Mount McCoy, CA

In order to determine what geologic themes I should consider, I decided to do a little digging. I looked at topographic and geologic maps of Mt. McCoy (the place I previously wrote about) and some of the surrounding area to get an idea of some geologic history or systems at work. I also did some research into the geologic history of Simi Valley (my hometown and location of the mountain), with some exciting results.

The geologic maps I looked at pointed me in two different directions: river systems and, surprisingly, volcanism. Deposited alluvium, indicating fluvial processes, was not a surprise-southern California is a mountainous place, so I expected some level of sedimentary deposition. Indeed, in the topographic maps, I saw plenty of carved waterways leading down from the mountains, including down from Mt. McCoy, towards the valley floor. I couldn’t find out specifically what type of rock makes up Mt. McCoy. I’m curious what it is, as I imagine any variations in its composition will affect its competency and where water flows and alluvium is deposited. If I was there now, I’d bring a shovel to the mountain and dig to see what the soil horizons look like, to see if I could uncover any rock outcrops, and to help me overturn any larger rocks jumbled about to get fresh faces. Because of all the alluvium deposits, I expect I’d mostly find unconsolidated sediment that is too deep to dig through for bedrock. However, between the composition of the alluvium, the soil horizons, and the composition of any lone rocks or outcrops, I’d be able to make some determinations about the underlying bedrock, and as a result, why the topography is the way it is.

I was surprised and thrilled to learn that there are exposed volcanic rock outcrops where I lived. Based on some light research, they’re Miocene andesites (implying subduction of an oceanic plate) from back when this part of the continent was underwater. Since this area was underwater, I wonder what sedimentary deposits might’ve been placed atop the volcanic rocks and whether or not there’s any preserved fossil history. There are sedimentary rocks in the area, but I don’t know if they predate the volcanic rocks or not. If I were at a volcanic rock outcrop, I could try and find the contact between it and surrounding rock bodies. Any contacts could then be relatively dated, and in the case of tilted beds, I could look for contact metamorphism or lack thereof to help determine what came first. If I found any fossils in any sedimentary rocks, I could also use those to date the rock beds more precisely around the andesite. Mt. McCoy, from what I remember, is definitely not made up of volcanic rock. I wonder then if the bedrock is a limestone or other marine deposit. If I could find a bedrock outcrop, I could test for limestone, look for fossils, and potentially relatively date the mountain with the volcanic outcrops nearby. 

In class we discussed rain shadows and wet versus dry sides of mountains. Because Simi Valley is so close to the Pacific coast, I wondered if I could see this effect with relation to my township and Mt. McCoy. After looking at satellite images of this area, I think this effect is present. There’s a clear difference in vegetation from one side of the greater mountain range to the other. Simi Valley is on the dry side, at a little over 30 degrees North latitude. The global pressure gradient and the Coriolis effect have the Westerlies blowing around here, so air is collecting evaporated water over the surface of the Pacific before bringing it to the coastline. Then it meets this incipient mountain range where this warm, humid air rises, cools, rains out, and sinks over the other side as newly dry and cool air. This then results in the very desert-like town I grew up in.

If I were back there, I’d go and observe as much of this process as possible. I’d go to the “wet” side of the mountains and see if I could tell a difference in the air quality. I’d also observe the difference in the soil texture. I expect that on the “wet” side, the top organic layer of soil would be much thicker, richer, and clumpier with organic matter. I’d have a harder time finding larger rocks just around and about because there’s more chemical erosion occurring. On the “dry” side, I remember that the top organic layer of soil is much thinner (we often had to supplement ours in the garden) and has more pebbles. The vegetation is less tropical and more suited to desert life. There are more random rocks about because there’s less chemical weathering. The air is definitely drier too (we were often at risk of wildfires). This is all indicative of rain shadows and these greater geologic processes.

I remember having to dust nearly every day as a chore because it was so dry outside that dust would just blow off the mountain. We would play childhood games that revolved around various adventures on Mt. McCoy, as well as hike to the top (not very far, but when you’re small with short legs, it was more daunting) often. I would never have attributed these tiny day-to-day things I observed or experienced with global processes like Hadley cells, the Coriolis effect, energy gradients, etc.. This is one of the biggest perspective shifts I’ve had, and why geology is so fascinating. It’s hard for me to connect myself and my actions with the long, slow processes like, for example, subduction, because they don’t have an effect on me and I can’t place myself as easily on that timescale in a way that actually hits past what logic tells me. I can only trust that what I read in textbooks or hear in lectures is true, and then when I see some hard evidence (i.e., volcanism or some other related rock-producing process) I can logically convince my brain that whatever I’m seeing and understanding is real. But with (relatively) fast-acting global processes like those mentioned above (Hadley cells, etc.), I can place myself in this cycle. I can actually see the difference in vegetation, soil, rocks, and actually feel the difference in the air humidity. I can extrapolate beyond what I see in just my town and connect what I observe to what other places experience.

This new level of awareness has definitely changed how I view Mt. McCoy and everywhere else. I no longer take for granted that there’s a mountain there, that this mountain is in the desert, and that it has eroded in places to accommodate the flow and deposition of alluvium, because I recognize how each of those situations and processes is impossible to occur in isolation and is connected to quite literally everything else.

The other day, I picked up a rock outside while on a walk with my family. I started to explain why this singular, insignificant looking piece of sandstone (I may or may not have had my hand lens on me) was so special. It meant that wherever this rock was from, there was water that sorted sediment, was high enough energy that the grains were rounded out, and then lost energy and was deposited. Then, more had to have been piled on. Then somehow, this rock was brought back up to the surface, and so on and so forth until I picked it up off the ground. And that doesn’t even get into where the grains might’ve come from! 


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May 13th, 2020