Permission to Play: Picking up Rocks in Central California

Andrew Alden over at About.com Geology mentions a skill and privilege that geologists and rock/mineral lovers need to remind themselves they have: Permission to Play! In other words, get out and have some fun looking for stuff. I live in the middle of California, in a place that at first glance does not offer many chances to build a collection of nice specimens of rocks. I thought about this a few years ago when I was trying to help some teachers build collections of classroom specimens of common rocks and minerals. I put together a brief guide to hunting for rocks in Central California. I've reproduced it below in the hope that some of our local rock-lovers will go out and play!

Where to find Rocks and Minerals in and around Stanislaus County

Living in a flat valley as we do, one may be tempted to think that it will require a lot of money and effort to accumulate samples of common rocks and minerals for the classroom or for collections. In actuality, we live near excellent exposures of a large variety of rocks and minerals that could be collected in two or three Saturday trips from the Modesto area.

Volcanic Rocks:

Basalt: Table Mountain lavas are made of a black volcanic rock that looks like basalt (it is actually a rare type of lava called latite). It can be found along Highway 108, especially near Knights Ferry, and Yosemite Junction.

Andesite: Gray volcanic rock that originates in eruptions from mountains like Shasta, Rainier or Fuji. It was erupted from vents around the Sierra Crest from Sonora Pass north around 20 million years ago, and can be found as cobbles in streambeds like the Stanislaus River.

Rhyolite: Light colored or pinkish volcanic rock that is produced by violent eruptions. The Valley Springs Formation can be found in the Sierra Nevada foothills from the Tuolumne River drainage north to, well, Valley Springs. It was once used as building stone in some Gold Rush-era buildings. The light colored cliffs along Highway 132 between Waterford and LaGrange are rhyolite ash deposits. Other exposures are found along the Stanislaus River upstream of Knight’s Ferry.

Obsidian and Pumice: These are both forms of volcanic glass, the first solid and shiny, the second, frothy. Obsidian is not normally found in the region, except as artifacts like arrowheads that were carried into the area. The rocks are found east of the Sierra Nevada in the Mono Craters area, and at Medicine Lake Highland northeast of Mt. Shasta.

Plutonic Rocks:

Granite: Granite is well-known as the rock of the Sierra Nevada, although the rocks found in Yosemite National Park include many types of similar “granitic” rocks such as granodiorite, tonalite, and diorite, and actually not a whole lot of granite! Collecting is not permitted in the national park, but many cobbles of granitic rocks can be found in the Merced River on Highway 140 just outside the park, and in the upper drainages of the Stanislaus River on Highway 108.

Diorite: See above!

Gabbro: The dark plutonic igneous rock is sometimes incorrectly called “Black Granite”. It is relatively rare, but can be found locally in at least two places, in Del Puerto Canyon above Patterson (just beyond the quartz mine), and along Highway 140 between Cathy’s Valley and Mariposa.

Peridotite: This rock comes from deep within the earth, and is quite rare at the surface. It is often altered even when it is exposed. Some relatively unaltered peridotite can be found in the uppermost reaches of Del Puerto Canyon (about three miles beyond Frank Raines Park), and near the site of the old Pinetree Mine at the brink of Hells Hollow on Highway 49 about 12-13 miles north of the town of Mariposa.

Sedimentary Rocks:

Sandstone and Siltstone: The Central Valley is underlain by several miles of layered sediments ranging in age from 140 million years to the most recent floods of our local rivers. Luckily, these layers have been pushed up to form the eastern edge of the Coast Ranges where they can be easily observed. Del Puerto Canyon is an ideal place to find a variety of shale, siltstone, conglomerate and sandstone samples. The layers are exposed in the lower 7-8 miles of the canyon. Fossils are occasionally found, especially in the first two miles of the canyon, where leaves can be discovered with a bit of work. The partial remains of a Hadrosaur dinosaur were found in the 1930’s.

Gypsum, Coal, and Limestone are not common in the area. Vast limestone exposures can be found in the White-Inyo Mountains east of the Sierra Nevada.

Metamorphic Rocks:

Slate and Greenstone: The Sierra Nevada foothills are composed almost entirely of slate and greenstone, the products of heating and deforming oceanic muds and volcanic flows. Individual sites are widespread. Dredge tailings, such as those found between Snelling and Merced Falls on the Merced River, are good places to find cobble-sized samples.

Marble: The Sierra Nevada foothills east of Highway 49 contain a number of excellent exposures of marble. Not surprisingly, the location of caverns is an excellent clue. Columbia, Camp Nine Road, Moaning Caverns, and California Caverns are good places to start.

Quartzite: Though less common than the previous rocks, quartzite is occasionally found in the higher parts of the Sierra foothills. Because it is a very hard and resistant rock, the best place to find samples is in streambeds and dredge piles (Merced and Stanislaus rivers).

Serpentine: The California State Rock! The former mantle rock is found along major faults in the Sierra Nevada foothills along Highway 49, and in the Coast Ranges, especially in the upper part of Del Puerto Canyon, just beyond Frank Raines Park. Two of the best exposures are at the base of Bagby Grade where Highway 49 crosses the Merced River, and just north of the town of San Andreas.

Schist and Gneiss: These are not common in our area. The extreme southern end of the Sierra Nevada near Tehachapi Pass, and the San Gabriel Mountains of southern California have exposures of these rocks. Further afield, Death Valley (no collecting), and several mountain ranges in the vicinity of Las Vegas are other good localities for finding them.

Have fun!