For much of its history, the landmass that is now Texas lay submerged beneath ancient, shallow continental seas. This phenomenon, known as marine transgression, was a repeated cycle occurring over hundreds of millions of years. The term “underwater” refers to a shallow, inland sea covering large portions of the state, distinct from the modern, deep ocean. This history is permanently recorded in the state’s bedrock, connecting us to a time when giant marine reptiles swam where cities now stand.
Geological Eras of Inundation
The history of Texas being covered by water centers on two primary periods of marine transgression: the Paleozoic and the Mesozoic Eras. During the late Paleozoic, the Permian Period (about 299 to 252 million years ago) saw a shallow sea cover much of West Texas, including the Permian Basin. This sea was bordered by a massive reef system, a relic of which is exposed today in the Guadalupe Mountains. As the Permian sea retreated, the remaining water evaporated, leaving behind thick deposits of salt and gypsum in an arid environment.
The second major period of inundation occurred during the Mesozoic Era, primarily the Cretaceous Period (about 145 to 66 million years ago). This was when the immense Western Interior Seaway formed, a vast body of water that split North America in two, connecting the Gulf of Mexico to the Arctic Ocean. The seaway covered large areas of Central, North, and East Texas, leaving nearly a quarter of the state underwater at its peak. Its presence lasted for millions of years, depositing layers of marine sediment across the region before retreating toward the modern Gulf Coast.
Driving Forces: Why Texas Flooded
The repeated flooding of the Texas landmass resulted from large-scale geological forces, primarily global sea level changes and regional tectonic activity. One main driver was eustatic sea level change, referring to global variations in ocean basin water levels. During the mid-Cretaceous, extreme global warmth eliminated large polar ice caps, contributing to higher sea levels worldwide.
Rapid seafloor spreading, particularly in the Atlantic, also contributed to rising sea levels. Newly formed, hot oceanic crust is more buoyant and takes up more space than older crust. This process effectively displaced ocean water onto the continents, which was a major factor in forming the vast Western Interior Seaway.
Regional tectonics also played a significant role by causing the land itself to sink, a process called subsidence. The rifting that opened the Gulf of Mexico basin created structural weaknesses in the continental crust. Furthermore, the immense weight of accumulating sediment, eroded from rising mountains, caused the crust in the Texas region to subside. This downward warping of the land, coupled with rising sea levels, allowed marine waters to transgress far inland.
Fossil and Stratigraphic Evidence
Geologists confirm the past inundation of Texas through specific rock types and the fossils they contain. The most prominent evidence is the vast amount of limestone found across the state, particularly in Central Texas. Limestone is a sedimentary rock composed of calcium carbonate, formed from the accumulated shells of ancient marine organisms. The Edwards Plateau, a massive geological feature, is largely composed of this Cretaceous-age limestone.
These marine rock layers are rich with the remains of sea life that once populated the ancient waters. Fossils of cephalopods like ammonites, which are shelled creatures related to modern squids, are common finds in Cretaceous rocks. Remains of large marine reptiles, such as mosasaurs and plesiosaurs, have also been discovered far from the modern coastline, confirming the deep penetration of the inland seas.
The legacy of these ancient seas is also tied directly to Texas’s economy through its petroleum reserves. Organic matter from countless marine organisms and algae that lived in these shallow basins was buried under layers of sediment. Over millions of years, the heat and pressure transformed this biomass into the oil and natural gas that now fills the porous layers of the Permian Basin and the Gulf Coast. These hydrocarbon deposits indicate Texas’s long history as a submerged marine environment.