The Blackland Prairie is a tall grass prairie characterized by black, calcareous, clay soil that forms upon weathering of underlying marine chalks, marls, and shales. Topographically, the area is a nearly level to gently rolling dissected plain. This type of soil shrinks in droughts and expands during the rainy seasons, making it difficult for trees or large plants to grow (and houses to remain level). That is why the Blackland Prairie is prairie vegetation and not wooded like the Cross Timbers, as larger plants can’t handle the heavy soils and constant movement of the shrinking-expanding clays.
Blackland Prairie historically supported a tall grass prairie dominated by little bluestem (Schizachyrium scoparium), big bluestem (Andropogon gerardii), Indian grass (Sorghastrum nutans), switch grass (Panicum virgatum), Eastern gamma grass (Tripsacum dactyloides), Florida paspalum (Paspalum foridanum), and long-spike tridens (Tridens strictus). Wooded areas occured along rivers, ravines, slopes, and in protected areas.
Geologically, Blackland Prairie is built on the back of the PawPaw Formation, Weno Limestone, and Denton Clay that are too closesly integrated to be mapped seperately. Denton Clay lies above the Fort Worth limestone and below the Weno marl. It is a shelly, blue-grayish marl formation aproximately 25 feet thick in Fort Worth. It is characterized by an abundance of oyster-like shell fossils which form a hard cemented shell surface near the top of the formation. Denton clay was formed as a shallow water deposit. Directly above the Denton clay lies the Weno Limestone. The formation consists of a series of chalky limestone and brown or yellow calcareous marls above the hard shell rock of the Denton clay. The top of the Weno limestone often forms a projecting shelf or terrace below the PawPaw formation above. The Paw Paw formation consists of brown clay containing sandy slabs in the lower half and has a characteristic assortment of fossils. This sandy-textured, acidic soil supports areas of sparse vegetation on typically steep slopes. The Paw Paw formation is about 20 feet thick in Fort Worth, becoming progressively thicker further to the north.