What are the Most Important Geologic Roadcuts in California? A Candidate in Coulterville

In a recent post, I explored the geology of the Charlie Brown outcrop. It included excellent examples of normal faults, welded tuff, and airfall tuff, features that served as a microcosm of the geology of the entire province. I was on a field trip this weekend and saw another famous roadcut that encapsulates the geology of a different region. It was the Coulterville exposure of the Mother Lode quartz veins. It can be seen a few hundred feet west of the main junction in the village on Highway 132.

Despite extending for more than a hundred miles through the western Sierra Nevada foothills, the quartz veins of the Mother Lode are surprisingly hard to see in detail. In many places the rock has simply been removed by mining. In other instances the veins are on private property and are not accessible. But in Coulterville, the full extent of the minerals and faults of the Mother Lode are wonderfully exposed.

During the Jurassic Period, a vast subduction zone formed the margin of the western edge of the North American continent. Oceanic crust was being driven downward into the mantle, where parts of it melted to form granitic intrusions. In the Cretaceous, the focus of subduction moved west to where the Coast Ranges are today. The old subduction zone provided an avenue for mineralized solutions to rise towards the surface, where they precipitated quartz and metal deposits including gold and copper. The Melones fault was part of this system of fractures, and runs from Mariposa to Sierra City. Much of the Gold Rush gold was found in the vicinity of this fault zone. Highway 49 follows closely along the trace of the fault as well.

Mariposite at the Coulterville roadcut

The outcrop at Coulterville is the boundary between two terranes, bits of crust that were carried across the Pacific and added to the edge of the continent. The Calaveras Complex lies east of the fault, and the Foothills Terrane is found to the west. The fault zone exposes quartz veins, pyrite, and a unique mineral assemblage of quartz, ankerite, and mariposite. The ankerite is an iron-magnesium carbonate mineral similar to dolomite. The mariposite is an exceedingly rare mineral elsewhere in the world, but is found at many sites along the Melones fault. It is a chrome mica mineral formed when serpentine rock was altered by hot fluids moving up through the fault system. It was a good indicator of possible gold mineralization.

So, I'm wondering what your candidates are for the most significant geological roadcuts in California? I've reviewed the Charlie Brown outcrop, and the Big Pumice Cut that was critical to understanding the age relationships between the glacial tills and volcanic deposits in the eastern Sierra Nevada. I imagine the spectacular folded beds adjacent to the San Andreas fault along Highway 14 in Palmdale might be included. What others would you suggest?

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