I saw these links posted on Reddit (/r/geology), and wanted to post them here for future reference. Here are some great resources for geological and other science illustration:
Geologic Map Patterns for Canvas (R), Illustrator (R) and Freehand (R) , from Andreas Plesch and the USGS
Integration and Application Network (IAN) symbol library, which contains 2604 science/ecology symbols (vector format), including lots of geologic environments.