440 J. D. Dana— On Southern Westchester County 
5. ORIGIN OF VARIOUS TOPOGRAPHICAL FEATURES OF NEW YORK ISLAND. 
Valleys and Low Areas.—From the distribution of the lime- 
stone, as exhibited on the map, and the fact of its easy wear or 
erosion, we derive explanations of several topographical features 
of New York Island and the adjoining region. For example 
we learn— 
Why Harlem river has its present position and depth, and its 
north and south course; Why there is an “‘ Highth Avenue val- 
ley;” Why the “Inwood parade grounds” are a broad rolling 
region from the Harlem to the King’s Bridge Road ; Why, south 
of the Inwood Presbyterian Church, there was a King’s Bridge 
Road valley, to fix the position of that old highway; Why 
Sherman’s Creek bends around the Fort George heights; Why 
Cromwell’s Creek exists and the valley or “Clove” to the 
north; Why Fleetwood Park is low and nearly flat, except its 
western side; Why Third Avenue in Harlem and the region 
east of it is low; Why wide flats (with small exceptions), extend 
from East River more than two-thirds of the way across the 
island, just north of Central Park; and, perhaps, why there is 
an Kast River channel. 
The limestone lands that are not low may owe their height 
to the fact that erosion follows water courses; but, besides, the 
rock when in nearly vertical beds—usually the fact in such 
places—is generally of a firmer kind, because the pressure 
which gave the beds this position, served to compact the rock 
and so favored closer and better consolidation. 
Trends of Ledges and City Avenues.—We also find a good 
reason for the precise direction given by the city surveyors to 
the New York avenues—it being the mean direction of the 
strike or direction of the bedding in the gneiss, and thence of 
the rocky ledges of the island. Many parts of the avenues in the 
northern half of the island have now a low evén wall on one 
side or the other,.made by a flat and nearly vertical cleavage- 
surface of the schist.* . 
The Manhattanville Valley.—The Manhattanville Valley (or 
Manhattanville and Harlem, as called by Dr. Gale), which cuts 
across the island obliquely from Manhattanville on the Hud- 
son in a nearly southeastward direction, is one of its most ex- 
traordinary geological and topographical features, as mentioned 
on page 427. It reached Kast River, in a broad creek and 
marsh south of 108th Street; and so low lay the surface along 
it from one side of the island to the other, that in 1826 a canal 
was projected that should here connect Harlem River with the 
Hudson ; and the canal was so far constructed that a celebra- 
tion took place of the completion of the first loc 
* The avenues of New York run N. 28° 504’ E._ This parallelism between the 
strike of the beds and the avenues is referred to by Dr. Gale. 
