W ATKINS GLEN 393 



the continental glacier buried this land beneath ice, which moved with 

 especial freedom through the north-south valleys, scouring them and 

 leaving them both broader and deeper. The amount of deepening by 

 this ice invasion was at least five hundred feet, and probably much 

 more, the exact amount being impossible of determination at present, 

 since much of the evidence was erased by later ice erosion. Moreover, 

 the first ice advance may have left lakes in the valleys, whose surfaces 

 acted as temporary base-levels, below which the interglacial gorges 

 could not be cut. 



With the recession of this ice sheet, the upland tributaries were 

 left hanging five hundred feet or more above the overdeepened main- 

 valley bottom, and the streams descending the steepened main-valley 

 slope began to cut gorges in it. This condition lasted through inter- 

 glacial times and resulted in the production of fairly broad and deep 

 gorges. Then came a readvance of the ice, which again broadened 

 and deepened the valleys and on its recession left the interglacial gorges, 

 partly buried and erased, hanging high above the newly made bottom 

 of the main valley. Since the retreat of the Wisconsin ice sheet the 

 streams have been engaged again in gorge cutting on the steepened 

 slope, in some places along the lines of the older gorges, but more 

 commonly partly or completely independent of them. 



There are some facts which indicate possible greater complexity 

 of ice erosion, for in some of the valleys there is apparently more 

 than one buried gorge; but the evidence on this point is not as yet 

 convincing, and for the present we can point with certainty to no 

 greater complexity than that of two periods, one the Wisconsin, the 

 other of some one of the earlier ice advances with which the work of 

 the glacial geologists of the Mississippi valley have made us familiar. 



The various glens of the Cayuga and Seneca Lake valleys, whose 

 general cause is as above stated, differ greatly in detail. They are all 

 wild and picturesque, and they are all narrow gorges with many 

 cascades and waterfalls. Their variations depend upon the varying 

 combinations of effects from several causes. One of these is the in- 

 fluence of the buried gorges. Wherever the postglacial stream enters 

 one of these its valley abruptly broadens. Where the postglacial course 

 coincides with the buried gorge the expansion is continuous ; but where 

 it merely crosses the older gorge, the narrow rock- walled and ro k- 

 bottomed postglacial gorge is replaced by an expansion, forming an 

 ' amphitheater ' with drift walls and bottom. The valley again con- 

 tracts where the stream leaves the buried gorge and has cut a post- 

 glacial glen in the rock of the other wall of the older gorge. 



A second cause for differences in the gorges is the influence of the 

 variation in resistance to erosion of the nearly horizontal strata of 

 Devonian shales and sandstones in which the gorges arc cut. The 



