BOUROPEAN GLACIAL DEPOSITS. 245 
interglacial deposits referred to rest upon and are covered 
by glacial and fluvio-glacial accumulations. Here, then, we 
have to do not with mere local oscillations of an_ice-front, but 
with great climatic changes extending over protracted periods 
of time. 
But this is not all,—the long persistence of interglacial con- 
ditions is further shown by the amount of denudation and valley- 
erosion accomplished during interglacial epochs. The best 
examples I can cite are those described by Professor Penck and 
others as characteristic of the Alpine Lands. In the valleys 
| descending to the north three conspicuous gravel-terraces may 
be noted, rising one above the other. The highest displays an 
average thickness of about 100 feet—its upper surface being 
some 250 feet above the level of the present rivers. The second 
terrace (eighty feet or so in thickness) rests like the former 
upon solid rock, and its upper surface is 130 feet or thereabout 
below that of the older terrace. The third and lowest terrace, 
occurring inside the preceding, rests also upon solid rock, and 
its upper surface is some 125 feet below that of the second ter- 
race. Each of these terraces is of fluvio-glacial origin, and 
directly connected with a separate series of glacial moraines. 
Obviously the tale they tell is one of glacial accumulation and 
interglacial erosion. After the formation of the highest terrace the 
glaciers retreated, and fluvio-glacial accumulation ceased. The 
rivers then slowly dug their way down through the fluvio- glacial 
gravels (100 feet thick) and, thereafter, proceeded to excavate 
the=solid rock sto! the depth) of “another 1oo feet at least. 
Thereafter ensued a return to glacial conditions, and over the 
newly excavated valley was accumulated another sheet of gravels 
reaching an average thickness of eighty feet. Once more the 
glaciers retreated, accumulation ceased, and valley-erosion was 
resumed, the rivers cutting down through the second series of 
gravels into the solid rock as before, which they trenched to a 
depth of sixty or seventy feet. Then a third advance of the 
glaciers took place and a corresponding series of fluvio-glacial 
gravels was deposited—the upper surface of which now rises 
