Wave-cut Terraces in Keuka Valley 
43 
As a distinct feature of the slope, this terrace disappears where 
the valley wall becomes very steep towards the southern end of 
the Bluff. The till at the end of the Bluff is made up largely of 
local material; there is other evidence also of vigorous corrasive 
work by the glacier on the slopes near the end of Bluff Point. 
No. 3. On the supposition that these terraces represent a 
body of water that fell successively to the levels indicated, terrace 
No. 3 is the oldest; but the difference in the degree of weathering 
attained, or in the sharpness of profile, is not noticeable. This 
terrace apparently does not extend as far north as no. 2; there is, 
however, some obscurity in this direction due to its disappearing 
beneath a wide band of drift. Furthermore its identification is 
not obvious quite as far south as Ogoyago; so terrace No. 3, in 
linear extent, falls short of the next lower terrace. 
TIME PERIODS OF THESE CLIFFS. 
The measure of post-Pleistocene time has been attempted 
through several lines of observations: The years involved in the 
carving of the Niagara and other gorges, in the construction of 
flood plains, etc., have been estimated relatively to units which do 
not admit of very accurate determination because of the interde- 
pendence of degradational activities, a variation in any one of which 
would give the units quite different values. Time-ratios of the 
continuity of certain phases of geological activities are less objec- 
tionable. 
From a study of the extent to which erosion has effected the 
several sheets of till, certain ratios have been deduced using the 
erosion period of the Late Wisconsin drift at a time-datum. The 
approximate value of this ratio, which may be subject to altera- 
tion through the acquirement of new facts, for the Early Wiscon- 
sin is 2; for the Iowan, 4; for the Illinoian, 8; for the Kansan, 16.2® 
The drift of the Mississippi Basin has furnished most of the data 
concerning these epochs of glaciation. It has already been estab- 
Chamberlin and Salisbury, Geology, vol. iii, pp. 413-421, 1906. Here is found 
a succinct presentation of the data on which are based the relative time-periods 
of the stages of the Glacial Period. 
