SURVEY OF THE ISLAND OF NEW-YORK. 
589 
northwest and southeast, happening to correspond exactly with the line of the grooves, or 
perhaps a medium of all those hitherto examined, which it will be seen vary but little from N 
45° W. It is proper to notice this fact here. The remaining half of the southern border of 
this great valley, or that from the 8th avenue on the west, to the 3d avenue on the east, lies, 
though somewhat irregularly, more nearly east and west. The northern border of this valley 
extends from the village of Manhattanville on the west, where the valley scarcely exceeds 
two hundred or three hundred yards in width, to the northern extremity of Harlem village. 
On the east side of the island, therefore, the Hudson and Mohawk valley extends from 110th- 
street to 133d-street, while on the western extreme it only extends to the width of about two 
blocks. The whole is a continuous bed of sand, gravel, pebbles, boulders, and other travelled 
materials, not unlike those of the southern extremity of the island. 
First Fortification. —We reach the first fortification at the distance of some two hundred 
or three hundred yards from the road, as it descends northward to Manhattanville, and at a 
distance of about two hundred yards south of the village, and one hundred feet above the 
waters of the Hudson, and seventy or eighty feet above the village. 
On the rock which forms the basis of this fort, and which is gneiss without much foreign 
matter, are abundance of drift grooves, but none very large or deep. These grooves cover 
the rock, which is uncovered two hundred feet north and south, and thirty to sixty east and 
west. The direction of the grooves is N 45° W, and the strike N 25° E. Dip 85° W. 
This fortification rock is from ten feet to forty feet above the adjoining grounds, being most 
elevated on the north. 
Second Fortification. —Proceeding eastward about two hundred or three hundred yards, 
we came to another, which we call the second fortification. It is ten or fifteen feet lower than 
the first. The basis rock (gneiss) is uncovered two hundred and fifty feet north and south, 
and from thirty to fifty feet east and west. Strike is N 25° E. Dip vertical. Grooves N 
45° W. The gneiss is very full of iron rust, apparently having resulted from the decompo¬ 
sition of pyrites. The grooves cover the whole surface of the rock, and are quite distinct, 
notwithstanding the long and continued action of the elements. 
Third Fortification {faced stone). —This is on almost the same level as the last, and in 
direction from it nearly southeast. There is a large surface of gneiss uncovered, the longest 
diameter being north and south two hundred feet. Strike N 25° E. Direction of grooves 
N 45° W. Dip W 80°. In this rock are tolerable specimens of adularia. 
In the valley between the third and fourth fortifications are grooves N 45° W. 
Fourth Fortification {entirely in ruins). —This stands on a promontory of gneiss, which pro¬ 
jects far into the valley, and lies still farther southeast of the others. It is on this line that 
the Croton aqueduct is to enter the city, crossing the valley a little east of the valley of Man¬ 
hattanville. Drift grooves are here seen in abundance, but have been much defaced by 
exposure. Direction N 45° W. Dip and strike as before. 
Proceeding S 45° E, or on a line with the southern border of the valley to its junction 
