GLACIATION IN THE TELLURIDE QUADRANGLE 735 
considered as an indication of “‘plucking”’ in the bottom of glaciated 
valleys. It is conceivable that the process of sapping might result 
in large blocks of rock outcropping above the surface of the ice to 
fall and be carried on the surface; and there can be no doubt that 
this sometimes occurs. But in the case of the cirques tributary to 
the San Miguel River, the summit of their bounding walls consists 
of an almost continuous ridge of Potosi rhyolite ranging up to 
about 1,000 feet in vertical extent, 500 feet or more’of which was 
probably all the time above the surface of the glaciers filling the 
cirques. Yet in the drift left by the ice from these valleys, the 
Potosi rhyolite is indistinguishable except in small fragments. On 
the other hand, the San Juan formation, which occurs in the drift 
in abundance in bowlders up to 18 feet in diameter, outcrops in the 
lower part of the cirque valleys from 10,000 to 12,000 feet in eleva- 
tion, and presents in many places the precipitous walls in the 
bottoms of the valleys transverse to the streams which have been 
referred to as giving to these valleys a roughened, unglaciated 
appearance when viewed from below. ‘The appearance of the faces 
of these cliffs transverse to the valleys is such as would result if the 
rock were plucked off in large masses. If sapping had any con- 
siderable share in supplying the large bowlders of the-San Juan 
formation found near Keystone, it must have occurred at the points 
where valleys such as Bridal Veil Creek and Marshall Creek join 
the San Miguel. Here the San Juan formation was exposed in 
precipitous walls 1,500 to 2,000 feet above the surface reached by 
theice. There is no evidence, however, that lateral erosion at these 
points was sufficient to amount to undercutting; the walls are 
precipitous but not roughened as if by the removal of large blocks, 
and the débris which is now falling is for the most part small 
fragments. 
The other drift deposits of the more recent epoch are not 
characterized by large bowlders. Drift brought by the Lake Fork 
Glacier is characterized by diorite-monzonite bowlders rarely over 
3 or 4feetin diameter. This formation outcrops in the bottom and 
sides of the valley in the neighborhood of Ophir Station where 
precipitous plucked faces occur. Here also the upper slopes of the 
sides of the valley above the elevation reached by the surface of the 
