354 
Culver—The Erosive Action of Ice. 
and is there made to do duty as a measure of the amount and 
rate of glacial erosion. In many cases, also, springs come out 
in the valleys under the ice and add another factor to the 
problem. 
In view of these evident and palpable sources of error affect¬ 
ing the most careful measurements that have thus far been made 
it seems clear that little confidence can be placed in any con¬ 
clusions as to the rate of glacial erosion, which are based on 
such measurements. 
Still less weight attaches to the opinion of those who only 
see the turbidity of the glacial stream and draw their conclu¬ 
sions hastily. On this point Heim says (p. 364): 
“Those who see the glacial stream continually turbid in 
summer, while other streams run clear, who go into the moun¬ 
tains only in fine weather, such may with Penk 30 be led through 
these momentary superficial glances, to the false conclusion that 
a more intense erosion takes place under the glacier than in val¬ 
leys not glaciated. (The glacial sediment, owing to its ex¬ 
ceeding fineness, produces a relatively strong optical effect.) 
Tf'ose who see the wild mountain torrents during heavy 
rain-storms will come to quite different conclusions. ” 
He further says: 
“A system of unglaciated valleys furnishes its drift only 
periodically, in one day accomplishing more in the way of 
drift transport than the same area of a glaciated mountain in a 
year. 
We already possess a number of measurements which show us 
that the ordinary alpine torrents whose united area is only one- 
tenth that of the Unter Aar glacier, by the annual floods, which 
in the natural course of things occur, carry into the valley in 
one or two days from 10,000 to 100,000 cubic meters. And 
the cases are not rare in which such torrents, in a few days re¬ 
move one, two, and even three million cubic meters from the 
drift hills to the principal stream or into lakes. ” 
[Compare this with the measurements of Dolfuss on the stream 
from the Unter Aar glacier, 60 cubic meters in a July day, and 
remember the corrections to be applied which reduce this amount 
to from ten to twenty meters per day.—C.] 
According to Heim the annual transport by the Unter Aar 
glacier is only 6,000 cubic meters or 1-20,000th part by weight 
of the water in the glacial stream. 
30 Mitt. d. Vereine fur Erdkunde, Leipzig, 1879, p, 16. 
