Glacial Polish

Most of the websites on Granite Chief gloss over the geology. Only Jeffrey Schaffer’s The Tahoe Sierra book goes into depth. He has a nine page chapter on Geology, and detailed references to geology in many of the hike descriptions.

Plutonic (igneous intrusive), volcanic (igneous extrusive) and metamorphic rocks underlie the wilderness. The plutonic is mostly classic light colored Sierra granodiorite, the dark grey and varicolored volcanic rock caps many of the ridges including the Sierra crest along the eastern boundary, and the usually reddish or black metamorphic rock is common along the west side. The metamorphic rocks were here first, about 350-400 millions years old, and are metamorphosed sediments and lavas. The plutonic rocks intruded the metamorphic rock about 100 million years ago, as part of the great Sierra Nevada batholith. Most is what is commonly called granite, more specifically granodiorite, but there are some darker intrusive rocks and of course many dikes which cut across both metamorphic rock and the granite itself. With the rise of the Sierra Nevada, much of the metamorphic rock was eroded from the crest, though not the western foothills, but the wilderness contains quite a bit of metamorphic rock. About 4 million years ago a variety of volcanic rocks were deposited over the existing rock and terrain. And much of it was then eroded away, remaining mostly on the ridge crests.

Two adjacent peaks show off the plutonic and volcanic geology. Granite Chief is granite, while Squaw Peak is volcanic.

Glaciation occurred in several periods from 2.5 million years ago to 13,000 years ago. Though much of the wilderness was glaciated, the ridge tops were above the glaciers and so the soft, easily eroded volcanic rocks have remained. One of the longest Sierra Nevada glaciers went down the Rubicon River canyon, which bounds the south edge of the wilderness.

The volcanic debris eroded off the mountains often covers the slopes and valley bottoms of the wilderness, and it is on these deep soils that the thick forests develop. There are some remarkably large trees, particularly red fir and Jeffrey pine, in the Five Lakes Creek drainage.

fan dike in Rubicon River Canyon

The most amazing geological feature I’ve found is in the Rubicon River Canyon, a radially shaped fan of columns from cooling in a dike. Though the columnar flows are common in the wilderness, with the rim of Powderhorn being the best example, this is an intrusive feature, found in an area of metamorphic rock. Similar structures are not uncommon in pillow lavas, which are formed in water, but they are pretty unusual as an intrusion.

2 thoughts on “Geology

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