Скачать или смотреть Dallas Paleontological Society at Ash Grove Quarry - Midlothian, Texas, 3/16/2025

"Underlying Midlothian, Texas are ancient sea floor sediments composed of shales and chalks deposited about 90 million years ago. The Eagle Ford Shale was formed from muds washed into a relatively shallow sea. The sea retreated creating a shallower and more turbulent environment eroding the top of the Eagle Ford and allowing little deposition of calcareous material. This activity resulted in what is now referred to as the ‘condensed zone’ or simply the ‘Austin-Eagle Ford contact’ as the ocean became much deeper and eventually produced the overlying Austin Group (Atco Formation and Austin Chalk). Cement manufactures employ the high-quality Austin Chalk to make their cement with pits dug into the underlying Eagle Ford Shale for its aluminum content – a catalyst in the cement cracking process. This foot-thick contact zone has gained popularity because of its relatively high fossil content comprised mostly of shark and ray teeth as well as disarticulated bones and teeth of other fishes, turtles, plesiosaurs, mosasaurs and crocodiles with some invertebrate material including snails, clams, gastropods, inoceramids and rarely ammonites. Also, of interest to collectors are sizable Marcasite nuggets that formed sometime after deposition of the Basal Atco Formation."

From "Geology of the Cretaceous Period Around Midlothian, Texas" DPS