Dr. Mac Culloch on the Parallel Roads of Glen Roy. 351 



by the transportation of rubbish. Thus while it is engaged in deep- 

 ening its own bed it is also changing its horizontal place, under- 

 mining its bank on one side and in succession quitting it to attack 

 some other point ; while at the same time it is flattening a larger 

 portion of the plane on which it flows than the breadth of its own 

 water, and reducing it to one uniform broad space. Thus a terrace 

 with a profile is formed and left at a distance from the course of the 

 stream ; while a series of such capricious changes, acting through a 

 considerable space of time both horizontally and vertically, fills the 

 bottoms of the straths with terraces so numerous and complicated 

 as almost to bewilder us in attempting their explanation. Such 

 a cause may be conceived to have produced the lines in Glen 

 Roy. We must imagine the glen filled to the depth of its upper 

 level with alluvial matter, and suppose that a wasting stream has held 

 its course through it for a space of time so long as to remove the 

 whole matter to its present depth, leaving the lines which are now 

 marked on its sides, together with the terraces that are to be seen 

 at its upper end, as memorials of its destroying force. This is the 

 second hypothesis depending on the action of water which has been 

 offered in explanation of the appearances in Glen Roy. 



The third and last method of explaining these appearances by the 

 the same agent, is founded on the form assumed by the alluvial matter 

 which in many cases is found at the edges of a lake, and on the 

 probable consequences which would arise from draining it. On this 

 view it is conceived that a lake had existed at the uppermost level of 

 Glen Roy, for so long a period as to have accumulated on its margin 

 that alluvium which now forms the uppermost of the lines in question, 

 and that, by a subsequent sinking through two successive and similar 

 periods, the two lower ones had been formed in the same manner. 

 As the sinking of the waters must have been the consequence of the 



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