1896.] KAIRCIIILD GEOLOGY OF IRONDEQUOIT BAY. 237 



Niagara river and Sodus bay which can compare in dimensions, or 

 cross-section, with the Genesee valley at Geneseo. A very rough 

 estimate of the gradient gives a result not inconsistent with the above 

 idea. Taking the altitude of the Genesee Forks as 1,624 ^^et above 

 tide, the valley at the mouth of Canaseraga creek below Mt. Morris 

 as 574 feet altitude, and the bottom of Irondequoit bay as 169 feet, 

 we find the upper section to have a direct-line grade of about 17 feet 

 per mile, and the lower section about 10 feet per mile. But to 

 establish the connection or continuity of the Genesee and the Ironde- 

 quoit valleys will require a considerable number of well borings. 



The glacier left a heavy kame-moraine in the southern part of 

 the visible Irondequoit valley, or between Pittsford and Fishers, above 

 which the valley is filled and lost. As the ice front receded deep 

 glacial waters were held between it and the high ground upon the 

 south, and these waters continued the work of filling the valley. 

 Lake Warren, with its surface at about 880 feet altitude, helped to 

 bury the southernmost section. Lake Dana, which represents a long 

 pause in the subsiding waters, with its height about 700 feet, also 

 performed its part of the work. Later came Lake Iroquois, which as 

 a gulf occupied Irondequoit valley as far south as Pittsford, with 

 hight in the gulf of 430-435 feet. (Altitude at the " Ridge Road" 

 440 feet. ) 



In this gulf of Iroquois, which probably existed several thousand 

 years, the process of filling with silts and sand by stream-wash pro- 

 ceeded so far as to entirely fill the gulf, as far north as the parallel of 

 Rochester, to within 30 or 35 feet of the water surface. Doubtless 

 the glacial drift (moraine, kame and esker) which the ice had left in 

 the valley lessened the task of the waters, for such deposits are found 

 beneath the lake silts. In other words, the delta deposits of the 

 Irondequoit river and its tributaries, like Allen creek, completed the 

 filling of a large stretch of the valley which the glacier had only 

 partially filled. This lake deposit was mostly subaqueous, as the 

 remnants are only about 400 feet altitude and chiefly fine sandy 

 silts, as if spread out by the gentle agitation of lake waters. The 

 Rochester sheet (plate 3) shows very clearly this plain at 400 feet 

 all about the Irondequoit valley, and the remnants of it are left as 

 tables or " sugar-loaves" in the midst of the valley. This silt plain 

 slopes gently northward, being somewhat under 400 feet at the Ridge 

 Road, and about 420 feet at Allen creek, giving a northward slope of 



