You're looking at history. The creekbed is solid limestone, a layer geologists call the Goodland layer. It runs through the GlenMar Ranch in Caddo, OK.
My Darlin' and I spent a day together, walking the creek, looking carefully at the limestone beneath our feet. It had just rained the night before, and the creek was flowing.
We were looking for a specific fossil, an ancient animal that lived under the sea. It's called an Ammonite. Most of them are two or three inches in diameter.
Bigger Ammonites existed, though. I've seen fossilized Ammonites the size of steering wheels at the Tucson Gem and Mineral Show in February.
They're right there, buried in the limestone. Sometimes all you'll see is a tiny ridged curve in the stone.
We each found one. We're driving back to Tucson with our prizes carefully wrapped. We've been converted to amateur paleontologists!