Yes. It comprises periglacial Löß and new rock, not mountain (Destiny)

by Funkmon , Friday, January 08, 2016, 07:53 (3337 days ago) @ marmot 1333
edited by Funkmon, Friday, January 08, 2016, 07:58

I'm no expert, but I was just going to bed, checked my laptop, and there was some geology talk. My degree requires a couple geology courses at undergrad level, and it's super interesting, so I went and got some USGS quadrangles from the university's online library, since the remnant of the Appalachians thing didn't make sense to me, and I wanted to know about the boulders.

Here's a layman overview, from a layman, who may know nothing. Take this with a grain of salt.

The Appalachian mountains were created during a few mountain building events, the main ones being the Alleghenian and Acadian orogenies, from the Silurian period to the Permian period. That is, just before dinosaurs showed up in the fossil record. This was the result of a few continental collisions, and the Appalachians originally extended into Arkansas. The Ouachita mountains are a remnant, and originally Louisiana was a basin. These go too far north for the area to be a part of them.

The area's formation goes north in an unbroken line to the Ohio river along the east side of the Mississippi, and appears to be löß. This stuff is windblown sediment that showed up during the ice age, millions and millions of years after the Appalachians were formed. It shows up in the floodplains of glacial rivers which were seasonal. When the glaciers were no longer melting, the rivers would dry up, exposing the entire floodplain to wind, which made these big piles of silt sized junk on the side.

This occurred all down the east side of the Mississippi, creating the blufflands that exist there. Let's look at the rocks.

At the top we see the löß, and underneath we see more sand and crap. There is a good chance this is the Citronelle formation. Underneath that, we need a better look to confirm it.

This is a waterfall. It is weird to have a waterfall here, since rock like this is normally very easily eroded. Indeed, it's barely even really rock. However, if it's is undergoing lithification, it will be cemented together and become more difficult to erode.

Cementation is pretty weird; it works with ions, but it turns sand into sandstone, and silt into siltstone. And mud into mudstone. And lime into limestone. And Sly and The Family into Sly and The Familystone.

Anyway, if these parts are becoming cemented, they appear to cause the waterfalls, and even the boulders. As waterfalls erode the cliff, other parts fall off, creating the boulders, as seen in the picture. It's also *technically* possible for big ass rocks called glacial erratics to form the boulders. These are usually big rocks glaciers scrape away and carry downstream, then leave there when they recede. Mississippi is too far south for that, but they can also be carried in ice chunks borne by rivers during floods. I haven't seen any in the pictures I searched for and looked at, but they may exist. If the only boulders you saw were in the creek, it's just due to erosion. Here's the search I performed.

https://www.bing.com/images/search?q=clark+creek+natural+area

Using the maps and some information on the hopelessly recondite USGS website, I found out that this is probably the Catahoula formation in Mississippi, and the Fleming formation in Louisiana. Here's some info on it. Sandstone and junk.

http://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=MSMIc%253B0

So yep. That's why you have boulders.

That's two hours of my life I'm never getting back.