While none of the sections are set up to replace textbooks, many of the guides in their extensive homework help section have written the equivalent of introductory textbooks, broken up into manageable articles and accompanied by extensive links. Among the best is the Geology section. Guide Andrew Alden has written hundreds of informative articles which will help students and amateur geologists at any level. He also has thousands of photographs, a huge collection of maps, and more than 700 carefully selected links. This site could easily replace several introductory geology textbooks.