Collapse of Mountain Summits 



291 



were the results of similar failures of less magnitude ; the lower one 

 is covered by a fair growth of mountain mahogany. 



Rock avalanches of greater or less magnitude are undoubtedly 

 continually occurring. Talus-piles receive a steady supply from the 

 slow prying off of small blocks under the action of snow, ice, and 

 frost. Pinnacles are worn out until they collapse, but it must be con- 

 cluded that the failure of a peak is a comparatively rare occurrence. 

 Of the seven instances noted in our trip of some one hundred and 

 eighty miles, there was nothing to indicate that any one of these was 

 of recent occurrence. The conclusion is that all belonged to or oc- 

 curred in a period considerably before the present. Undoubtedly in 

 each case there was local weakening in the form of a fissure or crack. 

 Such fissures and cracks can be observed in many places. One no- 

 table example of a partially open fissure was observed on the south 

 side of Piute Creek Canon about three miles above the junction of 

 Piute Creek and the South Fork of the San Joaquin. This fissure 

 dipped to the west at a steep angle and extended from the bottom of 

 the creek to the top of the hill. From my position on the north side 

 of the creek, many open crevices were visible. Altogether, it was a 

 remarkable and rare example of a partially open fissure. It was, 

 however, not related to any summit, but was a parting in a mountain 

 mass indicating the result of tension. 



While frost and ice very likely cause numerous small avalanches, 

 some force greater than this must be required for the overcoming of 

 the equilibrium of large peaks. Probably earthquakes are in the 

 main responsible. That they are not always so is shown by the col- 

 lapse of Turtle Mountain at Frank, British Columbia. 



The fact that there are a number of collapsed peaks in this part 

 of the Sierra should be of interest to geologists and suggests more 

 detailed examination to discover evidences of the time at which they 

 occurred and whether a number of them belong to a single time 

 period. Simultaneous occurrence would indicate a common cause. 

 Such occurrences may also coincide in time with the formation of 

 the blocky taluses along the walls of the Yosemite Valley. 



