34 ROCHESTER ACADEMY OF SCIENCE. [Oct. 8, 



thick drift sheet which buries the region is of very heterogeneous 

 composition and admirably adapted for sustaining vegetable growth. 



At the beginning of the Glacial period the northern part of the 

 continent was much above the present height in relation to the sea. 

 During at least the closing part of the Glacial period western New 

 York was depressed far below its present level, and following and 

 laving the retreating ice-front was a huge glacial lake, which buried 

 most of Monroe county to a depth of 350 to 400 feet, having its outlet 

 past the site of Chicago to the Mississippi. This lake, produced by 

 the closing of the lower eastern outlets by the ice-sheet, is called 

 Lake Warren.* Subsequently, as the ice retreated northward and 

 eastward, so as to uncover the Mohawk valley, this became an outlet 

 of the glacial waters, and the water-surface fell to the level of the 

 " Ridge Road", which is simply the beach of the glacial lake Iroquois, f 

 with its outlet at Rome, N. Y., to the Mohawk and Hudson valleys. 



Deposits of silts occur over the areas once covered by the glacial 

 lakes. In some localities they form extensive flats, with clayey soil. 

 In the depressions near Lake Ontario and in the valley of Irondequoit 

 bay they constitute the soft deposits which by erosion have produced 

 the conspicuous terraces and the mounds called "sugar loaves". 



The superficial geology of the region is thus a complex result of 

 the action of atmospheric agencies, glacial ice, stream drainage of the 

 glacier, lake action at the ice-front and subsequent to the removal of 

 the ice ; and these followed by a resumption of the destructive forces 

 of the atmosphere. 



Over the geological formations of harder rock, like the Niagara 

 limestone and Corniferous limestone, the glacial drift (till or boulder- 

 clay) is relatively thin, and sometimes leaves the rock almost bare, 

 while over the softer Salina there is a greater depth of the drift, which 

 is largely piled into the elongated parallel hills called drumlins or 

 drumloids, already described. 



A frontal moraine, marking a pause in the recession of the ice- 

 sheet, traverses the county from Brockport to Brighton. This is not 

 strong, but well defined west of Rochester as an irregular ridge, cut 

 by the main line of the New York Central railroad one mile north-east 

 of Coldwater station. Along the Rapids road in the south-west part of 



*See numerous writings of J. W. Spencer and Warren Upham in Amer. Jour. Sci.; Bull. Geol. 

 Soc. Am., and other journals ; Abstract by the writer in Bull. Geol. Soc. Am., Vol. 6, pp. 462-466. 



1" History of the Niagara River 1 ', by G. K. Gilbert, in 6th Ann. Rep. of the Commissioners 

 of the State Reservation at Niagara, for 1889, pp. 61-84, with plates, (also in the Smithsonian 

 A;in. Rep. for 1890 ) 



