In reply to James Jackson:
> (In reply to Mick Ryan - UKClimbing.com)
>
> Doesn't look like the fell off the nearby crag...
Quite near the explanation I think. I have photos of small cliff faces in the Buttermilk made up of giant orbs, cemented together......
Eastern Sierra Geology
The granitic peaks of the Sierra Nevada, favored by Sierran climbers from John Muir to Peter Croft, formed from multiple mountain-building events. Part of the Sierra Nevada Batholith (a 400 mile long province of rock extending from Tehachapi to Mt. Lassen), these granitic rocks resulted when magma rose from great depths in the earth's crust, melting and shouldering aside overlying rock, but solidifying without ever reaching the surface. The magma cooled slowly, making larger crystals and a coarser less porous texture than volcanic rocks. The Sierra Batholith is made of hundreds of individual bodies of rock called plutons (after Pluto, Roman god of the underworld). Each originated as a separate magma body, and each has distinguishing mineralogical and textural characteristics: The feldspar knobs so characteristic of the rock in Whitney Portal are absent on the Third Pillar of Dana--they are in different plutons. As a pluton cools, water and other compounds that don't fit into the crystal lattices of the major granite-forming minerals (quartz and feldspars) are distilled into metal-rich melts that fill fractures in the mostly-solidified pluton. When these melts cool in the fractures, they form dikes, such as at the Dike Wall.
About 120 to 85 million years old, the Sierran rocks are much older than the Long Valley volcanic rocks. The tectonic events of five million years ago that uplifted the Sierra Nevada to its current lofty height are much more recent than the granitic rocks themselves. The Sierra Nevada can be viewed as a giant slab of granite that has been tilted down on its west side and up on its east, resulting in the steep eastern escarpment that we see today. This tilting and lifting is related to regional stresses occurring throughout the mountainous Basin and Range tectonic province. The Sierra Nevada, at the western edge of the Basin and Range Province, is rising at a rate of about 4 centimeters per century.
Geologists refer to Sierran rocks as "granitic" rather than "granite," because there are actually several types of rock, such as quartz monzonite and granodiorite, that, along with granite, make up the granitic family of rocks. With difficulty, these various rocks can be distinguished from one another by their ratios of quartz to various feldspars. Geologists notwithstanding, we loosely call them granite, because it doesn't make much difference to climbers whether the rocks they climb are granite or quartz monzonite. Of much greater importance for climbers are the weathering processes that modify the rock surfaces exposed to the atmosphere. The clean sweeping faces of the Third Pillar and Cardinal Pinnacle are the result of frost wedging, where the expansion of freezing water in cracks gradually cleaves the rock into broad clean slabs. In contrast, the intricately featured faces of the Peabody boulders are the result of mildly acidic water gradually decomposing the minerals--chemical weathering. Expose the rocks of the Buttermilk or the Alabama Hills to the climate of Mt. Whitney, and after tens of thousands of years they would have athe blocky angular appearance typical of high-country granite.
Laurel Mountain (available as a free MiniGUIDE) has the only route composed neither of Long Valley volcanics nor Sierra granitics. It is composed of metamorphic rock, which originated on the seafloor 250 to 520 million years ago, prior to the intrusion of the Sierra Batholith. Highly deformed, subjected to prolonged heat and pressure by the plate collisions that generated the Sierra batholith, their fractured state and deformed layering attests to this tortuous history.
© Bob Harrington and Rockfax guidebooks.
