SUBSIDENCE OF THE LOWER MOHAWK WATERS. 205 
Aqueduct barrier—840 foot level —The next stage was the subsidence of 
the lower Mohawk waters to the plain of 340 feet. This is the altitude 
of the great mass of sands which girt the south and east of the broad 
basin in which Schenectady lies. A reference to the Schenectady sheet 
will render detailed description unnecessary. It was here, as has long 
been held, that the Mohawk was jostled from its more southerly pre- 
glacial course and sent toward Cohoes. For the most part rock lies at no 
great depth at Schenectady. The old channel is probably westward from 
the city, where at the city waterworks the following section was revealed 
by well boring.* The mouth of the well is 34 feet above the river. 
Feet 
Redaclay sand: loamy... sels cia «eters eters 12 
CANO botnet eo uae ame Os ance Miss Oh Peet OS 
Rock was not reached. Across the river from Schenectady westward 
a broad ridge of sand (so far as seen) about 60 feet high extends 3 miles 
up the valley between the river and the Central railway. It is perhaps 
a bar or shoal built into the lake at the mouth of the valley while the 
waters held the 340-foot level. This must have been maintained for some 
time, for the lowest point was found at Aqueduct, 4 miles northeast of 
Schenectady, where the river has cut a postglacial gorge 135 feet deep 
into rocks of Hudson Riverage. Traces of this level have been discerned 
up the valley, near Fort Plain and at other points. The Aqueduct bar- 
rier ponded the waters westward to the base of the gneiss at Little Falls. 
The valley of Alplaus kill, which joins the Mohawk valley above Aque- 
duct, offers a passage slightly lower than 340 feet. Whether this con- 
tinues northward and eastward to the Hudson is not known. In any 
case the receding ice-front in the Hudson valley would have served as a 
dam for a time. until erosion at Aqueduct provided at that place the 
lowest point of overflow. 
Considering the amount of postglacial work at Aqueduct, it is quite 
possible that a lake, contracting in depth and in westward extension, 
persisted until the end of the Iroquois stage, or even later. 
A striking similarity of conditions appears at the head of the upper 
and lower Mohawk deltas (see figure 2). In both cases the river emerges 
from the plateau by a narrow valley, swings broadly around through a 
bowl bordered by a steep escarpment of drift, and passes through a post- 
glacial rock gorge. In both cases the river seems to have been driven 
from a more westerly course by a blockade of drift and a lake has been 
maintained during the erosion of the gorge, except so far as the bowls 
* Obtained by the courtesy of Superintendent George T. Ingersoll. 
