294 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
descending its steep cascade, has only 1 :12,000,000 of the velocity 
of a large river on a similar slope (’ 97 , 203). The glacial channel is 
U-shaped, broad and deep, while the valley flares open, Y-like, 
above the ice surface. The ice surface slopes steadily down-stream, 
but the bed of its channel is unevenly scoured, here rising in knobs, 
there sinking in hollows or basins from which the bottom ice must 
ascend a little as it moves forward. When the ice melts away, 
lakes occupy the rock basins; the rocky knobs are seen to be 
rounded and plucked in a manner suggestive of heavily moving ice. 
The banks of the channel are scoured and fluted parallel to the ice 
motion; but above the ice-worn channel the flaring valley sides are 
ravined by descending water streams. At the junction of trunk and 
branch glaciers a strong discordance in the level of the channel beds 
may be expected; and the discordance becomes conspicuous when 
the glaciers melt away and leave their “ channels ” to be called 
“valleys.” Hanging side valleys are therefore appropriate as well 
as characteristic features of glaciated main valleys. They must 
come to be considered even more significant of glacial erosion than 
lake basins. 
The Cycle of Glacial Denudation. — The points of resemblance 
between rivers and glaciers, streams of water and streams of ice, 
are so numerous that they may be reasonably extended all through 
a cycle of denudation. Let us then inquire if glaciers may not, 
during their ideal life history, develop as orderly a succession of 
features as that which so well characterizes the normal development 
of rivers. The “ life history of a glacier ” need not be taken only 
in the sense so well illustrated in the last chapter of Russell’s “ Gla¬ 
ciers of North America,” where the glacier is called young when it 
is small at the beginning of a glacial climatic epoch; mature when 
it is largest during the full establishment of the glacial climate; and 
old when it is vanishing under the re-establishment of a milder cli¬ 
mate. Let us here consider the life history of a glacier under a 
constant glacial climate, from the beginning to the end of a cycle of 
denudation, just as Russell has considered the “life history of a 
river” under a constant pluvial climate, in his “Rivers of North 
America.” Thus young glaciers will be those which have been just 
established in courses that are consequent upon the slopes of a 
newly uplifted land surface ; mature glaciers will be those which 
have eroded their valleys to grade and thus dissected the uplifted 
surface; and old glaciers will be those which cloak the whole low¬ 
land to which the upland has been reduced, or which are slowly 
