GLACIAL LAKES 323 



he had been in his reasoning. Let us, then, first examine the facts, 

 and later their interpretation. The map of Fig. 350 will suffice 

 to set forth with sufficient clearness the course of the several 

 " roads." These " roads " are found in a number of glens tribu- 

 tary to Loch Lochy, and of the three neighboring valleys, Glen 

 Roy has three, Glen Glaster two, and Glen Spean one " road." 

 The facts of greatest significance in arriving at their interpretation 

 relate to their elevations with reference to the passes at the valley 

 heads, their abrupt terminations down-valleyward, and the mo- 

 rainic accumulations which are found where they terminate. The 

 single " road " of Glen Spean is found at an elevation of 898 

 feet, a height which corresponds to that of the pass or col at the 

 head of its valley and to the lowest of the " roads " in both Glens 

 Glaster and Roy. Similarly the upper of the two " roads " in 

 Glen Glaster is at the height of the pass at its head (1075 feet) 

 and corresponds in elevation to the middle one of the three " roads " 

 in Glen Roy. Lastly, the highest of the "roads " in Glen Roy is 

 found at an elevation of 1151 feet, the height of the col at the head 

 of the Glen. In the neighboring Glen Gloy is a still higher " road " 

 corresponding likewise in elevation to that of the pass through 

 which it connects with Glen Roy. 



To come now to the explanation of the " roads," it may be said 

 at the outset that they are, as Darwin supposed, beach terraces 

 cut by waves, not as he believed of the ocean, but of lakes which 

 once filled portions of the glens when glaciers proceeding from 

 Ben Nevis to the southwestward were blocking their lower por- 

 tions. The several episodes of this lake history will be clear from 

 a study of the three successive idealistic diagrams in Fig. 351. 



To derive the principles underlying this history, it is at once 

 seen that all changes are initiated by the retirement of the ice front 

 to such a point that it 'unblocks for the waters of a lake an outlet that 

 is lower than the one in service at the time. This is the principle 

 which explains nearly all episodes of glacial lake history. Thus, 

 when the ice front had retired so as to open direct connections 

 between Glen Roy and Glen Glaster, the col at the head of Glen 

 Roy was abandoned as an outlet, and the waters fell to the level 

 fixed for Glen Glaster. A still further retirement at last opened 

 direct connection between Glen Glaster and Glen Spean, so that 

 the lake common to Glens Glaster and Roy fell to the level of the 



