\2 NEW YORK STATE MUSEUM 



glacial out wash mainly in the low grounds where the ice border 

 drainage could not usually reach it. The extensive kame areas 

 in the Genesee valley and those named the Mendon, Irondequoit, 

 Victor and Junius [see title 20] are deposits formed at the ice edge 

 and not removed. 1 



The proglacial streams naturally swept along all the ice- 

 contributed and the ice-stream-contributed drift that came within 

 their grasp, and little evidences of such supply would be left along- 

 side the channels. The evidence which can be found is the consid- 

 erable proportion, in some delta masses, of rock materials of north- 

 ern origin, such as crystallines, Trenton limestone, Potsdam sand- 

 stone, etc. A few boulders and many cobbles of northward origin 

 are likely to be found in all deltas, as practically every stream 

 must have found some glacial material, either directly from the 

 ice, or from moraine deposits along the channel sides, or contrib- 

 uted by land wash from the southward. 



In the majority of pronounced deltas the greater part of the 

 mass consists of material derived from the excavation of the 

 stream channel by the corrasion of the stream itself. The amount 

 of preglacial and glacial deposits which the stream has removed 

 in any particular section before it could attack the rock strata 

 could hardly be estimated, but the amount of rock cutting is quite 

 clear. In the cases of the gorges and canyons, like the Gulf, Mar- 

 cellus and Railroad channels, the large volume of rock excavation 

 is evident, and it could be calculated approximately by measure- 

 ments. The deltas at the mouths of such gorges will have a com- 

 position like the wall rocks of the canyon, and a mass proportion- 

 ate to the channel excavation, provided of course that the delta 

 has not been eroded. Sometimes the large volume or hight of the 

 delta is the main evidence of large erosion by the stream, as in the 

 Cedarvale-South Onondaga delta, the channel being V-shaped on 

 account of weathering, and equivocal. Another interesting fact 

 is that narrow north and south valleys, like those at Jamesville 



: It should be understood that the kames (mounds of gravel and sand) are 

 essentially deltas in the manner of their origin, being formed at the mouths 

 of streams pouring out of the ice sheet. But they usually lack the form of 

 deltas because of the inconstant character of the stream channels, these 

 being walled in ice, and the shifting of the points of debouchment. Rarely 

 in this region the glacial deltas are broad and flat, as sand plains or glacial 

 outwash plains. One example has been described [p. 25] as the Shepard 

 Settlement plain, another at East Bethany. Genesee co., and yet another in 

 the same county near Darien station [see title 37]. 



