This is a fossilized gastropod, or marine snail. This was a big 'ol boy too.

It came from my place, in the bottom of a post hole. It is late Cretaceous, probably 60-65 MM years old and part of a Glen Rose formation (Trinity series) reef that puked up nearby and was covered with sea water. The depth of the water was likely only 30-40 feet over the very top of the reef and as the sea was in its transgressive period, going back and forth (before it finally receded to current day Gulf of Mexico), the reef was exposed to sunlight.

Where I am the top of the reef became calcified from very high salinity levels in the sea. The over millions of years mud washed down from the north and piled up on top of the reef. Under not very much pressure, nor heat, that mud turned into shale (mudstone), called the Eagle Ford shale. South of me, and down regional dip towards the Gulf, the Eagle Ford was deposited at much deeper depths, and over burden pressure, and heat, vegetation and organic matter got embedded in the mud and over time cooked into hydrocarbons. It's on the surface where I live and where EOG drained the hell out the Eagle Ford south of me, its 9,000 feet below the surface.

Below the Glen Rose behind my house, about 80 feet deep, is the top of the Edwards formation, the most prolific groundwater source in Central Texas with vast, vugular porosity, in many places, cavernous-like. The Edwards contains millions of acre feet of beautiful, clear water. Because the Edwards is so shallow a few miles NW of me, and the overlying Glen Rose reef system is so broken up and porous, when it rains in my part of Texas the Edwards can be recharged in a matter of days. Rain runoff goes straight down into it. There are caves in my neighborhood that I explored long ago that go down all the way into the top of the Edwards Aquifer. I swam in a few pools.

Mike S., PhD (post hole digger)