384 The American Geologist. June, 1894 
THE KAMES OF THE ORISKANY VALLEY. 
By T. W. Harris, Cambridge, Maes. 
When the great North American ice-sheet, in the process of 
its disappearance, shrunk away from over the northern sec- 
tions of the United States, its retreating edge left exposed to 
view a varied topography of hills and valleys, the latter 
stretching across the country in many different directions, de- 
pendent in their several cases upon a more or less complicated 
variety of considerations. In general, however, these con- 
siderations may be grouped into two great classes, ./fr*/, the 
preglacial topography of river courses and intervening hills,. 
which had been settled by the determining factors of decliv- 
ity, rock structure, etc.; and second, the distribution of the 
glacial detritus upon this more anciently sculptured surface. 
As the average thickness of the glacial deposits is relatively 
small, it is evident that where they were laid down upon a 
surface whose original topography manifested a considerable 
relief, they would produce but relatively slight modifications 
in the courses of streams, and in the general character of the 
drainage; while upon a surface of slight relief their effect in 
these respects might be very marked. A good example of 
this latter state of affairs is afforded by the Charles and other 
rivers of eastern Massachusetts, which wander about over 
drift deposits, with occasional excursions upon rocky ledges, 
in little relation to the preglacial drainage systems of that 
region. 
The valleys of the Oriskany, the Genesee and other streams 
of central and western New York are, however, cut to a depth 
of several hundred feet in the horizontal Paleozoic formations 
of that region ; and the topography being thus well marked 
these streams have therefore maintained, for the most part,, 
their preglacial courses, with few modifications of any great 
importance. The varied types of glacial deposit, therefore, 
instead of overwhelming the ancient topography and conceal- 
ing it under a new topography of their own devising, are 
themselves regulated and limited in distribution, according to 
the more powerful lines of environment, within which they 
find themselves laid down, but within which they can yet pro- 
duce a certain limited range of effect. Thus most of the riv- 
ers referred to still occupy their old valleys, but within those 
