548 



/. C. BRANNER 



Fig. 2. — Sketch to 

 show how the glacier 



valley was free from ice, or at least free enough to allow the 

 water flowing in from the side valleys to fall over precipices, 

 the waters from the melting snows and ice over the drainage 

 basin of Illilouette Creek flowed down along the west side of the 

 glacier that still filled the Illilouette valley above the fall. When 

 this water reached the canyon, it found the 

 bottom of the upper valley full of ice, and 

 it was thus obliged to fall into the canyon 

 — not from the bottom of the valley trough, 

 but at the west side of the glacier. (See 

 Fig. 2.) This stream cut the head of the 

 gorge back past the lower end of the axis 

 of the upper valley. When the ice disap- 

 peared, the water returned to the real bottom 

 of the valley, and was then obliged to fall 

 over the side of the gorge instead of at its 



above Illilouette Fall upper end. 



(shaded area) crowded t^i ^ i r -i a.t j r ii • 



the stream to the west ^he topography of the Nevada fall is 



and caused it to cut a even more striking than that of the lUi- 



gorge that overlaps the j^^^^^^ f^^j_ ^^^ ^^^^ Overlaps the 



axis 01 the upper valley. o a i 



stream and site of the present fall is so long 



and large that the trail leading to the upper valley and to Clouds 



Rest passes through it. It is much choked up with loose debris, 



while there are large quantities of waterworn material exposed 



at the upper lip of the gorge. 



Ice marks are abundant at the head of the Nevada fall, 

 though most of them have been obliterated from exposed sur- 

 faces. 



The explanation of this overlapping gorge is the same as that 

 for the Illilouette fall : towards the close of the glacial epoch, 

 but while the Little Yosemite — the valley just above the fall — 

 was still filled with ice, the water flowing down the north side of 

 the glacier there plunged into the lower gorge and gradually cut 

 its way back by the side of the glacier. When in time the ice 

 disappeared from the edge of the bluff, the stream abandoned 

 its old place beside the glacier and flowed over the cliff at the 

 site of the present Nevada fall. (Fig- 3-) 



