140 
GEOLOGY OF THE FIRST DISTRICT. 
the whole valley is comparatively shallow, inasmuch as the rock appears from ten to twenty 
feet above the surface, both to the north and south of the valley, and immediately on its 
borders. 
“ The extreme eastern portions, and especially the southeastern parts, are a salt marsh, 
which, along the borders of Harlem creek, approaches the Third avenue ; but as few, if any, 
excavations have been made in it, little can be said of its geology. 
“ Besides the above mentioned alluvial beds, others are found in different portions of the 
island, but they are so inconsiderable as to require no particular description. 
“ There is, however, a prolongation of the Harlem and Manhattanville valley, which extends 
northward, and embracing the Eighth avenue, till it strikes the Harlem river at McComb’s 
dam. It consists generally of sand and diluvial loam, and is almost a perfect level. Few 
boulders are to be seen on its surface, except on its northernmost part, where they are found 
in great abundance.”* 
The clay beds at Albany are more than one hundred feet thick in many places ; and between 
that city and Schenectady, sand thirty to fifty feet thick overlies the clay. 
At Troy, the gravel and sand beds overlie the clay to a depth of thirty to fifty feet. This 
may be seen at the south end of the town, where the land-slide of the first of January, 1837, 
occurred, by which several lives were lost, and some buildings destroyed. The top of these 
gravel and sand beds is judged to be about one hundred and fifty to one hundred and eighty, 
possibly two hundred feet above the Hudson; and the thickness must here be at least one 
hundred and fifty, probably two hundred feet.f 
At West-Point, the top of the gravel beds is one hundred and eighty-eight feet above tide¬ 
water ; and the main terraces of gravel through the Highlands, on both banks of the Hudson, 
is supposed to be about the same height. 
The sand beds in some parts of the southern portion of the island of New-York, are sup¬ 
posed to belong to the quaternary ; but in some places the excavations are through drift, and 
the subjacent tertiary beds, before reaching the gneiss rocks, on which all the superior deposits 
rest. 
At Roanoke point on Long island, and for two or three miles east, the shore is high, and 
composed of clay and sand of the quaternary above the layer containing the boulders ; and 
beds of sand, and clay of the tertiary era, underlie also this drift deposit. 
The terraces of the quaternary are not well developed in the immediate valley of the Hud¬ 
son, except in some few localities; but in the tributary valleys, which seem to have been 
* Prof. Gale’s Report to W. W. Mather, on the Geology of New-York island. Vide New-York Geological Report, 183G, pp. 
187, 190. 
I Mr. Finch mentions fossil wood as abundant in the clay marl at Troy. (Vide Silliman’s Journal, Vol. 10, p. 229.) This is 
presumed to be derived from Mr. Hale’s account of the geology of Troy, in which “ logs of wood, and other vegetable substances 
in a tolerable state of preservation,” were stated to he found at about twenty-eight feet below the town; but this is in the alluvion. 
(Vide Silliman’s Journal, Vol. 3, p. 73.) 
