NIAGARA FALLS AND VICINITY 



37 



sidence. The present surface of the Niagara plateau is therefore 

 not to be considered as identical with the old surface of deposition, 

 but as due to prolonged peneplanation, or erosion to near sealevel, 

 completed probably toward the close of Mesozoic or the beginning 

 of Cenozoic time. The following diagram (fig. 3) will illustrate the 

 relation between the strata and the surface of the land at 1) the close 

 of Siluric time, 2) late Mesozoic or early Cenozoic time, after the 

 completion of the last cycle of erosion and the reduction of the land 

 to peneplain condition, and 3) the present surface. 



Fig. 3 Diagram of ancient Paleozoic coastal plain, and its relation to the Mesozoic peneplain sur- 

 face and the present land surface. The numbering of the beds corresponds to that of the table. 



Between the close of the Siluric and beginning of Mesozoic time 



a long period intervened, during which this region was at first a 



land surface, suffering considerable erosion, but later was resub- 



merged, and covered with extensive deposits of Devonic limestones, 



shales and sandstones. The final emergence took place at the close 



of Paleozoic time, the succeeding Mesozoic time being in this region 



probably an uninterrupted period of erosion, during which the land 



suffered the combined attacks of the atmosphere and of running 



water. 



Development of the drainage features 



The water which falls as rain or snow on the land either evapor- 

 ates, runs off on the surface, or sinks into the ground, where it con- 

 stitutes the ground water. That which evaporates, accomplishes lit- 

 tle or no direct geologic work, but both the surface and under- 

 ground waters are important geologic agents. If the surface on 

 which the water falls is a perfectly smooth but inclined plain, the 

 water will run off in the form of a thin sheet. A perfectly smooth 



