May-June 1873, Yosemite Fall, Rainbows, Avalanches Image 10

16 (Avalanches of rocks in the Sierra Nevada) Most abundant in granite cans [cañons] where the meeting of tributary gls [glaciers] with a trunk gl [glacier] has produced deep cans [cañons] of the Yosem [Yosemite] type Vert [vertical] walls & projecting brows become weak by development of clvg [cleavage] planes through exposure to weather Smaller avalanches occur in the fall after rain sets in Many rocks ripe for change & when washed around seams give way thundering crunching turning sometimes over mass of a few hundred tons weight breaks on first projecting boss or pk [peak] & comes to valley in fragments none larger than 5 or 6 feet dia [diameter] but the size of the boulders depend upon the quality of granite. In some places only one seam will be disintegrated [bearing] all the mass [perhaps] 20 or 30 ft dia [diameter] perfectly solid This frequently [comes] 17 down in one block all the way with a depth of [tons] more easily imagined than described but the quantity of material brought down thus by ordinary [disintegration] is extremely small considered in relation to extent of wall in the valley The great avalanches occurred simultaneously through Sierra & were produced by earthquakes In my studies of gl [glacier] phenomena throughout this section of Sierra between Tuol [Tuolumne] & Merced I was long time puzzled in seeking to account for those immense taluses of [angled] fragments so universally found at the foot of all the steep walled cans [cañons] throughout the mtns [mountains] At first I thought with the state geologist that these avalanches slopes were the result of occasional falls of rock at any time by ordinary agents of [disenthrally] Never however for a moment believing that they were but the unburied top of [quantities] of talus so vast that they filled up an [imaginary] abysshttps://scholarlycommons.pacific.edu/jmj-all/1518/thumbnail.jp