Thread by @mike_malaska

Going to thread up some pix from today's #geology hike at #VasquezRocks.
(Why are those rocks right there? We'll get to that...) 🧵

Here is a descent into a canyon in S end of park. That's where the geology story gets really wild.

Here's a view of a tree from a ridge where I measured strike and dip of the layers using a compass.

Here is a zoom out view of the scene. It is hard to tell in the photo, you really have to be there, but those layers bend. The strike changes from 180 degrees at left to about 160 degrees at right. This is the hinge axis of a subtle synclinal fold.

Here I am standing at another location on the same hinge axis. But am now looking up a valley with a reverse fault. Can you see the fault?

Here is an annotated zoom of that scene. See that straight line where the color of the rocks don't match up? Also that big rock face right in line? Yup! That is a fault!
If you click to expand the photo you can see where I added yellow lines to show the fault location.

Here is hiking out of that area. This is roughly in line and to the west of that fault, and also sorta in line with that hinge. The hinge is no longer obvious, nor is the fault, so something has changed here. It is complex!

Setting sun lights up some of the highest rocks of #VasquezRocks County Park.
(So....why are those rocks right there? Time for a little #Geology!)

Maybe a better question, is not why those rocks are right there, but why aren't there other rocks in front of them or behind them?
It's counterintuitive, but Vasquez Rocks is a story of erosion, not uplift.

#VasquezRocks is...a valley!!!
Whut? But those rock poke up, right?
OK, BUT look at the rocks to E (lower older strata), and the rocks to W (younger higher strata). They are both HIGHER!
(Pic shows lower strata hill at right)
Vasquez Rocks are weaker rock than surrounding hills!

So, #VasquezRocks eroded FASTER than surrounding hills. But some were a little less weak than others. So they poke up where other weak sandstones got worn down. Great example of differential erosion.

So that explains why some layers poke up more than others. They are less weak than the others.
(But the higher volcanic hills at E end of park are still much stronger...)
So tha explains layers, but why those layers there, but not in front or behind??
Well, gotta look at topo...

Here's a map I made using #QGIS of the #VasquezRocks area. The park boundary I roughly indicated in black (may not be super accurate.) I colorized ASTGTM2 dem data, to highlight topography in near the park area. What do you see?
("479" is near one of those Big Rocks)

There is an overall pattern of valleys running SW to NE, going up valley to the NE. This is part of a much bigger pattern in this area of Southern California. Is orthogonal to spine of several mountain ranges, themselves controlled by being parallel to big faulting.
Big picture.

But now look closely, inside park boundary. Two parallel valleys go up SW to NE, but that high stuff is right between them. Aha, they are a drainage divide! They were sorta protected from erosion into a deeper valley! So maybe not just stronger, but actually just lucky!

And you can see little green valleys on either side of the layer as the weaker sandstone erodes out (see! I told you they are weak!). So eventually, they gonna erode down, just slower than everything else*.
*except the stronger hills on either side.

But...there's one more thing. See this area I've highlighted in red outline? Look at the topo. It's kinda high (yellowish) and also about the same level in that outline area.
(ignore that same elevation area at S edge, that is CA Highway 14. nice and level. Built up and roadcut.)

We're gonna look closer. Fortunatly, I've got a field notebook entry from what that wierd long plateau thing looks like on the ground. Here is that location called "470" on the same map. Is right in that zone.

And here is a ground image from "Station_470". The view is looking S. (that high hill in back right above the layers? Yeah that's the less eroded stronger materials younger than Vasquez Rocks sandstone.)
Boy is that plateau FLAT! almost....flat like a streambed, amiright?

This plateau is part of the Pacific Crest trail, is a nice plateau so you see a nice view of Vasquez Rocks on one side, and then a really steep valley to the S.
(A lot of my distant shots of Vasquez Rocks are from this plateau.)
Notice that really really really red dirt...

Here is an even closer view of Station_470 rocks.
What the.....???
Those rocks are round! Like they travelled a ways! And dark! And full of little vesicles. Like the older volcanics that are uplifted and make up the hills on E end of park!!!

Ohhhhh! So now we can put it together. That high plateau....used to be a VALLEY!
A long time ago, there was a valley (going NE down to SE) cutting through sandstone rocks. It carried volcanics from the E through the valley and plopped them here...

(must've been an impressive stream, some of those transported and redeposited rocks are pretty big. Not just gravels and cobbles....)

And it covered up sandstone at underneath the streambed. At some point, the stream shifted (my guess is just to S to make that deep valley). And left a filled valley of rounded volcanic rocks and gravels. Then erosion kept happening. And Vasquez Rocks sandstone is really weak..

So the sandstone eroded, but the stronger volcanic steambed fill DID NOT.
Differential erosion again!
Since the streambed didn't erode, but the landscape around it did, we get a remnant higher standing thin plateau!
Inverted topography!
#VasquezRocks #geology

By the way, this plateau really easy to get to. From the parking lots, just head S. Follow the largest trails (almost road sized) up to the top of the plateau. If the distance (0.5 km) and some rough-ish terrain isn't a problem, it is one of most accessible features in park.

This is why #VasquezRocks is such a wonderful park for #geology. It has great examples of layers, syncline, hinge axis, faults, differential erosion, inverted topography, and volcanics (all of you know where to look).
I love this place!
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