52 NEW YORK STATE MUSEUM 



and stagnate the movement of the ice which filled them. Hence it 

 came about that, on the melting of the ice, such east-west cross-valleys 

 were left with their lower ends " hanging " far up on the sides of 

 the much more deeply ice-eroded, north-south valleys to which they 

 are now tributary. In preglacial time these east-west tributaries no 

 doubt came into the main valleys at grade, that is, there was no de- 

 clivity, waterfall or gorge at their junctions with the main valleys, 

 because the preglacial weathering and stream-erosion processes of 

 valley-cutting proceeded concurrently and accordantly in the main 

 and tributary valleys, the smaller stream cutting down a narrow 

 valley as fast as the large stream cut down a broader one. More- 

 over, as will be noted by the cross-section of the hanging valley in 

 plate 7, this process of stream-valley development had proceeded to 

 a stage of maturity, that is, there had been sufficient time for weather- 

 ing to make the tributary valley wide open by wasting away the rock 

 of its sides. 



The present depth of the main valley below the level of the bottom 

 of the mature hanging valley gives, therefore, a measure of the 

 differential, glacial cutting-down of the north-south trough. The 

 visible portion of this amounts to 600 feet in depth and the rock 

 floor of the Onondaga trough is buried under loose drift of an 

 undetermined thickness, probably more than 100 feet. It may 

 therefore be safely reckoned that differential ice erosion lowered the 

 bottom of the north-south Onondaga valley by some 700 feet. 

 Plate 8 is introduced to illustrate the same process of hanging 

 valley creation in a region of existing glaciation, but this example 

 differs from the Onondaga occurrence in that both main and side 

 valleys were subjected to ice erosion, the former by a deep and broad 

 ice stream, the latter by a relatively thin and feeble one, hence the 

 difference in the extent of their erosion and the creation of the hang- 

 ing valley condition. 



Postglacial gorges. After the last withdrawal of the ice the 

 stream drainage of the present period was initiated. Where the 

 tributary streams encountered the glacially developed, precipitous 

 descents due to the oversteepened valley sides of the main trough, 

 waterfalls occurred. As postglacial time passed these waterfalls, 

 located on horizontally bedded stratified rocks of varying resistance 

 changed from an original single straight fall, first to a series of 

 step falls from one durable layer to the next and then, by recession 

 of their cascade crests, after the manner of waterfalls so conditioned, 

 there was cut a rock gorge in the lip of the hanging valley, as illus- 

 trated in plate 7. The occurrence of such rock gorges at the lower 



