Exfoliating domes of granite on Ryan Mountain

These domes of exfoliating granitic rock are on the northeast flank of Ryan Mountain. The domes are the remnants of Mesozoic-age molten rock material that melted its way, or was injected into, more ancient bedrock (the dark schist and gneiss visible on the upper-right side of the image) (Matti and others, 1994; Barth and others, 2004). The molten rock crystallize as bodies of granitic rock called plutons. The plutons probably formed many miles below the surface. Uplift and erosion over millions of years have exposed these rocks in the Little San Bernardino Mountains.

Exfoliation is a weathering and erosional process that affects rocks that formed under great pressure. As erosion strips away the overburden, the rock expands and fractures form parallel to the surface in rock masses that are of uniform texture/composition (like a granitic pluton). Weathering breaks down the rock along the fractures, and surface erosion and mass wasting remove the overburden, creating rounded domes -- such as these shown in the image (Gilbert, 1904).