50 OSNGAI Jals Jal EI S/aG NC 
amorphose variety, and the light oil-green schistose variety, and 
the blue tinting was derived from the grinding of this formation. 
It cannot be worked for its included gold asa placer deposit, 
because there has been no concentration of the precious metal 
by water action as in ordinary stream alluvium. 
This fine deposit of subglacial till or ground moraine attains 
its fullest development about midway of the course of the 
glacier where it must have a depth in places of not less than 
several hundred feet. At an altitude of about 5000 feet, the 
most prominent glacial features cease. Beyond this the valley 
contracts and descends rapidly over a series of high steps, which 
are strewn with a profusion of bowlders, some of which are 
striated. Everything here is confusion—there may be indis- 
_ tinct terminal moraines, lateral ridges, voches moutones, and some 
ground moraine, but the best expert cannot get much regularity 
out of the piles of bowlders heterogeneously distributed along 
the slopes of the bounding mountains and on the irregular 
valley floor. Here the creek descends rapidly in one long 
series of rapids and cascades, along its bowlder strewn bed, and 
in one place has cut a beautiful gorge thru the solid serpen- 
tine rock. It is several hundred feet in length and thirty to 
fifty feet in depth, and no wider than the stream. With its 
perpendicular and even overhanging walls, it is a veritable 
canyon. It abounds in remolinos (pot-holes) whose mode of 
formation can plainly be seen, from the clearness of the water. 
When the Swift Creek glacier issued from the deep valley in 
the high Sierra Costa Mountains and deployed across the 
Miocene basin, it did not spread out as an alluvial delta, but it 
maintained its narrowness to the end, five miles distant. Around 
this extra-montane portion it formed a beautiful moraine. The 
constitution of this is essentially similar to that of the cemented 
gravel farther up the creek, except that it contains less clay, is 
looser and coarser in texture, and has some large erratics on its 
surface. Where trenched by tributary creeks and its interior 
freshly exposed, polished and striated pebbles and bowlders are 
not difficult to find. Two parallel ridges of about equal height, 
