GLACIERS OF THE SIERRA COSTA MOUNTAINS 47 
Coffee Creek has been beheaded and added to the South Fork 
of Salmon River, but not many have clearly discerned that this 
was due to glacial action. In ascending the Upper Coffee Creek 
valley, after the great bend is passed, the floor widens to quite 
a plain, there being here a heavy filling of waterlaid gravel and 
sand, the extra-glacial deposit of the glacier above; on this, at 
the mouth of each tributary gulch, there is a beautiful alluvial 
fan. About one and one half miles below the head of the creek, 
a slight ridge crossing the valley and carrying granite erratics 
marks the extreme limit of the glacier. From here to the sum- 
mit stretches the ‘“ Big Flat,” a smooth plain of fine gravel 
and sand (with scattered granite erratics) about one and one 
half miles in length and one half mile in width. At its upper 
end (which is the summit of the Sierra Costa Mountains, the 
water-parting between the main Klamath and the Trinity River 
systems, and the Trinity-Siskiyou county line) there is the 
slightest tendency to a morainic character. This ‘“‘ Big Flat”’ 
has an altitude of 5500 feet while the mountains on either hand 
rise to 7000 and 7500 feet. Here the glacier made a filling sev- 
eral hundred feet in thickness, thus obstructing the valley. At 
the same time it wore the rock wall of the valley on the west 
(which had already been nearly cut thru by the head water 
erosion of the original South Fork of Salmon River) so thin that 
a glacial stream crossed the ridge in a col and soon cut down 
a gorge. Hence it is that the South Fork of Salmon River rises 
in the head of the original Coffee Creek valley, follows it for 
four or five miles until within a few hundred yards of the present 
head of Coffee Creek, then turns to the west at a right angle, 
and passing out of the broad valley thru a narrow gorge where 
it abounds in rapids and falls, it makes its way thru unglaciated 
gulches to the Klamath. This is one of the finest examples of 
the beheading of a stream by glacial action that I know of. 
As indicated by the granite erratics, the surface of this 
Salmon River glacier descended 1000 feet (and the glacier 
thinned to that amount) in the last one and one half miles of 
its course. 
