The principal geologic reference for Guam is the 1964 report by the United States Geological Survey:
Tracey, J.I., Jr., Schlanger, S.O., Stark, J.T., Doan, D.B. and May, H.G., 1964. General Geology of Guam. U.S. Geological Survey Professional Paper 403- A, U.S. Dept. of the Interior, Geological Survey. 104pp.
The following are standard abbreviations and color labels of different rock formations as used on the geologic map accompanying that report: