Geology & Fossils
Missouri’s lowlands and highlands alike harbor fossils of ancient marine invertebrates and coal.
The Ancient St. Francis Mountains
Southeast Missouri’s St. Francois Mountains include some of the oldest rocks in the Midwest, dating back 1.5 billion years. These rocks form a great dome, around which younger rocks from the Paleozoic Era dip away.
The already ancient St. Francois Mountains stood over seas as islands during the Cambrian Period, more than half a billion years ago. Because the St. Francois Mountains are volcanoes, they harbor no fossils in their older rocks. But sediments eroded from the mountains preserve fossils of marine animals that lived in the seas below. These include enormous colonies, or reefs, of plantlike algae that surrounded the islands during the Cambrian Period.
A few rock outcrops from the Cretaceous Period (65-230 million years ago) are found in this region, though most sediments are Cenozoic. Missouri’s lowest point is in this corner of the state, where most of the bedrock is less than two million years old.
The Paleozoic West
Most of Missouri’s bedrock dates back to the Paleozoic Era, much older than the first dinosaurs but not as old as the oldest rocks in the St. Francois Mountains. Missouri fossils mostly represent marine invertebrates, though coal is abundant in some areas.
The Mississippian and Pennsylvanian periods are represented in western Missouri. In the southwest is the Springfield Plateau. Limestones laid down during the Mississippian Period (310-345 million years ago) evidence of ancient seas. Farther north are the Pennsylvanian (280 to 310 million years ago) rocks of the Osage Plains. Here are found low, rolling plains with broad, shallow valleys. Coal forests which were repeatedly flooded by Pennsylvanian seas have left a “sandwich” of limestones, shales, and coal beds.
The Ice Age
During the early Pleistocene Epoch, or Ice Age, glaciers covered northern Missouri as far south as what is now the valley of the Missouri River twice. The force of rushing glacial meltwater probably carved the original valley for the Missouri River.
Under the weight of glacial ice, rocks were ground into dust. This glacial dust, or loess, drifted over rolling hills, evolving into the fertile soils that cover most of northern Missouri and the dissected plains region.
One of Missouri’s most famous Ice Age fossil sites is Mastodon State Historic Site, located about 20 miles south of St. Louis in Jefferson County, in eastern Missouri. This site contains the Kimmswick Bone Bed, a paleontological site listed on the National Register of Historic Places. Archaeological excavations established that Paleo-Indians hunted the American mastodon and other animals here during the ice age. The museum displays ancient artifacts, fossils and a mastodon skeleton replica.
Fossils in Our Lives
Missouri ranks third among the states as a producer of limestone. Mississippian limestones are widespread from northeast to southwest Missouri and are important commercially for building stone and agriculture. In southwest Missouri, limestones from the Warsaw Formation are cut and polished and sold as marble. They are widely used for floors and walls in larger buildings and wherever polished ornamental stone is desired.
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