CONCLUSIONS CONCERNING HARLEM RIVER 179 



A line of soundings to rock was made across the river, starting from a 

 point between Fifty-ninth and Sixtieth streets, with results which are 

 set forth in figure 24.* Mr Charles Macdonald, who made the borings 

 for this section, informed the writer that the apparatus used was not 

 sufficiently accurate to determine the profile all the way across the river. 

 The figure, which discloses the results of wash borings, records " rock 

 or boulder" at the bottom, inasmuch as the drill did not enter the 

 obstruction. Mr Macdonald has furnished the additional information 

 to that afforded by the section, that at a point 2,000 feet east of the 

 bulkhead line on the New Jerse}' shore, or near the middle of the river, 

 rock was found at a depth of 300 feet below mean tide. Before reach- 

 ing the rock the boring tool passed through 240 feet of silt and sand. 

 By reason of the very meager information concerning the bottom of the 

 Hudson river, this section is of very considerable interest. 



Conclusions respecting the Origin of the Channels 

 harlem river 



From the above it appears that little correspondence between the di- 

 rections of belts of limestone or dolomite and of the New York water 

 front can be established, except within the stretch between Kings bridge 

 and McCombs Dam bridge, where the observed facts point to the occur- 

 rence of a narrow strip of limestone dropped down between nearly ver- 

 tical faults. In other portions of its course the Harlem river flows over 

 limestone or gneiss, but in all instances in a direction transverse to the 

 strike and to the probable longer axes of the rock areas. The two reefs 

 of gneiss over which the stream flows are located at McCombs dam and 

 between Third and Fourth avenues. At the first-mentioned locality 

 the backbone of the reef lies at a very moderate depth below the surface, 

 from which in both directions the slope plunges awaj^ to a very consid- 

 erable depth ; as shown by the fact that the piles beneath the bridge of 

 the New York Central and Hudson River Railroad Company over Crom- 

 wells creek were driven to a depth of 120 feet without meeting rock. 



The Harlem River sections, which are furnished by the numerous 

 bridges across it, show clearly that it is not a simple erosion valley result- 

 ing from the cutting of the stream. In the north-south and north-north- 

 west south-southeast stretches of the river the rock banks are generally 

 not in evidence, and even when they are they do not always correspond 

 in position with the present banks. The bed of the stream appears to 

 be not a uniform decline in a single direction, or made up of slopes in 



♦Specifications for a suspension bridge over the Hudson river at New York. Engineering News, 

 vol. xxxiii, March 7, 1895, pp. 159-160. (Figure.) 



