Geologic Map of Missouri, 1990, Missouri Department of Natural Resources, Division of Geology and Land Survey (now Geological
Survey and Resource Assessment Division).
Cretaceous Period (144 to 66 million year old rocks)
Cretaceous age rocks in Missouri are exposed in the southeast, along a an arc of high ground called Crowley's Ridge. During the Cretaceous, sea levels rose all around the world. Since no evidence exists for a pre-Cretaceous glaciation with subsequent melt, geologists believe the reason for the rise in sea level has to do with the ocean basins becoming shallower due to an increase in sea-floor spreading, and the expulsion of large amounts of subaqueous lava onto the sea floor. In any event, the North American continent was split through Utah and Colorado by a major seaway. Along the Gulf Coast, great banks of carbonates built up on the continental shelf.
During the Cretaceous, the New Madrid Rift Zone was reactivated. Igneous diatremes (pipelike structures which bring deep igneous quickly to the surface) were put in place in St. Francois and Ste. Genevieve counties, in one of the few post-Precambrian igneous incidents. The proto Gulf of Mexico reached to near Cape Girardeau, depositing marine and near-marine sands, sandstones and clay in the low area over the Rift Zone, covering the Bootheel. This reach of the sea, which intruded twice once in the early and once in the late Cretaceous, the latter about 75 million years ago, is known as the Mississippi Embayment. Instead of building up, this area subsided, resulting in up to 500 feet of buried Cretaceous deposits.
The Cretaceous is the only period of the Mesozoic (the Age of Dinosaurs) which has deposits in Missouri. And dinosaur fossils we have. The first bones were uncovered while the Chronister family was digging a well in Bollinger County in 1942. After having the bones identified by a scientist from the Smithsonian Institution, the resulting paper was ignored for 20 years, until Bruce Stinchcomb read it as a student at UM-Rolla in the 1960's. For the next 10 years, Stinchcomb pursued this dinosaur site, eventually purchasing it and beginning a scientific dig in the late 1970's. This site has yielded many fossils, including significant portions of a hadrosaur, and a tooth related believed to have come from a carnivorous species. The fossils are approximately 80 million years old.
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