1902. 



Wright. — The Glacial Origin of Gleiidoo. 



97 



upper part of the fall being cut back faster than the lower. 

 The total drop of the stream at this point must be 20 feet or 

 more. 



In considering the bearing of these facts, it occurred to the 

 writer that the most natural deduction was that the cutting of 

 the pass was due, not to the streams which flow in it at present, 

 bur to some other agencj^ which acted before the streams had 

 time to effect any erosion, and that the waterfall up the lateral 



stream had been caused originally by its flowing over the edge 

 of the pass, and had since retreated by backward cutting into 

 its present position. 



At the close of the Glacial Period such an agency was 

 undoubtedly in existence in the form of the marginal waters 

 of the ice sheet. How the phenomenon takes place has been 

 made very clear by recent observations in Greenland. Cases 

 have been described as of very frequent occurrence, where 



