Database of geological group or formation exposures? Is there any sort of database that provides the locations of exposures of certain geologic groups or formations?

I ask because I have read that halite occurs in the Michigan Basin, and specifically occurs in beds of the Salina Formation (Silurian) and the Detroit River Group (Devonian).  I would love to find a source (preferably a map) that provides locations where these features (or really, halite) are at or near the surface.

Thanks.
You may also find localised geological guides, such as the Roadside Geology series.

http://geology.com/store/roadside-geology.shtml has a listing. While I have not used them, I have seen them crop up fairly often as a nice option when looking for interesting geology in more general terms, including for some outcrops.

(I would have made this a comment, rather than an answer, but have no rep yet.)