The region was volcanically active from approximately 35 to 17 million years ago. The province is characterized as an older, deeply eroded volcanic range lying west of the more recent snow-covered High Cascade Range. They range in elevation from 1,700 feet (520 m) on the western margin to 5,800 feet (1,800 m) on the eastern margin. The Western Cascades began to form 40 million years ago with eruptions from a chain of volcanoes near the Eocene shoreline. As the regional angle of subduction steepened, volcanic activity gradually shifted to the east in the Miocene and Pliocene.
The Western Cascades are made up almost entirely of slightly deformed and partly altered volcanic flows and pyroclastic rocks which range in age from late Eocene to late Miocene. These rocks have been heavily dissected by erosion and the only evidence remaining of the many volcanoes from which they were erupted are occasional remnants of volcanic necks or plugs which mark former vents. There are also minor Pliocene to Pleistocene intracanyon lavas derived from the High Cascades or rare local vents. From youngest to oldest, the Western Cascade Range consists of four main units:
- 0- to 4-million-year-old intracanyon basalt and basaltic andesite flows from the High Cascade Range.
- 4- to 9-million-year-old basalt and basaltic andesite flows with lesser amounts of andesite and dacite. These rocks which cap the highest Western Cascade ridges were erupted prior to uplift and faulting along the east margin of the Western Cascade Range. This unit is compositionally identical to unit 1 and other rocks of the High Cascade Range, and therefore was probably erupted from vents in and adjacent to the High Cascades.
- 9- to 18-million-year-old basalt, basaltic andesite, and andesite lavas with lesser amounts of dacite tuffs and lavas.
- 18- to 40-million-year-old silicic tuffs and lavas. Units 3 and 4 were erupted from many vents west of known High Cascade volcanic centers, although some of these older vents may be buried beneath the High Cascade Range.