I found these in Post Oak Creek near Sherman city limits. The formation within the creek is Alluvium which is Quaternary, Holocene, Cenozoic (in reverse order) I believe. It is surrounded by Austin chalk which is cretaceous. I have no idea what they are. I’m certain I’ve seen a modern version of this while Scuba diving, but for the life of me I can’t remember what it was or where I saw it. They look a little like a barnacle, but not exactly, maybe a tube worm colony? Please let me know