204 BRIGHAM—TOPOGRAPHY AND DEPOSITS OF MOHAWK VALLEY. 
of considerable thickness. Another section at Yankee hill cut on the 
West Shore railroad is as follows: 
Feet 
HAO WORN sso cca sec eds dec eights cuia seas eds 3 
Wraterlardiclay sive: Selinies stg icir ea enemy ani De Oe 5-10 
Blue stil]? 3 ac is occ tee Rares mene otc ere ete a tea aa 60 
The clays near this point are used for making brick. They are very 
fine and absolutely free from pebbles. Eastward from Amsterdam no 
noteworthy marginal drift is found until Schenectady is reached. 
Correlation of deposits in lower valley—440-foot level.—The dominant con- 
stituent is a massive basal till. The abundance of such material appears - 
- to be largely due to the free exposure of rocks by faulting. Overlying 
the till at several points we have found the fine clays, whose freedom 
from pebbles suggests that no glacier ice floated in the waters of the lake 
while the clays were laid down. The surface beds on all the marginal 
accumulations are sands and gravels. There is a striking accordance of 
levels thus: 
Feet. 
Bast Oreekwer aids eee eee eae eee 440 
SeNbate Holmen o> oouosoc sash scnueuoacee soos 430 
Honte elaine een ELEC ACE ee th ict eee a eh 431 
Ron da clee retin ce ee ee See 440 
Schohaniescree kom secsr ier 2 ey sire ne ne eeee ee 420 to 440 - 
PNGUNNS| Keio 2) ODay ean tetera tera Ii re fy. eee i 420 to 440 
This corresponds well with the Iroquois plane, though not believed to 
be genetically related toit. The waters may or may not have fallen below 
this level before the barrier was cut to the gneiss at Little Falls. When- 
ever it took place a waterfall of 100 feet or more was inaugurated at that 
point, and the postglacial breaching of the gneiss was actively begun. 
The dam that held the lower Mohawk lake in place is unknown. Most 
naturally it would be the retreating ice of the Hudson valley, perhaps 
extending up our valley toward Amsterdam. Stagnant ice may have 
filled much of the valley during the aggrading of the marginal drift, 
though the ice contacts have largely been cut away by later erosion. 
The benches, it is evident, are not kame terraces, nor are they in any 
true sense river terraces. The lake may not have originated and grown 
by uniform extension eastward, but may have begun at several points, 
as at the confluence of lateral valleys, these local lakes gradually merg- 
ing as the ice melted out. The lake may have come more or less fully 
into existence before the drainage of the great lakes found an eastward 
outlet. 
