442 J.D. Dana—On Southern Westchester County, etc. 
The valley is supposed by Dr. Gale to have had a glacial 
origin, since it follows quite closely the course of the glacial 
grooves over the island. But the rocks point to its earlier 
existence and a profounder cause. 
The general direction or strike of the beds of schist on the 
west side of 8th Avenue is about N. 29° E.—the dip being 
nearly vertical. But near this Manhattanville Valley, the 
strike, while normal to the north of it, to the south diverges 
eastern part of the Park and north of it. Whatever be true 
as to the nature of the flexure, the facts support the idea of 
a wrenching in the schists of the vicinity ; for these variations 
in pitch are not found south of the Manhattanville Valley.* 
Finally, we may conclude that the pre-determinations of the 
fundamental features of New York Island date back to the 
era of the Lower Silurian, and to the epoch of mountain-making 
at its close. No other rocks that now remain have been added 
by subsequent geological operations excepting the loose or 
* If the occurrence of limestone and serpentine on 157th Street near 10th 
Avenue, mentioned by Dr. Gale (see page 423)should be verified, it would give 
some support to the view that the western band of limestone of Area No. 3 is 
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