Guam consists of two geologically distinct parts. Northern Guam is an undulating limestone plateau, and Southern Guam is a rugged volcanic highland with some limestone outliers. This digital atlas focuses on the southern half of Guam.

Most of the southern Guam is covered by volcanic rocks that form the island's foundation. These rocks are locally overlain by limestone. The oldest exposed rocks on the island are of Eocene and Oligocene age and comprise the Facpi and Alutom Formations. They crop out in the highlands from central to southern Guam and underlie all other exposed rock units (Tracey and others, 1964; Reagan and Meijer, 1984). They contain a series of pillow basalts and pyroclastic rocks of volcanic origin, and range from tuffaceous shale to coarse boulder conglomerate and breccia.

Separate volcanic rocks of Oligocene to late Miocene age comprise the Umatac Formation and lay on top of the Alutom Formation. They crop out principally in the south-central highlands and plateaus and contain reef and forereef limestone, tuff breccia and volcanic conglomerate, and basalt flows (Meijer and others, 1983; Reagan and Meijer, 1984).

Volcanic rocks of southern Guam are locally overlain by limestone. The top of the mountainous ridge and central basin are covered by old limestone units. They are Miocene to Pliocene age and are known as Bonya and Alifan Limestone. Eastern coast and Orote Peninsula comprise of younger limestone. It is Pliocene to Pleistocene age and is called Mariana Limestone. This limestone is clay-rich in the vicinity of volcanic uplands.

Finally, there are minor reef limestone, beach deposits, and alluvium of Holocene age. The beach deposits are composed of poorly consolidated calcareous sand and gravel or volcanic sand. Alluvial deposits fill stream valleys and cover parts of the coastal lowlands.