The following is an excerpt from Wild DFW, published in July 2023.
Mick Tune, now a speaker on fossils for the Dallas Paleontological Society, relates his first exploration of the Ladonia Fossil Park: “About two hours, a couple of miles walking, maybe two thousand inspected and discarded rocks — plus four snakes, a pack of wild hogs, and a gang of turkey vultures eating something stinky — I noticed an odd and intricately curved rock. No digging or chiseling; this loose piece was just sitting there chilling after rolling around in the river for who knows how long.
“It was a good-sized mosasaur vertebrequent a. Even though I barely knew what I was looking for, the purposeful and exquisitely detailed shape and structure of a well-preserved fossil bone are unmistakable. I was holding a seventy-five million-year-old bone of a marine reptile that I found by myself by taking a hike just an hour from my house. How utterly amazing is that?”
“I had so many questions. What else lived in this ancient ocean with it? Why was there an ocean here in the first place? I had to find answers.”
Welcome to the Cretaceous Coast and the land of limestone. Cretaceous comes from the Latin word creta, meaning chalk, a fine-grained, porous type of limestone. The Cretaceous period, 145 to 66 million years ago (mya), featured a warm climate and fluctuating sea levels. North Texas was beneath the Western Interior Seaway, also called the Cretaceous Seaway, bisecting North America.
Over epochs, water repeatedly receded and rose with epic climate fluctuations and tectonic events, creating aquatic environments from deeper than skyscrapers, to vast shallow seas and swamps, to sandy shores. Layer upon layer of deceased microscopic calcareous sea creatures turned into Cretaceous limestone.
“Beneath the surface, limestone bedrock holds echoes of an ancient inland ocean in which monstrous sea lizards swam,” writes University of Tex'as at Dallas geosciences professor Robert J. Stern in Geology of the Dallas–Fort Worth Metroplex. These aquatic reptiles include mosasaurs — such as Dallasaurus turneri, named for the city and Van Turner, who discovered the specimen — and plesiosaurs, whose long-necked variety evokes the mythical Loch Ness monster. The continent’s oldest mosasaur fossil was found north of I-30 near Chalk Hill Road.
Sedimentary layers of the inland sea ended up slightly inclined, bringing older layers to the surface in the west. On the western edge are 300 mya shales of the Fort Worth Basin and Early Cretaceous limestone of 100 mya. Dallas’ Austin Chalk from the Late Cretaceous is 85 mya, and the Ozan Formation further east represents 75 mya.
These overlay a far more ancient geologic record, including remnants of the Ouachita Mountains, formed during the assembly of the supercontinent Pangaea, and the Texas Craton, dating back over one billion years. The past is not behind us; it’s below us, and it’s full of dramatic storytelling.
Land of Limestone
Everyone in North Texas knows limestone. It’s the white dust coating cars after driving rock roads and the chalk rocks kids use to make sidewalk drawings. The bluffs and escarpments so visible on the flat prairie terrain are usually limestone since shale erodes faster. Defined as at least half calcium carbonate, limestone is highly alkaline. Acidic rain associated with pollution hastens its erosion.
If it’s a ridge road, chances are it’s on limestone. Austin Chalk, which extends northward from that city into North Texas, traces the Balcones Fault Zone and forms the roadbed for I-35 down to Hillsboro when it shifts to shale. Mammoth and then bison herds, indigenous people and then Native Americans, and finally settlers traveled the curved diagonal route.
Austin Chalk of the eastern half of DFW prompts the frequent use of “white rock” in place names, including White Rock Lake. The soft pale limestone transmutes with weathering and decomposition into deep black clay-rich soil. Limestone in western DFW is exceptionally hard. Straddling sea to shore, the strata are flush with fossilized seashells of bivalves and other larger invertebrates.
The disparate personalities of Dallas and Fort Worth arise from bedrock. Soils formed from the soft Taylor Marl and Austin Chalk of the east side fostered a mid-1800s to 1900s agricultural economy of cotton and grain. The thinner, drier soils of the west were primarily suitable for less-prosperous cattle ranching. Fort Worth’s fortunes changed when petroleum reserves were discovered in regional sandstones and shales.
Shaped by Water
Unimaginably immense cross-continental rivers flowed to the inland sea, sometimes forming massive deltas where land and water blurred. The alluvial detritus spreads for miles. “The youngest rocks in the Metroplex are Ice Age river gravels found along the Trinity River and its tributaries. These loose, unconsolidated Pleistocene rocks are much less than a million years old. They contain the bones of mammoths, ground sloths, sabertoothed tigers and other Ice Age animals,” writes Stern.
