Need help identifying this rock!!

[bloodyerk]

This rock was discovered by my uncle while plowing a field about 6 miles from Sardis, Georgia. The field was sandy loam. The rock weighs about 50 pounds and is approximately 18" long with an unusual indentation. It looks like somewhat like clay but is extremely hard. Any help would be appreciated.

 

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[Back-of-the-boat]

It looks like concrete IMO

 

[SkySgt]

We call them petrified cow patties in Texas (aka Meadow Muffins)!

Seriously, I think BOTB may be correct. It looks like concrete that was laid in a circular pattern with a trowel, and ended up being below dirt level for quite awhile. I've dug out old wooden fence posts and the concrete that was used ended up looking much like your image.