Georef (Online)
UIUC access only. Index to articles, books and other publications on geology and earth sciences
National Geologic Map Database (NGMDB)
Information about over 90,000 products by more than 600 publishers including federal and state agencies, organizations and societies, universities, and the private sector
National Cooperative Geologic Mapping Program
Introduction to Geologic Mapping from the USGS
Maps from the USGS
Geologic Maps of the US States from the USGS including Illinois Geologic Map Data
USGS Publications Warehouse
Access to over 160,000 USGS publications
Geology Research Guide from Humboldt State University