Tertiary sources are excellent starting points!  They consist of information synthesized from primary and secondary sources.  Examples include:

• Almanacs
• Chronologies
• Dictionaries and Encyclopedias
• Directories
• Fact books
• Guidebooks
• Indexes, abstracts, bibliographies used to locate primary and secondary sources
• Manuals
• Textbooks

These resources give you succinct overviews of your topic, explain scholarly arguments, point out interesting questions, and refer you to especially key sources. 

Start your search for background information with these resources. Gale Virtual Reference Library includes the full text of over 100 subject encyclopedias spanning multiple disciplines, including the Encyclopedia of Geology.