DRAINAGE SYSTEMS AND THE GLACIAL PERIOD. 241 



posits should be found at the same elevation on other 

 mountains than those surrounding Glen Roy. Their 

 absence elsewhere points, therefore, to some local cause, 

 which was readily suggested to the trained eye of one like 

 Agassiz, then fresh from the study of Alpine glaciers, 

 who saw that these beaches were formed upon the margin 

 of temporary lakes, held back during the Glacial period 

 (as the Merjelen See now is) by a glacier which came out 

 of one glen and projected itself directly across the course 

 of another, and thus obstructed its drainage. The glacier 

 of Glen Spean had pushed itself across Glen Roy, as the 

 great Aletsch Glacier in Switzerland now pushes itself 

 across the little valley behind the Eggishorn. 



