TOPOGRAPHY. 69 



south from the brow of the escarpments, where in each case hmestone 

 forms the surface rock. 



The surface rock of the lowest plaiu is the soft shale of the Medina 

 gro^^p, and this formation extends northward beneath the lake. There is a 

 rise of 100 to 175 feet in passing southward across it from the lake shore 

 to the base of the escarpment. The plain is generally so smooth that its 

 slopes are barely detectible by the eye, but instrumental determinations 

 show that it has troughs and ridges of sufficient variation in altitude to con- 

 trol the drainage. This plain and the escarpment bordering it were made a 

 subject of special study by Gt-ilbert in 1898, and some results of the study, 

 presented to the Geological Society of America, have recently appeared.^ 

 Gilbert found that the courses of drainage are controlled by shallow 

 troughs which have a trend from northeast to southwest, in hannony with 

 the general direction of ice movement, as shown by strise. The depth of 

 the troughs is not definitely known, since they are usually drift filled to 

 such a degree that surface wells do not penetrate the filling . It is found, 

 however, that some of the troughs exceeded 40 feet in depth. Gilbert 

 considers it highly probable that this furrowing of the surface is the 

 result of glacial erosion. He thinks that the erosion much exceeds the 

 amount represented by these channels, for it appears to have obliterated 

 the preglacial topography, which presumably included a system of shallow 

 valleys descending northward with the general slope of the country. He 

 estimates that the general reduction of the surface must have been at least 

 40 to 50 feet, and it maj'' have been considerably greater. It is, liowever, 

 his opinion that the Niagara escarpment antedated the period of glacial 

 sculpture, though the ice erosion rendered it more prominent by excavation 

 along its base and by the general degradatioA of the lowland it overlooks. 



This plain was nearly covered by the glacial Lake Iroquois, the 

 Iroquois beach being situated either along the base of the escarpment or 

 within a mile or so north of it. The lake has removed to a perceptible 

 degree the fine material from the higher portions of the plain and silted up 

 the hollows with its sediment, thus producing an important subduing 

 influence on the topography. That it has degraded the surface of the ridges 

 is shown by the large amount of coarse pebblj^ material which remains as 

 a residue on the higher or more exposed parts of the lake plain. 



'Glacial sculpture in western New York, by G. K. Gilbert: Bull. Geol. Soc. America, Vol. X, 1899, 

 pp. 121-130. 



