National Geologic Map Database...

can be found at this link and looks like it will be a very useful tool. Here's the description from the web site:

"Welcome to our Data Portal. It is intended to: 1) give you a quick browse and query of our Nation's geologic maps, in a standardized format, and 2) link you to the source information found via the NGMDB Map Catalog and Geologic Names Lexicon. More than 80,000 maps and reports, by 370 publishers, are accessible. We anticipate expanding the Portal this year, and welcome your comments."