Dr. Mac Culloch on the Parallel Roads of Glen Roy. 385 



boundary into the Spey, and therefore that their course westward, to 

 which the removal of this barrier must be attributed, could not then 

 have taken place. Independently of this consideration I showed 

 that the difficulties of accounting for the failure of so many barriers 

 were increased in an unnecessary degree. It is therefore perhaps a 

 more reasonable supposition, that the barrier which dammed the 

 lake of Glen Roy to the east, existed beyond the point at which the 

 waters of Loch Laggan, if now elevated above their eastern boundary, 

 would fall into the Spey ; a point situated near Dalchully. Of the 

 nature of this boundary, as well as of the causes by which it has 

 been removed, it would be fruitless to speculate further. It is 

 more important to point out the magnitude of the change itself. 



The depression of the Spey at this point below the uppermost 

 line of Glen Roy, may be estimated without material error at 400 

 feet or more, and the valley here puts on the form of a flat strath 

 bounded on each hand by high rocky mountains ; varying from a 

 mile to half a mile in breadth. The necessary altitude of the ob- 

 struction may hence be readily computed, and it is equally obvious 

 that however much during the course of the Spey to the sea the 

 breadth of this imaginary barrier may vary, according to the position 

 in which we place it, its altitude must be constantly increasing 

 during the descent of a river so rapid. Variations in its supposed 

 position will produce correspondent effects on the spaces which 

 must have been inundated during this state of things. Thus its 

 removal a few miles lower would cause the supposed lake to fill 

 the valley of the Truim to a considerable height above Dalwhinnie. 

 But it is unnecessary to pursue these consequences further in this 

 direction. It is proper however, although we cannot assign the 

 real causes which may have produced the breaking down of this 

 barrier, to show that independently of the general reasons assigned 



