Cagayan has abundant water resources. This include the Philippine territorial waters of the South China Sea and the Pacific Ocean, with their bays, breakwaters, fishing banks, capes and points; the major rivers found in the province, their tributaries, and several creeks; and Cagayan's hydrosol and ground water. The whole province has artesian aquifers at 12 to 90 meters, which are recharged continually, especially in areas with alluvial bedrock whose permeability is high.
The bedrock foundation of the slopes of Cagayan is miocene to pliocene sedimentary bedding (formed 7 to 25 million years ago) which includes shale, sandstone, siltstone and limestone. Remnants of coral and other marine organisms are present in the bedding. The Sierra Madres and the foothills of the Cordilleras, however have Cretaceous bedding (formed in the Age of the Dinosaurs, some 135 million years ago), made of very extensive volcanic rocks composed mainly of coarse-grained igneous rock.
The lower relief areas of the province, found mainly along the rivers and creeks, have a bedding made of recent alluvium (quaternary to recent, formed some 0.01 to 2.5 million years ago). Composed of unconsolidated clay, silt, sand, gravel, cobble with occasional boulders derived from weathering and erosion of pre-existing rocks deposited in the floodplains, riverbeds and banks, and valley floor. Interspersed between the recent alluvium and the slopes are interlocking patches of Pliocene to Pleistocene bedding (formed 2.5 to 7 million years ago) of shale, sandstone, mudstone sequence and some corralline limestone.
Abutting the province of Isabela are small tongues of oligocene to miocene bedding (formed 25 to 36 million years ago) of shale and sandstone. This are found in the southern parts of Rizal, Tuao, and Peņablanca.
Along the northern coast in Sta. Praxedes, Claveria, Sanchez Mira and Sta. Ana are patches of miocene bedrock formation of lava flows of basalt-andesite series with pyroclastics. A patch of quarternary to recent igneous formation of pyroclastics is found in Gonzaga. It is composed mainly of volcanic ash, sand and boulders. Pyroclastics and quarternary volcanics are also found in the Babuyan group of islands, where the only recent alluvium found in the islands, are strips found along the seashore. These bedrock foundations are quite expected of the province, being located on the Pacific Ocean side of the country, and the Philippines belongs to the so-called "Pacific Rim of Fire," one of the earth's great belts of active volcanoes.
On the whole, the province is spared of major faultlines. Only the protruding west portion of the town of Rizal along the Cordilleras falls along the Digdig Fault which is considered the country's most active as manifested by the 1990 killer earthquake.