THEORIES OF ORIGIN 153 



of the subjacent rock, and not a few giving practically complete sections 

 across it. 



The present is, then, an especially favorable time to study the geology 

 of the channels, and it is of great importance that observations be now 

 made and recorded, lest the opportunity be forever lost. 



Theories of Origin of the New York Water Front 



The unexcelled harbor facilities of New York city have furnished a 

 fascinating subject for study, and writings of geologists on the structure 

 of Manhattan island have devoted considerable space to it. As is well 

 known, the water-courses surrounding the island form sharply incised 

 tideways of quite exceptional depth. Stevens accounted for the location 

 of the lower Hudson, the Harlem, and the East rivers both by the posi- 

 tion of supposed belts of limestone and by longitudinal faults along the 

 channels.* 



Dana ascribed their formation entirely to the presence of supposed 

 limestone belts, and formulated the theory which has since formed the 

 subject of many papers ; all, so far as is known, in support of the theory, 

 although comparatively little evidence has been adduced for it. Dana 

 stated his views as follows : f 



"From the distribution of the limestone, as exhibited on the map, and the fact 

 of its easy wear or erosion, we derive explanations of several topographic 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 ' Eighth Avenue valley ; ' why the ' Inwood parade grounds ' 

 are a broad rolling region from the Harlem to the Kings Bridge road ; why, south 

 of the Inwood Presbyterian church, there was a Kings Bridge road valley, to fix 

 the position of that old highway; why Shermans creek bends around the Fort 

 George heights; why Cromwells 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 East River 

 channel." 



An objection to the full acceptance of his theory led him to add : 



"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." 



♦History of the geology of New York island Ann. Lye. Nat. Hist. N. Y., vol. viii, 1865, pp. 108-120. 



t J. D. Dana: Geological relations of the limestone belts of Westchester county, New York, 

 southern Westchester county, and northern New York island. Am. Jour. Sci. (3), vol. xxi, 1881, pp, 

 26-443; also ibid., vol. xxii, 1881, pp. 313-315. 



