The odd looking structure of rock (on the other side of the gastropod) is petrified bone from the Cretaceous. I find lots of fossil bones, some quite large, most are small. I think that some are paddle bones and some are vertebrae from a Mosasaur. I have found several Mosasaur teeth in the creek; so, I’m guessing Mosasaur bones.
Could be J, There IS a lot of marine material in that creek. In her youth, Badger found a perfect vertebrae from the X fish there.
Before the flood control dams were built and nature ruled.. some of those gravel bars perfectly classified
out various density of materials in long rows...we once found about 20 ray teeth ( 1/4 inch ) all along one
I was surprised by a similar classification at C3 last week..in a rapids area about 15 inches deep I found 2 concentrated areas of just pure hemetite ( iron )