NIAGARA FALLS AND VICINITY 47 



glacial Genesee river is the only other that can be mentioned here. 

 Though now flowing northward on account of the tilting of the 

 land, we may assume that much of its valley was carved by a south- 

 ward flowing stream, the bottom of which, as shown by borings, 

 was considerably below the floor of the present river. Whether 

 Irondequoit bay is a part of this ancient channel, or whether it 

 marks the position of an obsequent stream, must remain for the 

 present an open question. Soundings in Irondequoit bay show a 

 depth of 70 feet, though the rock bottom is probably much deeper. 

 As soon as the consequent streams began cutting down their 

 valleys again after the continental uplift which followed the period 

 of peneplanation, the lateral subsequent streams began once more to 

 open out broad lowlands in the weaker beds which now had become 

 extensively exposed. These lowlands, in part now filled by drift 

 deposits, are the Ontario and Georgian bay valleys, the latter con- 

 tinued in the North Passage, all carved out of the weak Medina and 

 Lorraine shales; the Tonawanda-Chippewa valley, with the deeper 

 portion of the Huron valley farther west, carved out of the soft 

 shales of the Salina group; and the valley of Lake Erie cut out of 

 the softer middle and upper Devonic shales. A few of these may 

 be considered in greater detail. 



Ontario valley. It is a well known fact that Lake Ontario is deeper 

 in its eastern than its western part. In the following six cross- 

 sections (fig. 7), constructed from the lake survey charts, the greatest 

 depths from west to east are 456, 528, 570, 738, 684 and 576 feet. 

 The section showing the greatest depth is that from Pultneyville 

 to Point Peter light, in the eastern third of the lake. As the present 

 level of Lake Ontario is 247 feet above the sea, the deepest sounding 

 recorded in these sections is 491 feet below present sealevel. From 

 this point of greatest depth, the floor of the lake rises eastward, 

 at first at the rate of 3 feet in the mile, and later at an average rate 

 of 9 feet a mile. The valley appears to be continued south of the 

 Adirondacks in New York along the present course of the Mohawk 

 river, which flows at present several hundred feet above the rocky 

 floor of the valley.^ This floor ascends eastward, till at Littlefalls 



Carll. Pa. geol. sur. V-.s^S- 



