It was a beautiful warm day, the last day of March, and perfect for introducing a couple of people to my favorite prairie creek in western Tarrant County. Flowing across the dark earth of the Fort Worth prairie, the water cuts down to the layers of limestone, clay, and shale, so that the creek bed is relatively wide and flat. I was introduced to Mary’s Creek in the 1960’s by museum buddies, and we walked and waded along its course finding plenty of the reptiles and amphibians that we loved. On this day, Clint and his nine-year-old son Zev would make its acquaintance, wading through clear water and walking over jumbled limestone rocks that the creek piles up like coarse sandbars. Zev is just a little younger than I was when I first explored this place.
In the early days, it was almost a sure thing that you would find a ribbonsnake or two threading their way through the vegetation at the water’s edge, hunting for cricket frogs. Similarly, plain-bellied watersnakes (then called blotched watersnakes) were common, though the creek has gone through periods when diamond-backed watersnakes dominated. When I first wandered this creek, it was common to see Texas earless lizards darting between chunks of limestone and stopping to raise their tail and wave the exposed black-and-white bars underneath in a display meant to distract predators. These pale gray or slightly beige lizards are still found here and there in central and west Texas, but they are evidently gone from Tarrant County, at least.
A lizard that may still put in an appearance from time to time, especially in areas where sandy soil borders the creek bed, is the Texas spotted whiptail. These striped lizards burrow in sandy soil and easily live up to the speed implied by “whiptail” and “racerunner” (a closely related species). Red-eared slider turtles have always been there, and occasionally you find a Texas river cooter. Spiny soft-shelled turtles turn up here and there, as do snapping turtles.
When we started our walk, Clint went ahead in search of insects, while Zev and I spent quite a bit of time wading through broad, shallow pools and narrow places where the water surged along rapidly. We talked about how the exposed flat areas of limestone are quite slippery because of the algae growing there, and he quickly learned to use submerged patches of gravel for more sure footing. His curiosity is unquenchable, and he brought up a pebble of shale and marveled at the gooey surface of the rock. These “mudstones” are really just compressed silt and clay, and they easily deteriorate after being in the water. We probably both would have enjoyed wading on into the deeper areas and dunking below the water, had we been dressed for it. But we tried to stay upright in our trek across the slippery limestone and somehow we both succeeded.
It became largely a fossil-finding walk for Zev, as we looked for fragments of the late-Cretaceous ammonites that are fairly common in the limestone bed of the creek. “Lower” Cretaceous means late in that geologic period when dinosaurs were alive and a shallow sea covered a good portion of Texas. A little over 100 million years ago, the area around Fort Worth was at or near the shores of that sea, and to the southwest the dinosaur tracks around Glen Rose were evidently made in the mud around the water’s edge. The limestone of western Tarrant County contains fossils of many sea creatures, including oysters, gastropods whose shells made a spiral cone, rounded sea urchins, and ammonites. While the overall shape of an ammonite shell suggests that of a chambered nautilus, they are said to have been more closely related to today’s squids and cuttlefish. In my many walks and wades along this creek, I used to find intact ammonites up to about 18 inches across (which is not their maximum size). Whether because the creek has eroded down to layers with fewer fossils or for some other reason, a walk today will usually turn up no more than a segment of that ribbed spiral shell.
We each spotted several segments of ammonites, and I found an oyster or two while Clint picked up a couple of gastropods for Zev’s inspection. I think Zev is ready to return to the creek, and I would sure be happy if I have handed down a tradition that he can pick up and carry on. These prairie streams are magical, whether they are running with clear water after rains or drying in the summer sun into isolated pools where sunfish dart under submerged ledges and herons prowl the water’s edge for frogs and fish.
Spearing, D. 1991. Roadside geology of Texas. Missoula: Mountain Press Publishing Co.
Wikipedia: Ammonoidea. https://en.wikipedia.org/wiki/Ammonoidea (accessed 4/1/18)