EVOLUTION OF IRONDEOUOIT VALLEY. 151 



would have possessed // it had stood somewhere near the Ridge 

 Road level it later assumed. Its chief value, then, is for comparison 

 with the next map, to show the contour of the final shore line before 

 its modification had begun, thus giving a more graphic conception 

 of the sum total of that modification. 



In map C (Plate V'l) we have the cHmax of the sedimentary 

 cycle, at or just after the full height stage of Iroquois. All that 

 remains of the ancient valley is the small but remarkable unfilled 

 area in the center, surrounded on every side by heavy masses of silt 

 with steep borders. In these the deposits made by the several 

 streams are comparable to their respective watersheds ; the Genesee 

 river, on the west (just oft' our map), furnishing the largest bodv 

 of sediment. The small distributaries ramifying over the delta of 

 the Irondequoit have mostly some echo to-day in the courses of 

 the brooklets that are now carving these silts ; but they never all 

 functioned at once in the way the map has to picture them. . In 

 general, however, everything expressed on this map has a sig- 

 nificance relative to the later development, and in general it must 

 come pretty near to being an actual reconstruction of the true 

 geograph}' of those days. With it should be compared the longi- 

 tudinal section of this stage given in figure 5. 



Rc-excavatiou — Lake Irondequoit. The phase of the later his- 

 tory to which we now pass is signalized by a succession of falling 

 wacter-levels, in which the individual stages are of less interest 

 than the general fact that sedimentation in the large has ceased and 

 re-excavation commenced. After a venerably long life Iroquois 

 became a memory, and its plane was lowered rapidly to the Emmons 

 pause, map D (Plate VII). By this drop of a little over one hundred 

 feet our map becomes mostly land instead of mostly water. The 

 main shore line falls back two miles or more to the north, and it no 

 longer has an embayment into the Irondequoit valley. All that re- 

 mains of the latter is the water held in the unfilled central space 

 to which attention has just been called ; and this was isolated from 

 the main water body before its level had fallen thirty feet. To the 

 steeply walled lake or pond thus produced, unique in type and with 

 a very real and rather long existence albeit at no fixed level, the name 

 Irondequoit adheres spontaneously and as "Lake Irondequoit" it 



