EVOLUTION OF IROXDEOUOIT VALLEY. 153 



inequality of deposition will need be looked for. such for example 

 as the familiar case of a delta higher in the middle than at either 

 side so that its stream eventually skirts one of its lateral margins 

 as we have observed in Allen's Creek and the Irondequoit. Possibly 

 another modifying factor is to be acknowledged if the squeezing out 

 and plication of the deeper portion of the silts (sec f^agc Ij8) took 

 place chiefly in consequence of the augmentation of load as the waters 

 lowered, yet the areas that so slumped would credibly be governed 

 by subjacent depressions in the till and such movements would 

 therefore merely accentuate the effects of the normal settling. 



Now the Genesee silts, heaped across the Irondequoit chasm, 

 were pounded down to wave base and accurately levelled by the 

 surf of Iroquois. But with the withdrawal of that lake the results 

 of settling must have been conspicuous here if anywhere and we 

 should expect to find the outlet of Lake Irondequoit superposed 

 over the axis of the buried canyon beneath. Its actual position lies 

 slightly to the west of this, but that is after all in keeping with our 

 experience in other portions of the valley, — where an esker core pre- 

 empts the midline and the drainage channels are displaced laterally. 

 There is no good reason to suppose that the esker was confined to 

 the southern section where its presence is visible, and there is some 

 evidence, as yet unsifted, that an esker fan of early Iroquois times is 

 incorporated in the silt mesa. What determined on which side of it 

 the new flow should go remains an open cjuestion. Certain features 

 suggest that in the primary outflow, just as the Iroquois waters were 

 sundered from those of the Irondequoit lagoon and before the silts 

 had had opportunity to settle, a temporary escape was actually af- 

 forded around the east margin of the Genesee delta, across the nar- 

 row neck of the 400 foot contour on the map, where a weak and 

 shallow channel appears to have been partially refilled by wind work. 



The Irondequoit outlet as finally adopted passed over soft silts 

 its entire length. Into these it must have notched itself almost as 

 rapidly as the lake levels lowered. Thus the decline of Lake Ironde- 

 quoit kept i)ace with that of the larger body, and as the former 

 in turn controlled its affluents on the south and west they in their 

 turn became free to intrench themselves in ecjually soft materials. 

 By the time of the Emmons pause their grades were lowered so far 



