Author’s Note: This part of a weekly series on geology for a class I am taking this semester at BYU-Idaho. This week’s prompt required students find and take a picture of a sedimentary rock. Other posts from this assignment can be found under the “Geology 111” category.
After looking at the prompt, it was not hard for me to decide to head out to Carson Sink. I wrote a little about Carson Sink a few weeks ago, but that was mostly talking about salt that can be found there. Salt is not the only thing you can find at the terminal lakes in the Great Basin.
Since water doesn’t flow out of these terminal lakes, the sediment they contain gets deposited there in a way similar to how it gets deposited in the ocean. Easily viewed examples of sediment being deposited in the ocean can be found in various river deltas, like the Mississippi and the Nile. The material deposited in these river deltas can range from soft clay to large boulders.
Much of the water in Carson Sink is sourced from the Carson River, which begins high in the Sierra Nevada in far eastern California. It then flows through Carson City and Fallon, before terminating in the sink. Because of the arid environment of northern Nevada and the east slopes of the Sierra, the terminal lake in Carson Sink is seasonal – Carson Sink doesn’t consistently have water in it.
Along with the deposits of the Carson River, there are other streams that flow into the sink. These streams flow very rarely – generally following good thunderstorms. When they do flow, they don’t flow for long. The rock that I picked would have come from one of these water sources, since I pulled it from the northern edge of the sink rather than near where the Carson River comes in. I didn’t pick this spot for any special reason other than that it has super easy access to Interstate 80.
A lot of the material here is soft clay. Last fall, we drove out to Carson Sink and found that it had water in it. About a mile and a half from I-80 there were two trucks and two tow trucks. Despite being over 100 feet from the water itself, the truck drivers had managed to get stuck in the muddy clay down to their axles.
There’s more than just clay here, though (obviously, since I found a rock and took a picture of it). It’s interesting to me to consider the journey this particular rock took to get here. It was embedded in the clay, so I imagine it had been there for a while. That, or it was mashed down by a car.
The hills above where I got my rock are scarred by numerous mining activities. Was my rock dislodged in one of the gold mines or rock quarries? Is my rock from the narrow, but very small, canyon that I have to pass through to get to the ghost town? Was it deposited in the creek bed by some ancient landslide? How long did it sit in the creek bed before finally being washed to the sink?
Well, based on its color it isn’t a part of the underlying rock that form the structure of the hills. Those rocks are more reddish in color. This area was home to volcanic activity during the Jurassic Era, but it’s definitely sediment and not an igneous rock. May this rock have once been along the shoreline of ancient Lake Lahontan? I’ve not been up to the bench where the old lake shore once was, so I couldn’t tell you the answer to that. Either way, just by virtue of this rock being in the sink I can tell you it’s probably seen quite a lot.