Finding Gold in the Mother Lode (but not that kind): A bit of color in drought-stricken California

Our search for gold in the Mother Lode of the Sierra Nevada foothills was successful, but we weren't looking for that kind of gold. Yes, these gentle hills once hosted one of the greatest gold rushes in history, but the mines have been silent for many decades. Today, the Mother Lode is a tourist destination, and one of the attractions is the spring wildflower show. In the last post, we saw how a few endemics are showing up this week on the serpentine soils in the Red Hills Area of Critical Environmental Concern. Serpentine occurs in bands associated with the Melones Fault Zone which divides two major terranes, the Calaveras Complex and the Foothills Terranes. The Mother Lode gold-bearing quartz veins occur along this same boundary.

Highway 49 winds for 150 miles or so along the Mother Lode, and as we saw in the last post, the early flower shows are patchy, and are helped in places by runoff from the asphalt. The hills are green for the moment, but the green will not last long. In most normal years, 90% of the rain has fallen by now, and we have received maybe 25% of the rain we usually expect. It's the worst drought ever recorded. The green grass in these pictures will start turning brown in a few short weeks, and we can look forward to a hot, dry and fiery summer season.

So the early wildflowers are out, and if the expected storm arrives tomorrow, they might receive enough water to last a few more weeks. One thing I know from years of exploring southwest deserts, flowers in a dry place are precious to behold. So enjoy some of the color we discovered last weekend.

 There were occasional patches of Foothills Poppies.

The Redbuds were in bloom, adding bright splotches of purple or pink to the hillsides.

 Lupines seem to do well in lots of environments along the Mother Lode.
These pleasing-looking leaves are not good for touching or picking; this is the ever-present Poison Oak. The leaves are colorful and eye-catching throughout much of the summer season and into the fall.

We reached the Moccasin Creek area and saw where the Marshes Flat Road diverges from Highway 49 and heads west through the metavolcanics and metasedimentary rocks of the Foothills Terranes. These Jurassic rocks were deposited on the sea floor and scraped off into the vast subduction zone complex that once extended from Canada to Mexico and beyond. Today the rocks are tilted to an almost vertical attitude. On the meadowlands around Marshes Flat the rocks are rarely seen, as they are covered with fairly deep soils.

The road is a pleasant backcountry avenue that serves as access for several ranches. It's usually an uncrowded drive.

The metavolcanic rocks show up when the road starts down the steep slopes towards Don Pedro Reservoir. Although there was almost no water in the creeks, some of the slopes had pockets of blooming flowers, including these Shooting Stars.

 If these are Shooting Stars, are the two flowers below star-crossed lovers?

 Near a creek we found some wild onion blooming.

 And a moving rock. Seriously, why is there a track here, and what moved the rock?

We saw that the sun was getting low in the sky, and I had a lot of grading to attend to, so we headed home. I hope the blooms last long enough for another trip before summer sets in!

Adventures in a Roadcut: It's All in Your Point of View

It's March in Central California, which makes for a really fine time for a geology field trip. We headed east into the Sierra Nevada foothills for a look at some Mother Lode history. For those of you not familiar with the history of California, the Mother Lode was the site of one of the world's greatest gold rushes, starting in 1848 with the discovery of gold flakes in the American River by James Marshall. Ultimately several hundred thousand people made the difficult journey to California to try and find their fortune. As is often the case with such things, only a few ever really prospered. We traveled through Hornitos, an off-the-beaten-track gold era town with some beautiful old ruins, and then followed Highway 49 from Mariposa to Coulterville and then to the hills near Jamestown.

The sun was getting low in the sky when we reached our last stop of the day on Peoria Flat Road outside of Jamestown. It's a neat little mystery for the students to work with, and a great lesson in the need for alternate points of view when problem-solving.

We had spent much of the day in typical rolling hills of the Mother Lode, and crossed several deep gorges, including the Merced River which flows out of Yosemite Valley. When we reached Peoria Flat, the landscape was different. The hilltops were flat and barren, even mesa-like. The flat hills were topped with some kind of dark brown rock that was unlike any of the the slate and serpentine outcrops we had been seeing all day. We found a spot where the road crossed through the flat-topped ridges and had a closer look at the dark rock and the sediments underneath it.

The students found that the dark rock was volcanic, and unweathered surfaces were nearly black. At their level (physical geology or historical geology), black volcanic rock usually equals basalt, though additional study would show the actual composition of the rock to be more along the lines of latite or trachyandesite. The rock underneath the ridge turned out to be conglomerate, with clasts composed mostly of a variety of rounded volcanic rocks. They were clearly deposited in a river. So we had something strange: a flat-topped hill composed of river gravels and topped by a volcanic flow. How did a riverbed and lava flow end up as a ridge?

The best clue to understanding this strange relationship is a different point of view. Several years back, a friend with a plane invited me on a flight over Sonora and Jamestown, and we flew right over the strange outcrop. I knew exactly what I was seeing because Table Mountain is a famous example of an inverted stream.

Nine or ten million years ago, the Sierra Nevada was a lower mountain range with a series of volcanic centers near the crest at Sonora Pass that would have resembled a region like Lassen Volcanic National Park. One lava flow was unusually long, some 60 miles, and as such had followed a stream canyon that had been occupied by the ancestral Stanislaus River. The lava displaced the river to something closer to its present pathway, and as the mountains rose and tilted to the west, erosion stripped away the softer rocks surrounding the lava flow. The old river bed became the flat summit of the Stanislaus Table Mountain; it had become an inverted stream.

You just have to look at the problem from another angle.

It was a beautiful day, and the flowers are ready to bloom. It's a really good time of year for geology problem-solving!