During the Silurian-Devonian periods (440 - 355 million years ago), much of North America lay under a shallow tropical sea, just south of the equator.

Photosynthetic organisms provided the bottom of the food chain.

Marine life such as trilobites, nautiloids, crinoids, corals, and sponges were abundant.

Their calcium-rich shells and skeletons accumulated over time into thick deposits that form the limestone bedrock of the continent.