GLACIAL WATERS IN CENTRAL NEW YORK 43 



and Manlius, were entirely filled with delta rubbish so that the 

 streams swept the later detritus clear across a valley into an east- 

 ward basin. In such case the delta may represent the cutting of 

 two or more channels on the west. 



The character and size of the delta depends not only on the 

 amount of stream erosion but also on the nature of the excavated 

 rock. To some extent limestone is removed in solution and the 

 shales are largely reduced to so minute division that it is borne 

 far away. The rock strata encountered by the proglacial streams 

 described in this paper do not include any sandstones. The higher 

 channels, stratigraphically, are in the Marcellus shale; the middle 

 ones in the Onondaga and upper Salina limestones; the lower ones 

 in the lower Salina (Camillus and Vernon) shales. The deltas are 

 found to have pronounced characteristics, dependent on the nature 

 of the materials. Examples will be cited later. 



(B) The building of a delta at all, and the form and extent of 

 the deposit, depended somewhat on the topography or form of the 

 receiving basin. When the western wall of the basin was a gentle 

 slope the condition was favorable to the construction of broad delta 

 plains, even of finer material, and for their preservation, like those in 

 the Genesee valley. When, on the other hand, the receiving basin 

 had a steep western wall, like the valleys east of Syracuse, this was 

 unfavorable to lodgment of the detritus near the lake level, specially 

 of the finer material, and a moderate volume of material might leave 

 no visible delta. 



If in addition to the steepness of the wall the valley is narrow and 

 V-shaped, like those at Jamesville and in the Chittenango-Oneida 

 district, the deposits even if large in volume may be subsequently 

 removed. 



(C) The changes to which the deltas have been subjected since 

 their formation are: (i) weathering; (2) storm-wash; and (3) 

 stream corrasion. Weathering and storm-wash have little effect on 

 deposits of gravel or sand, and are not rapidly severe on calcareous 

 material. It is surprising how quartzose sand deposits, beaches or 

 deltas, have preserved their perfection of form through all the 

 vicissitudes to which they have been exposed in the thousands of 

 years since they were made. 



If, however, the deposit is shale rubbish and therefore clayey, or 

 even with considerable admixture of silt, then it may suffer decom- 

 position and erosion. The delta in the Oneida valley is an excellent 

 illustration, and the one northwest of Geneva one of less degree. 



