Geological Overview of Fort Worth
Surface Geology: Dallas and Fort Worth Counties
During the Cretaceous Period, seas rose and left multiple layers of deposits with various compositions. These formations represent several deposition environments: shallow marine, deltaic, and coastal. Ancient land deposits to deeper-water sediments provide a rich variety of fossils: vertebrate specimens include marine reptiles, sharks, turtles, fishes and rarely dinosaurs. There are also plenty of invertebrate fossils: sea urchins, shells of clams, snails, and ammonites (related to the Nautilus).
After the Cretaceous Period the area tilted slightly resulting in a 1/2 degree dip due east. This slight dip across the thin deposits causes many formations to outcrop on the surface - much like slicing an onion. The exposures form bands that run generally north-south. Since the various rock units are deposited one on top of another, the oldest is on the bottom and youngest on top. The tilting exposes the oldest rocks in the western part of the area and the youngest in the east.
During the Quaternary Period, the Trinity River carved out terraces through the Cretaceous deposits. The river left behind clays, sands, and gravel that are of economic value. Today, the Trinity River headwaters form in western Tarrant County, fed by the many creeks flowing from Parker, Jack and Denton Counties. The river causes a dendritic drainage pattern across most of western Tarrant Country. Fossil mammoth, bison and other mammal bones can be found along the Trinity River.
Dallas County has six formations represented on the surface. Tarrant County has fifteen formations on the surface. The list below is from most recent to oldest. Source