. And the middle terrace of Glen Roy coincides with 

 a water-shed at the head of Glen Glaster. Ancient river-courses may 

 be traced leading from the different levels of the terraces into the 

 neighbouring glens and valleys of lower levels ; and it seems evident 

 that the waters which formed the several terraces flowed out of the 

 glens, and descended by river-courses into the low countries. Thus 

 the waters which formed the terrace in Glen Gluoy descended nearly 

 thirty feet by flowing into Glen Roy ; those of the upper shelf in Glen 

 Roy flowed in like manner into the valley of the Spey ; those of the 

 middle terrace were discharged over the head of Glen Glaster down a 

 slope of 212 feet in vertical height into Glen Spean ; and the waters 

 that produced the terrace or shelf in Glen Spean issued out of Lake 

 Loggan by the ancient river-course at Mukkul. 



It appears, therefore, that barriers originally existed, which pent up 

 the waters at different levels in the glens, and were lowered at inter- 

 vals ; till at length the lakes were dried up, from the waters sinking 

 from the level of the highest shelf to the next ; and thus, by successive 



* See Dr. Macculloch, On the Parallel Roads of Glen Roy, Geological Transactions, 

 vol. iv. p. 314. 



t On the Parallel Roads of Lochaber ; by David Milne, Esq., Edinburgh Philo- 

 sophicalJournal, October 1847. 



