Rocks and Minerals in Illinois
President Thomas Jefferson gave Lewis instructions to take samples of local minerals and describe them. Lewis described several minerals found in Illinois as the Corps of Discovery sailed down the Ohio River and up the Mississippi River.
On November 25, Lewis wrote about finding limestone in yellow flint at Cape Girardeau. This description fits Sexton Creek limestone, which is from the early Silurian period, but it could also have been Girardeau Limestone, a slightly older formations that contains black specks of chert. Many of the bluffs along the river north of Cape Girardeau had a similar composition.
Other resources that Lewis found had commercial uses. Southern Illinois had large deposits of coal. Along the shore near Grand Tower he also spotted Spanish Whiting (silica clay).