Geological mapping, including section measurement, in eastern Luzon Central Valley revealed six (6) sedimentary formations including five (5) member units disposed in an almost continuous north-south trending exposure belt along the foothills of the Sierra Madre Range. The section represents a more or less continuous depositional sequence within the outer neritic to bathyal zones from Early to Late Miocene and lagoonal to shallow marine to continental deposition from Pliocene to Pleistocene.

No structure of regional significance was mapped however, minor and localized disturbances are evident in small folds and faults manifested in the section.

A summary of the inferred geologic history is given.

The short-term cycles are demonstrated by the cyclic occurrences of transgression sedimentation followed by regression and orogenic movement yielding mountain systems and (or) elements of island arcs and emplacement at near surface of quasi-solid plutons, usually within geosyncline/platform background.

The span of time covering the better known Mesozoic-Tertiary geotectonic cycle is about 100 to 130 million years while that of the Paleozoic seems much longer. The short-term cycles involved periods of 10 to 60 million years becoming shorter from geosynclinal to platform stage.

The development of various cycles of activity are considered effects of reorganization of the earth’s interior region which seems to have coeval relationship to the earth’s orbital movement around the center of the Milky Way galaxy.

The essential differences of crustal development between the Paleozoic anal the Mesozoic-Tertiary geotectonic cycles may also be premised on the possible differences in orbital radius and speed of travel of the earth during the two cycles.

The recognized formation of Barrovian type metamorphics and granites during Paleozoic, being of an earlier orbital cycle, could have been due to weaker gravitational field setting and slower pace of orbital travel of the earth. Under such conditions conducive to more pronounced magmatic differentiation, anatexis and regional metamorphism even at shallow depths were developed in the presence of strong contractive force and greater hydrostatic pressure.