l894-i PLANTS OF MONROE COUNTY. 35 



the city the moraine becomes more broken, but between the river and 

 Brighton it forms the most conspicuous hills of the region, the famous 

 " Pinnacle Hills". These are mainly sand and gravel, with some 

 masses of till or unassorted glacial drift and many large boulders. 

 The sand and gravel beds in these hills show remarkably complex 

 structure. To glacialists the hills have been well known but very 

 puzzling. They are probably part of the frontal moraine, of the 

 nature known as "kame".* They consist chiefly of the materials 

 washed out of the glacier by the drainage and accumulated at the 

 front of the ice-wall in the deep water of the glacial Lake Warren. 

 Three other similar kame deposits are found in Monroe county, but 

 not directly connected with any morainic ridge. One is the group 

 of remarkable sand and gravel hills enclosing the Mendon ponds, 

 another the sand hills and plains extending from the head of Ironde- 

 quoit bay past Pittsford into the north-west corner of Ontario county. 

 Another immense deposit lies in the north-west part of Ontario county 

 and the extreme south-east corner of Monroe county. An area of 

 sand knolls also occurs south-west of Rochester, toward Chili Center, 

 the summits of which bear large ice-rafted boulders. 



Glacial gravels are found in hundreds of localities over the 

 county, and the lake silts are abundant, chiefly in depressions. 



Irondequoit bay probably represents a preglacial river valley 

 modified by ice-erosion, and then more or less filled by serving as a 

 catchment basin during the ice retreat and the episodes of lake 

 Warren and the 'ater lake Iroquois. The sand hills at the head of 

 the bay are remnants of the lake deposits, and the present conspic- 

 uous terraces at an elevation of about 400 feet on each side of the 

 bay probably represent the Iroquois lake bottom. 



Influence upon Plant Life. — The influence of the geologic condi- 

 tions upon the plant life should be considered. In regions beyond 

 the limits of ice-drift, where soils are the result of decay of rock in 

 place and consist of the insoluble residue of the rocks, the several 

 kinds of rock and consequent different soils are marked by more or 

 less dift'erences in the flora. In the area here considered such dift'er- 

 ences in the flora can scarcely be marked, because in the place of true 

 soil there is the sheet of complex drift which masks the rocks and 



♦"The Kame-Moraine at Rochester, N. ¥."' By H. L. Fairchild, Amer. Geol., Vol. XVI, pp. 

 39-5«, July, 1895. 



"Eskers near Rochester, N. Y." By Warren Upham, Proc. Roch. Acad. Science, Vol. II, 

 pp. 181-200. 



