152 ROCHESTER ACADEMY OF SCIENCE. 



Stands upon our map. To its history we shall return after consider- 

 ing what was happening to the sediments thus newly exposed. 



By comparing maps C and D it is seen that nearly all of the 

 newly added land surface consists of Iroquois sihs, that is, of fine 

 sandy material. Unless vegetation were able to clothe this surface 

 as rajjidly as the water laid it bare, it became temporarily the prey 

 of the winds. The dunes that rise so prominently above the 400 foot 

 l)lain just atop the east rim of the Bay at its porth end (sec Plate 

 III) may have originated at this period. In their situaton on the 

 very edge of an abyss they remind one of the "Hogback" dune on 

 the great Iroquois delta at the Pine Plains encampment near 

 Carthage, bordering" the Black River entrenchment. Besides wind- 

 work, many little streams set themselves "the task of carving the 

 reclaimed land, while the larger streams either bisected their former 

 deltas or deployed into the conveniently low ground at one side of 

 the delta. Densmore Creek and the Glen Haven Brook are examples 

 of the former ; the Irondequoit and more particularly Allen's Creek 

 exemplify the latter case. 



As for the sediments themselves, left far above the falling 

 waters, they must have settled considerably, even though not per- 

 ceptibly. In approaching this question of the behavior of sediments 

 when the waters of their deposition are withdrawn, we need to 

 recall that sand, with a specific gravity of 2.65, weighs 60'/ more 

 cut of water than in. Such moisture as remains in it temporarily 

 during the draining process merely swells this i)ercentage, though 

 it may however retard the resultant com])ression or settling of the 

 mass. It is evident that the amount of this response, even if not 

 great, will be everywhere proportional to the vertical thickness of 

 the dei)Osit ; thus if an irregular surface has been levelled up with 

 sediment, the settling will develop a faint apparition or bas-relief 

 of the underlying topography. A shallow surface valley will appear, 

 to mark the position of a deeply buried anterior valley. A vanished 

 knoll will cause a slight eminence on the silt surface above. Eleva- 

 tion or hollow, each will have its counterpart on the resulting sur- 

 face, sufficient to determine the drainage lines though perhaps un- 

 obtrusive to the eye. If, therefore, we find any notable departure 

 from this result, indicated by the rearranged drainage, some original 



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