NIAGARA FALLS AND VICINITY 29 



For many miles to the east and west of the Niagara river the 

 plain does not change perceptibly in elevation. Nevertheless, there 

 is a gradual eastward descent, till, on the Genesee river, the surface 

 of the plain, where not modified by superficial deposits, is fully a 

 hundred feet lower than at Niagara. Westward the plain rises 

 gradually, its elevation near Hamilton averaging 500 feet above 

 Lake Ontario, or considerably more than 700 feet above the sea. 



Owing to the southward inclination of the strata of this region, 

 the limestone bed which forms the capping rock at the escarpment, 

 eventually passes below the level of the plain, having previously in- 

 creased in thickness to over 200 feet. The disappearance of the 

 limestone as a surface rock occurs near the northern end of Grand 

 island, as shown by the accompanying geologic map, and from this 

 point southward the surface rock is formed by the soft gypsiferous 

 and salt-bearing shales of the Salina group, which overlie the lime- 

 stone and in turn pass below the higher strata in Buffalo, where 

 beds of limestone again become the surface rock. Throughout the 

 area where the shales form the surface rock, the plain is deeply ex- 

 cavated on both sides of the Niagara river, a longitudinal east and 

 west valley, now largely filled by surface accumulations of sand and 

 gravel, being revealed by borings. Tonawanda creek occupies this 

 valley on the east, though flowing on drift, considerably above its 

 floor, and Chippewa creek occupies it in part on the west of the Ni- 

 agara river. This valley, as will be shown later, can be traced west- 

 ward into Canada and eastward to where it joins the Mohawk valley, 

 with which it forms the great avenue of communication across the 

 state of New York. The northern boundary of the Tonawanda and 

 Chippewa valleys is formed by a limestone cliff similar to, though 

 less pronounced than, the Niagara escarpment. This cliff, gen- 

 erally known as the second limestone terrace of western New York 

 (the Niagara escarpment being the first), is formed by the upper 

 Siluric limestones (Waterlime and Manlius limestone) and the Onon- 

 daga limestone of the Devonic series. The latter is a very durable 

 rock and hence it forms a very resistant capping stone. This es- 

 carpment is scarcely visible at Black Rock, where it is crossed by the 

 Niagara river, for here it is low, and, in addition, extensive drift 



