GLACIATION OF THE ALASKA RANGE 423 
the summer, and to cause it to build extensive gravel flats in 
favorable portions of its course. 
YENTNA DRAINAGE 
Both forks of Yentna River are known to rise in glaciers, those 
on the west fork being of comparatively small size. The East Fork 
has its source in two large ice tongues, one of which probably heads 
on the slopes of Mount Dall, and flows eastward, and the other 
apparently drains the ice from the southern flank of Mount Russell, 
and moves in a southward direction. The two tongues meet at 
their distal ends at an elevation of about 400 feet above sea-level. 
As seen from a distance, both of these glaciers seem to be relatively 
free from morainal covering at their lower ends, though they are 
striped with longitudinal surface moraines. It is not known 
whether they are advancing or retreating as they have not been 
critically observed. 
Glacio-fluvial deposits and. moraines.—The forks of Yentna 
River both show the characteristic features of glacial streams above 
their junction, having wide, bare flood plains of sand and gravel, 
over which the stream flows in an intricate network of braided 
channels. Below their junction the stream maintains a much more 
definite channel to its mouth. The valley floor throughout its 
length, however, is covered with a heavy deposit of glacial outwash, 
and bed rock outcrops along the banks at only a few places. Some 
recognizable terminal moraine occurs between the forks for a few 
miles above their junction, but such deposits are not common. 
Extensive terraces are developed from the earlier outwash gravels 
and border the present stream flat in its lower course and are of wide 
extent between lower Skwentna River and the Yentna (Plate I). 
Kahiltna Valley —Kahiltna River, a tributary of the Yentna, 
some 25 miles above its mouth, heads in one of the largest glaciers 
of the Alaska Range. This glacier, which terminates 600 feet above 
sea-level, has pushed to the edge of the mountains and is expanded 
somewhat in its lower end into a piedmont lobe which is four miles 
wide at its face. For the lower 13 miles of its length this ice tongue 
averages 3 miles in width, has a smooth, even gradient, and is little 
broken by crevasses except near the edges (Fig. 1). The surface is 
