Glacier JVork. — Scoit. 241 
Whether a glacier charged at its base with debris shall erode 
or depo=^it at a given point, obviously depends upon its rate 
of flow. This rate of flow is influenced not only by the angle 
of slope, the pressure due to accumitlation of snow and ice. 
and the configuration of the enclosing valley, but also to a 
great extent by the amount of englacial debris carried at or 
near the base. "If the debris charged ice moves at all it will 
cause abrasion ;" says Russell * "but, if it is so heavily 
charged with debris that it. is rigid under the forces to which 
it is subjected, it will remain stationary, and wall not only 
cease to erode, but protect the rock beneath and lead to the 
accumulation of debris." Salisbury confirms this statement 
by his observations in the north where "the low^er part of the 
ice carrying debris in large quantities, seems to be motionless, 
and to become a bed for the upper part of the ice mass to 
move upon." f 
Running water is sometimes an important factor in 
erosion by Alpine glaciers. As the ice is melted dur- 
ing bright days, there appears a stream upon the upper 
surface Avhich may attain a considerable size before reaching 
a crevasse and tumbling in as a w^ater-fall. The sand, mud or 
stones carried by the stream wdll tumble in with it, and under 
certain conditions where the crevasse is a deep one, it may be- 
come still deeper and finally moulins are eroded in the bed 
rock. These moulins will naturally contain the rounded 
detritus after the crevasse at last closes up and the ice moves 
onward. Many cavities of this kind are found in Norway 
known as "giants' kettles," which have doubtless had an origin 
under the mass of ice which once overspread the country. 
It seems that a comparatively small amount has been done 
on the matter of actual measurement of erosion by different 
glaciers, though some idea of the amount of erosion is ob- 
tained from the fine sediment which gives the characteristic 
turbidity to the streams that escape from the melting end of 
Alpine glaciers. It is stated that the Aar glacier, which is one 
of the smaller glaciers of Switzerland, discharges every day 
in the month of August 440 million gallons of w'ater, wdiich 
contains 280 tons of sand, t The fine sand from one of the 
*Joumal of Geol. 1895. 
tJournal of Geo). 1896. 
tGeikie, Text-book ?,([ (Edition. 
