238 ROCHESTER ACADEMY OF SCIENCE. [Jnne 9, 



about three feet per mile. But this part of the continent has been 

 tilted since the Glacial Period, or differentially uplifted in a northerly 

 direction, at the rate of about three feet per mile, which makes the 

 original slope of the plain at least six feet per mile. 



When the St. Lawrence valley was slowly opened, by the 

 removal of the ice blockade north of the Adirondacks, lake Iroquois, 

 which had its outlet at Rome to the Mohawk valley, was slowly 

 drained down to the ocean level. As the Thousand Islands region was 

 then about 150 feet below sea level, the slow tilting of the great land 

 area has now raised the Thousand Islands about 400 feet, so that Lake 

 Ontario has an elevation of 246 feet above tide. With the falling of 

 the waters in the Irondequoit valley river action again came into play 

 and a large part of the Iroquois lake deposits have been eroded and 

 swept down into the present lake. The filling process is simply 

 shifted again to a lower level. The four definite planes of lake 

 action have been at 880 feet (Warren), 700 feet (Dana), 440 feet 

 (Iroquois) and 246 feet (Ontario). 



The Irondequoit topography is the most singular and interesting 

 of any area in the Rochester region. It is the product of atmospheric 

 and stream erosion acting on horizontally bedded lake-silts overlying 

 glacial drift. The wide-spread level, mesa-like stretches are con- 

 structional, being remnants of the sublacustrine Iroquois plain, while 

 the gullies and valleys (and the hills as a negative element) are ero- 

 sional. The Irondequoit district offers an unusually good field for 

 geographic and geologic study. The horizontal rocks, sandstone, 

 shale and limestone, which constitute the ancient valley walls are a 

 record of the far ancient time when all the region was beneath oceanic 

 waters. The broad valley represents the erosional work of an ancient 

 river (supposedly the preglacial Genesee) through many millions of 

 years of later Paleozoic, Mesozoic and Cenozoic time. The varied 

 features of the upland surfaces and the buried deposits in the valley 

 are the effects of the Pleistocene glacier. The horizontally bedded 

 sands and clays and the widely extended silt plain at or near 400 feet 

 elevation are the constructional work of the glacial lake .waters that 

 were held in front of the receding ice sheet. The present work of 

 lake waters may be seen in active operation, as wave erosion along 

 the Ontario and Irondequoit shores ; wave construction in the great 

 bar which now cuts the bay of? from the larger lake ; and the evident 

 filling of the bay at either end by detrital materials from the land 



