In Oregon, the young terrane is commonly called the High Cascades and the old terrane the Western Cascades, terms that reflect present physiography and geography. The terms are not useful in Washington, where young vents are scattered across the dominantly middle Miocene and older terrane. The High Cascades subprovince is built of rocks mainly younger than 3.5 million years and is the modern Cascade Range volcanic arc. In contrast, the Western Cascades encompass a deeply eroded pile of chiefly Oligocene to Pliocene volcanic and volcaniclastic rocks. The High Cascades in the areas south of Mount Jefferson to Santiam Pass is a broad ridge built up by several shield volcanoes and numerous cinder cones.
Most summits mark either relatively young vents or deeply eroded vent complexes.
Reversed polarized basaltic andesite, basalt, and andesite lava older than 0.73 million years are exposed in the walls of U-shaped canyons, but most of the area is mantled my normally polarized rocks younger than 0.73 million years. Most of the area is within the Mount Jefferson Wilderness.