34 NEW YORK STATE MUSEUM 
first mentioned by Smock *° who, in commenting on the Sterling and 
the Lake mines observed that the undulating foot walls, or the 
“rolls,” were remarkable features of the mines. They are not, 
however, confined to the mines, but are a common structural feature 
of the area in which the Sterling group of mines is situated, appear- 
ing as a series of cross-corrugations with short axial trend, spaced 
from crest to crest only a few yards, and whose axes vary in strike. 
They are irregular in distribution and are apparently due to cross- 
folding, as already suggested. 
Faulting. The Highlands area is complexly faulted, no less 
than three different periods of faulting being recorded. The earliest 
and the most obscure faults are Precambrian in age; these are com- 
pletely healed, and hence do not form zones of weakness or decay 
and have had but little influence in controlling the present topog- 
raphy, as they occur not only in the valleys but across the ridges 
as well. 
A later series of faults of distinctly Appalachian type may be 
much more easily observed because of the development of prominent 
crush zones, slickensides and other weaknesses. These faults follow 
the general strike of the structure and in some cases exhibit great 
displacement, especially the series or succession of faults which 
follows closely the northern and western borders of the Highlands 
ranges; particularly the one at the northern base of Storm King 
mountain and Breakneck ridge. ‘This fault is exposed a mile south- 
west of Cornwall station, where the walls show a fault-plane dipping 
steeply to the southeast, with the Storm King granite overthrust 
upon the Hudson River slates; the displacement is estimated as 2000 
feet or more.*° The strong overthrust tendency from the southeast, 
together with later block faulting and subsequent erosion, is like- 
wise responsible for the production of the isolated hills of gneiss 
surrounded by Cambro-Ordovician sediments, previously mentioned. 
Such are Bull hill and its neighbors, a few miles north of Monroe; 
Snake hill and Cronomer hill near Newburgh, Woodcock hill, and 
other smaller detached hills along the western margin of the High- 
lands area in Orange county. 
A third and still more recent period of deformation is repre- 
sented by block faults produced during Triassic times. The most 
prominent of these follows the west side of Peekskill creek to the 
85 Smock, J. C. First Report on the Iron Mines and Iron-ore Districts in 
the State of New York. N. Y. State Mus. Bul. 7, p. 16. 1880. 
86 Berkey, C. P. Structural and Stratigraphic Features of the Basal 
Gneisses of the Highlands. N. Y. State Mus. Bul. 107. 1907. 
