Help me identify these rocks.

Factual Questions
1

I’m restoring an old house in Calvert, Texas and I keep coming upon these big, round, flat rocks like this and this and this.

At first I thought they might be man-made because they were so round but the medium sized one has an obvious flair to one side which indicates more of a natural origin.

The largest one was not found on the site but about ten miles away by the Brazos River.

Calvert is located in central Texas between Bryan/College Station and Waco. The soil is mostly sandy loam, and there are very few actual rocks in the region other than sandstone and petrified wood.

So, can anyone tell me what these things are?

2

Since it’s in Texas, I would guess they are a by-product from digging wells. Just a wild guess.

3

I’m no expert, but they strike me as half-completed millstones. :confused:

4

That was my first thought, too. I’ve seen much smaller ones, like 2" diameter, from water wells that my Grandfather drilled. Maybe they are from big Texan oil wells.

5

I don’t think they’re related to oil wells. There just wasn’t that much drilling in that part of the country. Water well, maybe?

The millstone theory is intriguing especially since I found one right next to the bottom part of a millstone. The problem is that I can’t see any evidence of tool work on them. There are no grooves on either flat side as you would expect, and the round edge doesn’t show any scars from chisels or other tools.

6

Maybe just “potential” millstones that were rejected for some flaw in the stone? Or they were going to be millstones, but they were still in the rough stages of being shaped? Or just cast-away pieces from a millstone production process?

7

How are they located? If basically right at the surface or slightly buried, and with a flat surface parallel to the surface, and in close proximity to each other, maybe you are looking at stepping stones, deliberately manufactured and set to form a path across a muddy area.

8

OK. I took a close look at these things yesterday and I can tell you a little more. First of all, there are no tool markings anywhere. Also the flat parts are really not that flat - there are lots of contours - and the round parts are not that round. One would expect a man-made object to have these qualities. There are just too many imperfections to suggest the work of a human hand. I’m quite confident they are a natural phenomenon.

There is one other feature that is consistent with all three. One side is larger than the other by a diameter of about one or 2 inches. It is as though the original thing was cone-shaped and when it began to erode, it broke apart in layers.

9

Geologist educated in Central Texas here but I’ve never seen anything exactly like that before. Can you identify the rock type? Could it be coral, some type of stromatolite maybe?

I remember you’d said you owned this place and I looked for it last time I drove through Calvert, which happens 4 or 5 times a year. I’d love to stop by and check these out sometime. Beautiful place you got there as well.

I’ll keep trying to get an answer for you.

10

Could it be a section from one of these?

Stromatolites. Think “algal mat”.

11

I’ve got a few things which look roughly similar that result when I let a tub of mortar or concrete dry too much to be useful. When I pop it out of the mixing tub, it retains a smooth shape, but may have irregularities where it didn’t completely fill the tub or where it was poorly mixed.

12

Looking at the pics again, the small one may be too rounded to be a stromatolite. They are probably what are called cannonball concretions that have been broken in half. You might try to fit the two together to check for fit.

If they’re all limestone it could very well be a stromatolite, but if they’re mostly fine sand with calcite cement then probably it’s a concretion. People used to collect concretions and carry them around as curiosity items. However, once they broke it open they probably would have left it. It might not even be from there originally.

Off for more research…

13

Hmmm…Hmmmmm…

Nice house.
When I lived in AZ we’d come across these on occasion in the north…nearly identicle. Ever think that even though they may not look man made, they could be much older than the men you are thinking about? Just stepping out on a limb here, but I wouldn’t get too far from those being milling stones of the 15th century variety…

The only other time I have seen stones that circular is when they were in the water for an extended period of time, consistently whacked together by either rushing water (rapids) or a rolling motion…But those look waaay too big for that!

I’m with Lieu…off to do more research

14

It’s really hard to tell just from pictures, but of these things I am sure:

  1. It’s a sedimentary rock. If I had to guess from the pictures, it’s either a fine-grained sandstone or some kind of mudstone (i.e., siltstone, shale). This would also be consistent with the costal-plain geology. Help us out a bit: find some dilute HCl, give the rock a few drops, and report how it reacts. And if you get a chance, grab a hand lens: do you see any grains? Fossils?

  2. The roundedness of the blocks is man-made. That ain’t natural.

Pantellerite, professional geologist (albeit a hard-rock geologist).

15

I sent a link of your question to my brother who is into rocks. Here’s what he had to say:

"Hard to say what they are, I like the stromatalite theory, which is a type
of fossilized algae. I have a rock club meeting tomorrow I will show them
the pictures and see what they think.

I disagree with anyone who says they are definitely man made. There is no
proof of that."

I really do love my brother, but belonging to a “rock club”, how geeky is that?

He also belongs to this club that races those little HO electric cars. You should see these guys being all macho while racing toy cars.

Anyway, I’ll let you know what the rock club has to say

16

Update from my Geologist-type brother:

“I showed pictures of the rocks to two Geology professors from Winona St.
University. They say they are stromatolites.”

If they’re flat enough, they might make interesting stepping stones in a garden walk.

17

Well, I can’t help with the rock question…I just wanted to say Cool House, Cool Rocks,… Cool Beans!

18

They look like concretions.

I consulted the Geologic Atlas of Texas, Austin Sheet (1974) (available for purchase from the Texas Bureau of Economic Geology at UT) which barely includes Calvert. The map shows Calvert situated on the Calvert Bluff formation of the Wilcox Group. Here are some excerpts from the published description:

“…mostly mudstone with various amounts of sandstone, … ironstone concretions, … Sandstone, medium to fine grained, … crossbedded, lenticular…”

The crescent shaped features displayed on the round “sides” are small scale lenticular cross-beds which are typical of shallow marine (intra-tidal) sand deposits.

The shape is typical of concretions. In this case, one or more iron-bearing minerals precipitated in the sediment in an ellipsoidal region while the sediment was buried beneath overlying layers of younger sediment. The precipitated mineral is probably either hematite (Fe2O3) or goethite (FeOOH) or some mixture of the two. Hematite produces a red powder when crushed, goethite produces a yellow or gold colored powder when crushed.

During exhumation by erosion (in response to lowered sea level) the surrounding, less indurated sediment was washed away leaving these relatively more resistant “M&M’s” of iron concretion cemented sandstone.

As an aside, contact the Texas BEC for a catalogue of publications, order a geologic map, maybe some guidebooks, and go rock shopping.

Also, if you can find good intact exposures of the Calvert Bluff formation, look for thin layers of clay - you may be able to find plant fragments, fish bones, and shark and ray teeth in there. I haven’t been around Calvert, but from the map, I suspect there are probably some decent exposures in the cut-banks of the Brazos.