72 NEW YORK STATE MUSEUM 



of the ice sheet, as described above. The deserted and more or less 

 drift-buried stretches of the older valley must connect the ex- 

 isting open, mature stretches, passing around the new, ravine 

 stretches. 



The criteria for locating the abandoned stretches are as follows : 

 (i) Direction: which would be expected to He in fairly direct line 

 connecting the open or unobstructed stretches. (2) Width: which 

 should correspond to that of the old, open valley, making allowance 

 for any difference in the character of the rocks. (3) Walls: which 

 should have slope and height similar to the open stretches. (4) 

 Depth : the original bottom of the deserted stretches must have been 

 in accord or graded with that of the open parts. In want of deep 

 borings sufficient to fully prove the location, depth and form of the 

 buried stretches we have to rely at present on the general form and 

 relation of the broader valley features and the exposures of rock. 

 While these are not entirely satisfactory yet they are probably suffi- 

 cient to show the main facts of the preglacial drainage. 



Rochester district. Somewhere between Avon and Rochester 

 the Genesee river leaves its old valley and enters its modern course, 

 which becomes at Rochester a rock canyon with three cataracts, cut 

 in Niagara, Clinton and Medina strata. The ancient and wider 

 channel must have had a northward trend and somewhere must have 

 crossed the Niagara scarp in order to join the river or lake which 

 occupied the Ontario depression. From the St Davids valley to 

 Sodus bay there is no break in the horizontal strata which could 

 possibly have carried a large stream for a long time except the gap 

 now occupied by Irondequoit bay (or lake). Here is an open valley 

 over a mile wide, in hard rocks, and extending southward as a 

 traceable depression some 15 miles. The depth of the Irondequoit 

 valley to rock is unknown but the depth of water is given as 87 feet. 

 The rock bottom of the old valley must be graded to the depth of 

 the Ontario basin which runs into hundreds of feet a few miles out 

 from the south shore. 



The conclusion is unavoidable that here we have a portion of the 

 old, preglacial Genesee valley, and the most northerly stretch now 

 above the waters of Ontario. From the neighborhood of Fishers, 

 in the Irondequoit valley, westward to the Genesee valley near West 

 Rush, or to the mouth of the Honeoye creek, the old valley is so 

 completely obscured that no confident suggestion of its course can 

 be based on surface features. It appears most probable that the 

 course of the river had been adjusted to the underlying rocks and 



