CONDITIONS AFFECTING ALTITUDES OF DEPOSITS. 201 
level appears inadequate, it is to be remembered that the conditions did 
not favor uniformity in deposition or stability of waterlevel. The bar- 
rier which held the waters at this stage was doubtless of a somewhat 
transient nature. A strageling water body received contributions from 
fluctuating glacial drainage streams on its border, and perhaps suffered 
invasion from glacial ice itself. A progressive reduction of the baselevel 
may have exposed the heads of the deltas to dissection while their frontal 
portions were still receiving accessions of material. We are thus led to 
inquire as to the location and nature of the barrier. Gilbert suggests an 
ice dam in the lower Mohawk valley. This would be entirely adequate, 
but from his present knowledge of the facts the writer inclines to place 
the obstruction at Little Falls and to view it as composite in character. 
It is to be noted that the gorge is long, narrow, and sinuous. At the time 
in question the gneiss barrier would have stood at an altitude of at least 
440 feet if the relation to the sealevel had been the same as at present, 
but the country was lower by whatever share this locality had in the 
Champlain depression. This diminished the rate of flow and may have 
made ice-gorging a most effective agent. Dana thus urges the impor- 
tance of this means of blockade: 
““The obstructions in particular cases might have existed for a very long era, in_ 
stead of for a few weeks, such as happens after a modern winter. Again, the 
slackened or suspended flow of the water, caused by such ice obstructions, would 
have favored the deposition and accumulation about them of drift, and some may 
have thus been converted into complete dams.” * 
Chamberlin and Salisbury admit blockade by ice-gorging to the extent 
of considerable flooding of the Driftless area.t 
At Little Falls we have the most favorable conditions—a narrow gorge; 
toward which abundant waters moved—waters which in turn were bor- 
dered by glacial ice. It is asif Glacier bay, Alaska, were to be raised 
above sealevel and given a long, devious outlet a fraction of a mile in 
width. There is, however, an added consideration. The drift shoulder 
on the northwest of Little Falls has suffered severe erosion. ‘The Calcif- 
erous cliffs on the south side have also been sapped to an unknown 
amount. The valley is now barely wide enough for the river and the 
various lines of travel. This passage was not unlikely completely blocked 
to near the present altitude of the shoulder or 808 feet. The barrier 
would then have been cut down like the blockade north of lake Bonne- 
ville, but probably much more slowly, being protected by grounded ice 
*J. D. Dana: On the damming of streams by drift ice during the melting of the great glacier. 
Am. Jour. Sei., vol. xi, March 1876. 
{Chamberlin and Salisbury: Driftless area of the upper Mississippi valley. Sixth An. Rep. U.S. 
Geol. Survey, p. 290. 
