Illinois Was An Ocean 400 Million Years Ago

Photo byMatt HardyonUnsplash

GALESBURG, IL - Long before it became the Midwest we know today, Illinois was submerged under a vast, shallow ocean.

According to the Illinois Department of Natural Resources, this dramatic chapter in Illinois’ geological history dates back around 480 million years, during the Ordovician Period.

At that time, much of North America, including Illinois, was positioned closer to the equator, creating a warm, tropical environment ideal for an inland sea.

According to the Illinois Department of Natural Resources, this ancient ocean was teeming with marine life, including trilobites, brachiopods, corals, and other now-extinct creatures.

Over millions of years, the remains of these organisms accumulated on the ocean floor, forming thick layers of sediment.

As the ocean receded, these sediments were compressed into limestone and other sedimentary rocks, which can still be found throughout Illinois today.

Evidence of Illinois’ oceanic past is visible in rock formations, fossils, and caves across the state, according to the Illinois Department of Natural Resources.

Places like the limestone bluffs along the Mississippi River and the fossil-rich Mazon Creek near Chicago offer glimpses into this ancient underwater world.

The geological legacy of this prehistoric ocean has significantly shaped Illinois’ landscape, contributing to the fertile soil that supports its agricultural industry today.