Looking Back on Ten Years of Geotripping: Permission to Play...Picking Up Rocks in Central California

Andrew Alden is one of the earliest and best of the geobloggers, and he provided much of the inspiration that lead me to consider blogging in the first place. He currently blogs at Oakland Geology, and much of his earlier material can be found at ThoughtCo. One of his posts inspired me to put together a list of rock localities accessible to the folks in and around my county. Folks who live in the Great Valley may think of their world as flat and featureless (as well as rockless), but we are surrounded by some pretty spectacular mountain ranges with plenty exposures of very cool rocks. They are diverse enough that teachers who want to put together a locally derived teaching collection can get enough to work with in a day or two.

This is a continuing exploration of some of my favorite posts from ten years of geoblogging here at Geotripper. This post originally appeared on January 11, 2011.

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!

You call that little bag of samples rock collecting? THIS is rock collecting!

Had a short diversion out of a busy week a few days ago. I went rock collecting. But not just rock collecting. I went ROCK collecting. We're are in the final stages of completing our new Science Community Center (which will house the science programs and a big new Great Valley Museum) and it was our desire to have some landscaping around the building that reflected some of the unique rock types we have in our region. But the idea isn't to have a little chunk of rock laying on the ground. We intend to have a couple of ton-sized boulders.

Choosing what type of rock is the trick. This is California; we have a great many geological environments, and many rock types could be described as "typical". We live next to the Sierra Nevada batholith, so a big chunk of granitic rock could be used. But that's kind of plain. We are in the Great Valley, so we could go with a big piece of sandstone or siltstone from the Great Valley Sequence, but such rocks are not very durable, and the rocks we choose may very well be in place fifty years from now.

We decided to seek out two rocks that are found very close to our campus, in the Sierra Nevada foothills. The first is serpentine, the metamorphic product of bringing the Earth's mantle peridotite up through the crust under high pressure to the surface. It is relatively rare in most places, but is common throughout much of California. It is unique enough that it was declared our state rock in the 1960s, joining our state mineral gold, and our state gemstone benitoite.

The problem was finding an appropriate piece of the right size. There are lots of outcrops along the highway 49, but few active quarries. Serpentine was an important source of chromite, magnesite and mercury, but those ores are mostly imported these days. They were usually mined during wartime (WWI and WWII especially). Serpentine was an important source of asbestos, which was a wonderful fireproofing material, but later it was also found to be a cause of lung cancer and mesothelioma. Asbestos became a scary material (all those lawyer commercials on late night TV may have clued you in to this), and serpentine is no longer mined for that purpose. We finally located a nearby quarry that was still active, grinding the material for road beds.

The other problem with serpentine is that on the journey from the depths of the mantle to the Earth's surface, the rock is internally sheared and fractured. It is not always durable enough. So we were invited to search the quarry for a good piece, one that could survive a bit of rough handling. Looking at the steep quarry face, and all the talus that has fallen from the edge, a possible candidate emerged. Look at the photo above (note Mrs. Geotripper up on the side of the quarry for scale): are there any chunks of rocks that have fallen and survived the impact? Yup, I saw it too, looking a bit like a turtle shell.

Here it is up close (below). Not just a big chunk, but one that is covered with the beautiful polished green surfaces that resemble jade. Just the perfect rock for school kids to touch when they are walking into the entrance of the museum. We discussed price with the quarry operators.

"How much per pound?"

"Oh, we sell by the ton. Say $30 per ton."

"We'll take two!"

 First mission accomplished!

Then we were off to the Carson Hill Mine Quarry to seek our next objective, a much rarer prize: A big piece of mariposite. Mariposite is a mineral that is practically unique to our region (Mariposa is the Spanish word for "butterfly"; it is the name of the southernmost town in the Mother Lode). It is a chromium bearing mica that is somewhat related to fuschite, but the color is a very bright apple-green. It is often closely related to the gold-bearing quartz veins of the Mother Lode, and I have seen some beautiful samples that contain visible gold. It's a bit harder to find large samples, but the Carson Hill Mine, a Gold Rush era gold mine turned stone quarry, said they had some candidates we might like.

Photo by Mrs. Geotripper

And the other cool thing? This is one of the quarries where the Mythbusters blow things up! I'm pretty sure that this is the spot where they blew up the cement truck, for instance.

We pulled into the corner where they warehouse the big chunks of mariposite, and we started climbing over the boulder pile looking for the perfect sample. There was more in this one pile than I've ever seen anywhere else! How to choose?

We looked and looked and settled on several possibilities. Talking price? It's $0.20 a pound. You must choose wisely....

Our second mission was accomplished. Two multi-ton boulders collected! Of course, I couldn't resist collecting a slightly smaller piece of serpentine for the department... 



Photo by Mrs. Geotripper

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!