This is a very comprehensive web-based geology textbook. In scope, coverage, and topics covered, it is comparable to non-OER textbooks. Geology is a very visual science and many of the figures and photographs of this textbook are excellent.
The main topics generally covered in an introductory geology class are covered. The textbook also features "did I get it" quizzes at the end of many sections within a chapter, as well as review quizzes at the end of chapters. These quizzes are instantaneously checked and are a great interactive feature.
The textbook also has an online glossary that is very comprehensive.