44 NEW YORK STATE MUSEUM 



Deltas of coarse limestone rubbish, like those in the Butternut and 

 Limestone valleys [see pi. 26, 31], may have accumulated a soil, 

 which along with the frost work on the boulders, make the irregular 

 and perhaps eroded surface resemble moraine. (It should be remem- 

 bered that all these deposits have been covered with forests, which 

 facilitated decay and soil production.) 



The chief factor in the modification of the deltas has been corrasion 

 by the later proglacial streams and their successors, the valley 

 streams. As described earlier in this writing, the deltas of coarse 

 limestone rubbish which once must have filled the Butternut valley 

 at Jamesville, have been largely removed and the material swept 

 down the creek (northward) to form extensive deposits on the edge 

 of the Oneida lowland. In the cases of the narrow valleys on the 

 east the same sort of work has been done in even greater degree. 

 The excavation of the deltas was at first by the later flow of the 

 same streams which had built the deltas, as the ice barrier receded 

 and the base levels of the streams were lowered, and since then con- 

 tinuously by the existing valley streams. The lake waters in the 

 steep-walled valleys had a minor destructive effect . as the agitation of 

 the lowering waters tended to shift the detritus down the steep slopes. 



With the principles of formation and destruction of the proglacial 

 stream deltas before us, as outlined above, we will now review the 

 several delta tracts in order from west to east. 



Description 



Genesee valley. The deltas are extensive in the Genesee valley, 

 though the mapping, plate 2, is somewhat hypothetical as the limits 

 are difficult to determine, even on the ground. These deltas are too 

 broad to be conspicuous in appearance, and they blend into the 

 uplands. At a few points their delta structure, specially the foreset 

 beds, are grandly shown, as in the great gravel excavations at 

 Scottsville, Canawaugus and intermediate pits [see pi. 7-9]. The 

 gravels are a mixture from several sources, but those at Scottsville 

 have a preponderance of Salina material, in which the correlating 

 channels are cut, and are consequently of poor quality for road 

 metal though they answer better for railroad ballast. The lime- 

 stones were not deeply cut in the district west of the Genesee, though 

 a few short gorges with cascades lie north of Leroy, and the delta 

 material is not very coarse. The width of the Genesee valley is so 

 great and the west wall so gently sloping that the delta deposits 

 found favorable conditions for building and for preservation. 





