Of Cinders and Rainbows

Rainbow Basin, California

Since this semester I was fortunate enough to take a geology class I got to head out on a field trip. Not like I need an excuse to head out to the desert for some hiking but I’m happy to take advantage of one.

Our first stop was a brief stop along highway 58 to observe the way the landscape changed as we passed through the area where the Garlock fault crossed heading westward towards the San Andreas fault and East towards Death Valley. It was explained that this was a left lateral slip fault, unlike the San Andreas which would be a right lateral slip-fault. this means that if we looked across the Garlock fault when it moved we would observe it moving to the left. Since this was not a particularly active fault we observed no motion.

We then headed to Ludlow and on to Dish hill and its accompanying saucer hill. Both are basalt cinder cones though Dish hill is a horseshoe shape as part of it is open. Perhaps due to a past eruption. The taller dish hill and shorter saucer are joined by a ‘saddle’, a ridge where the more reddish basalt from dish hill meets the darker basalt from saucer hill. We were told this was a good location to look for xenoliths. Chunks of the earths mantle, mostly made up of olivine that had been surrounded by magma which had cooled rapidly enough around the ‘chunk’ that it remained seperate from the surrounding basalt. I set off in the direction of the saddle in search of some. I stopped in a small ravine where I spotted a structure dug into the surrounding basalt. Later we speculated later that this might have been storage for explosives used when cinders were being actively mined from dish hill. About two thirds of the way up the ridge I found a nice chunk of slightly weathered Olivine. It has small green crystals possibly peridot, and bright black crystals. It is ovoid in shape and measures approximately 4cmx3cmx2cm. I continued up the hill and found a large chunk of basalt that was split open it had a great example of a xenolith embedded in it. since it was a little large I took a photo instead of collecting it, being sure to include my rock hammer for scale.

High desert

Train passing by Dish hill

Proceeding up to the top of the ridge I found another good example of a xenolith as well as a view of the opening of the horseshoe shape of dish hill. This xenolith was embedded in basalt that also included vesicular basalt. There were a lot of large(10-20cm) pieces of vesicular basalt in the area which were very light, making me think they might even be light enough to float the way pumice does. Several people had climbed the main hill above the old mining ruins and I took some pictures as they made their way back down. Some had found pieces of obsidian though I did not find any of these.

Xenolith

We returned back through Barstow on our way to our next stop which was Rainbow basin. Here we observed that most of the surrounding was made not of basalt or granite but of various layers of mud. This sedimentary rock was in various layers each with distinctive colors and textures, though almost all was fine enough to have been deposits of mud, silt or clay, not sand. Here we saw excellent examples of how the sediment layers which would have been layed horizontally originally had been shifted to different angles and folds and then even more layers had built up. Some of which would have been ash from nearby volcanic eruptions. This explains some of the dramatic colors we observed in the different layers. Climbing around I spotted some openings in the mud which turned out to be small mud caves formed by erosion. At least one of these proceeded back far enough to justify the title of cave as I was in total darkness well before reaching the back of the cave which then slopped downhill for anothe 7 or eight feet to where there appeared to be a drainage sump. Outside of the cave we spotted a small opening approximately 30 feet away that appeared to be where the water would enter the stream bed. Walking upstream I found what appeared to be a wonderful example of calcite ( approx 20x20x20cm) It was white/clear with a bright green layer in the center. There was also a lot of small granite and feldspar chunks in the stream bed presumably washed there from up above the mud ridge.

Rainbow basin

Inside a mud cave

Mud cave entrance

Examining rock samples inside the mud cave

After all this fun I had a chilly ride home on my motorcycle( I had traveled over 400 miles) but I broke it up into several stops for dinner and after dinner snacks so it wasn’t bad.