Much of the concrete produced here contains alluvial sands. Gravel mining pits dot the broad river floodplains. Once played out, the quarries fill with rain, becoming ponds and wetlands, especially abundant in southeastern Dallas County. As a result, habitat for water-loving birds increases.
Between Eagle Ford Shale and western limestones resides the Woodbine Sandstone, forming the sandy soil of the Eastern Cross Timbers. The porous sandstone’s ability to trap water in its layers creates water tables accessible through seeps, springs and shallow wells — crucial to early inhabitants.
“The mountains were here 300 million years ago, a broad white sand beach was here 110 million years ago and the great river was here 100 million years ago. These beautiful scenes all existed where we are today; we just arrived too late to enjoy them,” writes Professor Stern. “But it’s not too late to see evidence that these beautiful scenes once existed where we live today.”
Let’s car surf the Cretaceous Coast. Many travelers along I-30 on the western edge of Dallas notice a historic building high on a ridge. It once housed the Eagle Ford School, a mid- to late-1800s community founded around a sloping shale crossing of the West Fork.
Navigate to the south side of I-30 and Chalk Hill Road. Head up the hill and pass the old school. The easily accessible limestone and alluvial gravel led to the area’s nickname, Cement City.
Continue and wave at the rise where Dallasaurus turneri was found. Chalk Hill ends at West Davis and the Mount Carmel monastery. Take a moment to appreciate the 640-plus-foot elevation. Navigate to Loop 12, also called South Walton Walker Blvd. Traveling south and then veering southwest on TX 308, you follow the Austin Chalk ridge along Mountain Creek Lake — the mountain of its name.
South of I-20, navigate to FM 1382, also called Belt Line Road. Heading southeast to US 67, the highway splits around a massive limestone knob in a broad central median. One past it, Eagle Ford Shale spreads to the right, and Austin Chalk to the left.
Find Your Own Fossil
Ladonia Fossil Park
Located north of Greenville near the town of Ladonia, the park consists of a parking area and a path to the North Sulphur River, which takes a steep climb to access. Voluminous rains expose Late Cretaceous and Pleistocene fossils such as mosasaur vertebrae (which local ranchers call “dinosaur knuckles”), ammonites, bivalves, gastropods, nautilus and shark teeth. The river’s western half will be inundated by 2025 for Lake Ralph Hall, significantly reducing fossil collecting opportunities.
Mineral Wells Fossil Park
Excavation exposed a treasure trove of 300 mya Pennsylvanian-era marine fossils, including crinoids, urchins, bivalves, corals, trilobites and sharks. The Dallas Paleontological Society lobbied for the creation of a public fossil park.
Dinosaur Valley State Park
Once on the edge of the Western Interior Seaway, Cretaceous epoch sauropods left their elephant-like tracks and theropods imprinted their distinct three-toed tracks in what is now the Paluxy River. The hilly park with wading in the river is home to golden-cheeked warblers and black-capped vireos.
Or simply enthrall yourself by experiencing fabulous local fossil finds.
Heard Natural Science Museum
See a mosasaur found in Garland and the partial skeleton of Texas Nessie, a plesiosaur fossil discovered in 1991 in Collin County by Mike Donovan, showcased against a colorful mural of ancient sea life. At Grapevine Lake, nine-year-old Ty Leslie Goble noticed the fossil of an ancient aquatic turtle, Trinitichelys maini, first discovered in Tarrant County by Dr. Derek Main and exclusive to the Woodbine Formation. Now dubbed Ruby, it’s on display after being excavated by professionals and prepared by volunteers in the Heard Paleontology Lab.
Perot Museum of Nature and Science
The paleontology hall showcases several impressive locally found fossils, including Ellie May, a Columbian mammoth discovered in an Ellis County gravel pit. Flexomornis howei — a pheasant-sized bird and the oldest known bird in North America (about ninety-five mya) — was found by Kris Howe at Grapevine Lake. Plus a Tylosaurus skeleton from the shores of Lake Ray Hubbard and Dallasaurus turneri, found in West Dallas.
Waco Mammoth National Monument
Gain insight into the Ice Age with Pleistocene-epoch fossils of Columbian mammoths that weighed 20,000 pounds and stood fourteen feet tall. See a paleontological dig site preserved under a shelter and a museum with excavated fossils.