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PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
ner. Every lake or fiord is an effective lowering of base-level for 
the stream above it; for the level of a body of standing water is 
essentially the same at both ends. As fast as the inflowing river 
builds its delta forward at the head of the lake or fiord, its flood 
plain must rise up-stream and aggrade the valley floor. This process 
is very pronounced in many Alpine valleys, where the aggraded 
valley floor has a relatively rapid descent on account of the plentiful 
and coarse detritus furnished by the active side streams. Indeed, 
every ravine furnishes a great quantity of rock waste, whose descent 
is analogous to repeated landslides of small dimensions. The valley 
floor beneath the ravines is invaded by great alluvial fans, and the 
main stream is driven away toward the further valley wall by their 
rapid advance. At every flood, the waste supplied from the fans is 
swept abundantly into the main stream, whose flood-plain grows 
rapidly as a delta in the upper end of each lake that it enters. The 
delta of the Ticino seems to have advanced so far into what was 
originally the basin of Lake Maggiore that the apparent height of 
the hanging lateral valleys steadily decreases toward the lake; and 
for several miles above the head of the lake the lateral valleys seem 
to enter the main valley almost at grade, although there can be little 
doubt that if all the delta alluvium were removed, the lateral valleys 
would be found to stand high above the rock floor of the main val¬ 
ley. The standing lakes, the aggrading flood-plains, and the grow¬ 
ing fans all show that the bed of the glacial channel has been worn 
too deep to serve as a valley floor for the existing river; the river 
must aggrade, with water or with waste, the bed of the channel that 
the glacier degraded; hence Penck has suggested that glaciated 
valleys of the Alpine kind should be called “overdeepened.” In 
the same way, the waterfalls from the hanging valleys, the shower¬ 
ing waste that forms the fans, and the landslides from the basal 
cliffs, all show that the banks of the glacial channel — the lower 
walls of the existing valleys — are too steep; and they may be 
therefore called “ oversteepened.” Much glacial work had to be 
done upon the mature preglacial valleys of river erosion, to bring 
them into mature adjustment with the needs of glaciers; much river 
work must likewise now be done upon the overdeepened glaciated 
valleys, and upon their oversteepened walls and their hanging 
branches, before they can be maturely adjusted again to the needs 
of rivers. 
Practical Utility of the Ideal Glacial Cycle. — In every case, the 
full understanding of the conditions developed by any system of 
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