a 
ae ele ae Na ee ee ee ee bet ee | an * 
W. H. Niles—Erosion of Valleys. 369 
failed to notice the small fragments of stone which often darken 
and sometimes cover the surface. When these are examined 
they are found to be sharply angular; and if the examina- 
tion is extended to the medial and lateral moraines, they wi 
be found to contain immense quantities of similar materials. 
These small fragments, as well as large ones, find their way into 
the sub-glacial streams, in which by the sharpness of their 
angles they become most effective instruments in the work of 
erosion. The materials transported by ordinary streams, even 
when swollen by heavy rains, are of a different nature. The 
small stones and gravels which they receive are usually more 
or less rounded, while the finer materials are chiefly loam, clay, 
soil, or well-worn sand. The erosive power of a current carry- 
ing such old, worn, and often soft materials, is much less than 
that of one charged with the new and sharp instruments of the 
sub-glacial streams; hence the denuding agency of the latter 
should not be estimated by observations upon the former. 
The excavating power of these streams is shown in the num- 
ber of pot-holes which they produce. The steepness and irreg- 
ularity of their courses, the abundance of water with stones and 
sand, and in many places the presence of ice causing gyratory 
movements of the water, make these streams peculiarly efficient 
In this work. Sometimes these pot-holes succeed each other so 
closely in the course of the stream, that as they increase in size 
they unite and form a deep, narrow gorge, whose walls present 
a succession of their concave surfaces. 
_ furthermore, the ice of the glaciers often exercises a control- 
ling influence upon the positions and courses of these streams. 
It is not uncommon to find a stream flowing along the edge of 
the glacier considerably below its surface, in a channel one side 
of which is ice and the other side rock. In such instances the 
streams are often supported by the ice at a considerable eleva- 
tion above the bottom of the valley where they would otherwise 
The power which a glacier may have for preventing water 
from flowing directly into the lower portion of its valley, is 
well illustrated by the Marjelen See, a lake which owes its ex- 
istence to the ice-wall of the side of the Great Aletsch Glacier 
Which forms one end of the basin which it occupies. 
The lateral streams above described are abundantly supplied 
- with small and large pieces of stone from the lateral moraines, 
remembered that the ice is constantly renewed by the motion 
