The Eastern Cagayan region, as treated in this report, includes all the municipalities east of the Cagayan River from Tuguegarao to Buguey in Cagayan province and west of the Sierra Madre mountain range, Intermediate to basic volcanic rocks and rnetasedimentary rocks of probable pre-Tertiary age constitute the basement upon which three Miocene and one Pliocene sedimentary formational rock units were laid. Comprising the sedimentary rocks in conformable sequence are the Lubuagan coal measures, composed of sandstone, shale, limestone, conglomerate and some coal lenses; the Callao formation, subdivided into a limestone and clastic member, composed of coralline limestone^, sandy limestone and interbedded sandstone, shale and conglomerate; the Baliwag formation, composed of shale and mudstone with minor interbeds of sandstone; and the Ilagan sandstone (Pliocene), composed mostly of sandstone with interbeds of shale and some lenses of conglomerate.

The region is a part of the eastern side of a structural basin in which the Cagayan valley is disposed. Superimposed upon the basin are minor folds that trend and plunge generally to the west. The region is traversed by faults in several places, but northeast-northwest faults constitute the most prominent set. Major faults are found only on the northern part of the region at the vicinity of the Casigayan nose.