Big rock covered with round nodules?

[JohnRich]

Ran across this rock on the Brazos River near College Station, Texas. Covered with round nodules.  Dimensions are about three feet by two feet by 18 inches.  Any ideas what it is?  

[Planko]

No idea but how cool is that?!?!  Having a drink while i wait for others. 

[Planko]

Sorry, is there two of them as it looks like another in the back ground? 

[JohnRich]

Yes, two of them. There was also a lot of large petrified  wood logs in this part of the river.

 

[supertramp]

Weird and interesting finds.

Are they on the dry riverbed? Is it possible to check whether those globular structures are also present at the base of the boulders? Can you turn them over?

Some of those bumps are broken/fractured: how do they look inside? Seem they are partially hollow/not compact; is it correct?

Other images from different angles may help.

ciao

[Darbi]

That's cool-looking rock! I think it's possibly a speleothem, a type of cave formation.

[JohnJ]

@JohnRich

 

I'm very familiar with the stretch of river you were in...there are some very weird rocks and the petrified wood logjams are incredible.    

 

I think these are septarian formations.  In the immediate area, there are more lenticular nodules comprised of the same materials and from the same stratigraphic level.  

[garyc]

29 minutes ago, JohnJ said:

@JohnRich

 

I'm very familiar with the stretch of river you were in...there are some very weird rocks and the petrified wood logjams are incredible.    

 

I think these are septarian formations.  In the immediate area, there are more lenticular nodules comprised of the same materials and from the same stratigraphic level.  

I’m also quite familiar with the petrified log jams. I always wonder how small a log it would take to sink my kayak. I haven’t seen any of the weird bulbous rocks though. All I can say about it is “it’s not a tumor “!!

[westcoast]

I'm getting a pisolithic, concretionary nodules vibe from this. 

[fifbrindacier]

I also think this is a geologic pattern. An odd one.

[FranzBernhard]

Very nice rocks, thanks for posting.

Lots of good hypothesis already out there, just to sum up:

Speleothem / Tufa.

Concretion / Septarian nodule(s).

Pisolite

As already stated, more data are required. Nice puzzle, btw!

Franz Bernhard

[JohnRich]

16 hours ago, JohnJ said:

"the petrified wood logjams are incredible." 

 

Attached is a photo of one of those. There were several other places where the riverbank had clusters of large petrified logs like this. Amazing sights.  

[JohnRich]

Thanks for all the thoughtful comments so far. I'll try and respond to them all at once here.

 

"Are they on the dry riverbed?" 

 

It's on the water's edge at a low water level. At higher water levels after rain they would be underwater. The water level goes up and down 8 to 10 feet routinely. So, sometimes they'll be dry, and sometimes under water.

 

"Is it possible to check whether those globular structures are also present at the base of the boulders? Can you turn them over?"

 

They are too big and heavy to move.

 

"Some of those bumps are broken/fractured: how do they look inside? Seem they are partially hollow/not compact; is it correct?"

 

I didn't think to look or to try and break one open. Now I wish I had. They kind of remind me of barnacles.

 

"Other images from different angles may help."

 

This was kind of a one-time view. It was a 15-mile canoe trip down the river, and there is no other easy access to this site. Private property farm and ranch land on both riverbanks, with no roads into the area. We were passing by, and stopped to look, but then moved on. If I ever get back there, I'll spend more time examining them.

 

"possibly a speleothem"

 

No caves in the area that I know of. Flatland prairie, rolling hills.

[JohnRich]

15 hours ago, garyc said:

I’m also quite familiar with the petrified log jams. I always wonder how small a log it would take to sink my kayak. 

Even the small ones are quite dense and heavy. Too big to fit in a kayak hatch. And if you strap it down on top you'll be top-heavy and more likely to tip over.

You would need a barge and a crane, and since the water is shallow in many places, you won't get that kind of equipment in there.  Even the public access points at SH21 and FM60 are steep banks, where you can't get a regular boat in and out of the water.

Besides, the State of Texas owns the river and half-way up the bank. Collecting from public lands is illegal.

They are just there for all passers-by to marvel at and enjoy.

[JohnRich]

A close-up view of the nodules. Some of the larger ones appear to be cracked and hollow inside.

[Darbi]

6 hours ago, JohnRich said:

 

"possibly a speleothem"

 

No caves in the area that I know of. Flatland prairie, rolling hills.

I was thinking more in line with speleothem being broken free when the river eroded the underground cave at the bottom of river or when the river eroded the filled-in cave at the bank, something like this.

[JohnJ]

1 hour ago, Darbi said:

I was thinking more in line with speleothem being broken free when the river eroded the underground cave at the bottom of river or when the river eroded the filled-in cave at the bank, something like this.

 

Not a real possibility in that area due to the local geology.

[Darbi]

1 hour ago, JohnJ said:

 

Not a real possibility in that area due to the local geology.

Just my guess. You're far more familiar to this area than I am, so...

[Caallison]

8 hours ago, JohnRich said:

A close-up view of the nodules. Some of the larger ones appear to be cracked and hollow inside.

 

I find A lot of similar looking rocks here i Gainesville, Fl. They range in size from a dime to a baseball, they are usually around red clay and turtle fossils for some reason. The ones i have can be solid crystalline dark gray, clay-tan colored, some are orange, and some that have calcite crystals in them. I haven't been able to find out what they are yet, but it is nice to see im not the only one finding these. I've just been running with the theory that they are silica nodules forming from basic ground water and over saturation. 

 

I call them sand poops because I have the maturity of a third grader.  

[DPS Ammonite]

@JohnRich Bring a piece back if you go there again. Determine if it is calcite by putting acid on it and seeing if a knife blade can scratch it. I am guessing that these are quartz nodules that grew in the limestone. They even may be partially hollow geodes. I doubt that it is calcite. The river and sediments would chew up soft calcite and not leave such a detailed exterior. 

[JohnJ]

@DPS Ammonite they are not calcite or quartz based on my observations and acid testing.  These are siliclastic, middle Eocene formations; limestone is not present.

[DPS Ammonite]

14 minutes ago, JohnJ said:

@DPS Ammonite they are not calcite or quartz based on my observations and acid testing.  These are siliclastic, middle Eocene formations; limestone is not present.

By quartz I meant forms of silica such as chalcedony, chert and silica cement. They look like botryoidal silica spheres that grew and don’t look like clasts. Without a hardness they could be something more exotic that grew/precipitated.

 

Do we know the Eocene formation that they came from? Maybe some research will reveal what they are.

[JohnJ]

Likely Yegua Formation.

 

 

4 minutes ago, DPS Ammonite said:

Do we know the Eocene formation that they came from?

 

[supertramp]

On 22/7/2020 at 7:17 PM, JohnRich said:

"Is it possible to check whether those globular structures are also present at the base of the boulders? Can you turn them over?"

 

They are too big and heavy to move.

Thank you,

I asked you that question because maybe it can’t be taken for granted that the boulders have a different rock type core rather then beeing composed of nodules at all.

Have you seen somewhere around those nodules growing on a clearly different surface? maybe here?

 

The close-up of your last picture is good, but gives rise to doubt that the nodules have a compact dark gray fill, rather than beeing hollow inside.

Anyway, I have seen something similar at the Samaria gorge in Crete: it was botryoidal chalcedony on a limestone surface (as DPS said) but on a much smaller scale;

something similar to the image below. Not sure that this is the case, considering the local geology as described by JohnJ

https://www.agefotostock.com/age/en/Stock-Images/Rights-Managed/DAE-BL036481

[abyssunder]

Maybe, there's a possibility of clay bubbles expanded by gas sources?