l893-] FAIRCHILD — GEOLOGICAL HISTORY OF ROCHESTER, N. Y. 223 



lake called Lake Iroquois, (") covered portions of this region and left 

 shore lines or beaches, of which the " Ridge Road " is the most 

 conspicuous near Rochester. Areas below the level of that beach 

 have- their glacial drift in many places covered thinly with lacustrine 

 silt. Irondequoit bay was probably filled with such lake deposit. 

 The final retreat of the ice and the lowering of the lake left the 

 atmospheric agencies free to produce the surface configuration and 

 drainage which we now have. The lacustrine deposits and under- 

 lying glacial drift have been eroded, producing the sand hills and the 

 peculiar topography about Irondequoit, while the Genesee has carved 

 for itself through the drift and rock strata the ravine in which it now 

 flows. Niagara canyon has a history similar to that of our Genesee, 

 and estimates upon its rate of formation allow several thousand years. 

 The conclusions from all sources as to the length of time since the 

 Glacial period seem to indicate that it has been only some 6,000 to 

 10,000 years. ('") 



As exposed land above the sea our locality is very old, and has 

 witnessed the greatest physical changes of the continents, in land 

 expansion and mountain-making, and the evolution of the higher 

 forms of life. ' 



The geologic history of this locality may be summarized as 

 consisting of four great divisions ; first, the time of continuous 

 submergence beneath the sea and sedimentation, lasting from the 

 Archean to perhaps the Devonian ; second, the time of exposure above 

 the sea and erosion, from the Devonian to the Glacial period ; third, 

 the relatively short time of glaciation ; and fourth, the present brief 

 period of the renewal of the conditions of subaerial erosion. 



The paper was discussed by Dr. E. V. Stoddard, Professor A. L. 

 Arey and others. 



(q) See articles : Warren Upham, Bulletin of the Geological Society of A iiierica. Vol. 2, 

 pp. 260-264. G. K. Gilbert, "Changes of Level in the Great Lakes," T/ie Forum, Vol. 5, June, 1888, 

 "The History of the Niagara River," 6th Ann. Rep. Commissioners of the State Reservation at 

 Niagara, for 1889, Albany, 1890. J. W. Spencer, "The Deformation of Iroquois Beach and Birth of 

 Lake Ontario," Aiiicr. Jour, ^r/., Vol. 40, 1890, pp. 443-451. 



(10) For a succinct statement of the concurrence of geological evidence bearing upon this 

 point, see The Journal 0/ Geology, Vol.11, page 142, in article on "The Glacial Succession in 

 Norway," by Andr. M. Hansen. 



