I stumbled on this construction site while walking my dog the other day. There's a vertical cut showing about 4' of strata.

• this is half a mile from White Rock Creek in central Dallas county, a Trinity River tributary
• My research has discovered the Trinity was once much larger with a much greater flood plain

Here's another cross section from a few feet away showing a narrower, more distinctly red band:

• The top layer is 1-2 feet stiff clay(I'm pretty sure).
• At 3-4 foot depth, whitish band at the bottom, is "slightly weathered chalk" from the Austin chalk formation(I think), which would be 65 million years old. Formed from coral reefs when Texas was under the ancient North American Seaway.

I'm most interested in the redish/brownish band in the middle. Is this from the shallow sea, or are these river deposits? It's a redish sandy clay with smoothed rounded pebbles, these must have been formed either by beaches/shallow seas(millions of years ago), or by river runoff during glacial melting(10k to 20k years ago)So my question is, are these pebbles just 10s of thousand of years old, river pebbles from ice-age runoff? Or are these ancient beaches with sand and pebbles from 100s of thousands or millions of years ago?

Most pebbles are 0.5 inches to 2 inches. See below:



Anyway it's pretty cool to think DFW used to be on top of a vast mountain range with peaks as high as the contemporary Rockies, and was also a beach and shallow sea at times. I guess I didn't time my property investment properly.

cross posted on the history board
https://texags.com/forums/49/topics/2867318

Please expound

Download the nrcs web soil survey app. That should help.

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My research has discovered the Trinity was once much larger with a much greater flood plain

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Wouldn't this be true for any river that has levees systems and man-made dams in its path?

I don't think that 2nd layer is fill. Too many pebbles, and they're big. Plus you don't put a layer of expansive clay on top of sandy fill.

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Wouldn't this be true for any river that has levees systems and man-made dams in its path?

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Not necessarily. The river used to be greater because the climate was wetter in Texas 10k years ago. There's a 300' canyon under the trinity that was covered by sediment.

No idea. But why would you put clay on top of sandy fill? Sandy fill or select fill is always the top layer.

You're the expert. I'm only telling you what I have seen, not a geotech but I read a lot of geotech reports. Clay is more expansive than sandy fill, no? Why would you ever top clay over sandy fill. I've never seen that design.

Edit: I didn't mean to imply select fill is always the top layer, but that it's always the top layer when used.

Gary Johnson said:

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Wouldn't this be true for any river that has levees systems and man-made dams in its path?

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Not necessarily. The river used to be greater because the climate was wetter in Texas 10k years ago. There's a 300' canyon under the trinity that was covered by sediment.

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I see what you are saying. Very interesting.

So in your opinion this is probably man-made. Thus not particularly interesting?

Oh well.

I still found the exchange quite interesting! Especially the canyon reference!

Thanks. I guess you know you're getting old when geology is suddenly interesting. As to the point about flooding, man made dams/lakes do prevent flooding along with providing water sources. But the prehistoric Trinity was more like the Amazon than the lazy river we have today when the tx climate was warmer/wetter.

Monkey knife fighter, I think the site is an old abandoned railroad track, if that's a helpful clue. It's otherwise undeveloped land in a probable flood plain.

You might find this interesting. It's near dfw airport.

http://www.arlingtonarchosaursite.com/index.html

Gary Johnson said:

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climate was wetter in Texas 10k years ago.

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Since you were around 10K years ago, tell me, was there still construction going on on Central Expressway?

pepe the dog said:

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Gary Johnson said:

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climate was wetter in Texas 10k years ago.

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Since you were around 10K years ago, tell me, was there still construction going on on Central Expressway?

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No, but I believe the first iteration of the Trinity River Tollway was put forth about that time.