ROCKSLIDE ORIGIN OF THE BUTTERMILK BOULDERS, EASTERN CALIFORNIA

The main Buttermilk climbing area, located about 15 km west of Bishop, California, hosts some of the best bouldering in the world. Individual boulders over 10 m across are common, and the Peabody boulders, before separating along a joint, formed an ellipsoidal giant 15 m tall, 20 m wide, and 27 m long. Although these boulders are typically described as erratics, several lines of evidence demonstrate that they are locally derived and formed by rolling and sliding down from bedrock above, perhaps during large earthquakes.

The boulders, underlying bedrock, and cliffs above are all composed of weakly porphyritic Mesozoic granite. There are no boulders of the abundant mafic and metamorphic rocks that would have been among those brought in by glaciers from the west. In-place bedrock uphill is strongly jointed along a dominant steep, NNE-striking set that forms tall fins, and top surfaces of bedrock joint blocks are riddled with shallow weathering pans up to 1-2 m across and with width:depth ratios of ~4. Boulders typically also have weathering pans on top, but some have planar surfaces with weathering pans on sides or undersides, and a few lack any pans on top. Boulders typically have smooth, curving undersides and limited contact area with underlying rock, which is typically deeply weathered, pulverized, cut by gently dipping fractures, and microbrecciated.

We interpret these observations to show that the climbing boulders rolled and slid from joint fins upslope. Smooth undersides and crushed rock beneath rule out a core stone origin. Side or bottom surfaces with weathering pans were likely top surfaces of in-place blocks, and smooth upper surfaces of some blocks indicate that they fell too recently to have developed weathering pans. In this area the 1872 Lone Pine earthquake produced ground shaking estimated at modified Mercalli intensity VIII with peak ground acceleration of ~0.5g. It is possible that the Buttermilk boulders record a history of this and earlier great earthquakes.