Essentially true. Apparently Michigan's stone goes down in layers almost a mile before it finally gets to non-sedimentary rock; the Detroit River Group in particular is known for its salt formations, which is why a quarter-mile directly beneath Detroit, they're chopping salt out of the ground. It doesn't mean there's no rock, though - just that it ain't 'solid, heat-treated' igneous or metamorphic rock. It's otherwise perfectly solid to build on.
This actually makes for some interesting reading for me; in one of my games / pieces of fiction, I placed a superhero school 'somewhere north-west of Detroit', and now I know how - and why - the area can serve as the hub of a 'hidden' transport system to the classified location. (The aliens had attacked Detroit, but either got driven off or else retreated after acquiring whatever it was they came for; the central part of the wrecked area became dedicated to teaching the teen supers how to fight in built-up areas, which is where the aliens are concentrated. The shaft to the high-speed rail system to and from the school descended at the edge of the combat area ... though only 194' below-ground, while the salt mines are 1200' down. Food for my thought.
