r34 



NEW YORK STATE MUSEUM 



Valley and river gradients 



The present grade of the Mohawk flood plain is from 360 feet 

 at Little Falls to 420 feet at Rome. This is 60 feet in 36 miles, 

 or 1 foot and S inches slope a mile. The outlet or wasteweir of 

 Lake Iroquois at Rome has been somewhat obscured by alluvial 

 deposit of the present Mohawk. The amount of filling is not de- 

 termined but is thought to be of small depth. The present 

 altitude of the col south of Rome is about 430 feet. If we 

 allow 20 feet depth of water over the present broad plain 

 at Rome, we have an altitude of the Iroquois waters at the 

 outlet of 450 feet, at the closing phase. The present Mohawk 

 river has not greatly changed the rock channel at Little Falls 

 from the condition in which the Iromohawk left it, and if we 

 take the water surface of the latter as about 390 feet, or 30 

 feet over the present Mohawk flood plain, we have a gradient of 

 the Iromohawk the same as given above for the present river. 



It is quite certain that postglacial deformation of the area 

 has not depressed the Rome region relatively to Little Falls, 

 as such movement would be in opposition to all the changes of 

 level over the Great lakes and New York State. It may be 

 assumed that the Iromohawk, a volume of water like the St Law- 

 rence, did not have any steeper gradient than the present valley 

 would allow. Indeed such gradient far exceeds that of the 

 great rivers in their graded sections. It is more likely that the 

 Iromohawk slope was less than the above estimate and that the 

 river was broad and shallow at the head and narrow and deep 

 at Little Falls. However, there are reasons connected with the 

 lacustrine levels, as will appear later, and with the Iroquois 

 levels in central New York, which make it probable that the 

 Rome locality has not been much, if at all, lifted relative to 

 Little Falls. Our working theory is that the present Mohawk 

 valley shows a gradation plain of the greater river, which in 

 form and altitude is practically as that greater river left it. The 

 early Iromohawk had a higher head, by whatever amount the 

 Rome outlet has been cut down, but on the other hand it had a 

 higher base-level, by whatever amount the rock barrier at Little 

 Falls has been trenched. 



