Detroit Salt Mine Formation
How did the salt get there in the first place?
Michigan, in geologic terms, is a basin - a large area of lower elevation than surrounding areas. 400 million years ago, scientists believe this area was filled with salty sea water, much like the Great Salt Lake in Salt Lake City, Utah.
Through several climate changes, a process of layering began, leaving deposits of salt and other minerals behind in Michigan.
These salt deposits were covered with other sediments over several millions of years, forming alternating layers of rocks.