The Tertiary Period, 65 to 2 million years ago, extends from the demise of the dinosaurs to the onset of the Pleistocene Ice Age. Bedrock from the Period, generally consisting of soft sediments or hardened lava, are found primarily from the High Plains westward; an exception is the limestone of north-central Florida, deposited on the Florida Platform during the early to mid Tertiary.
Tertiary sediments of the High Plains, typified by the Badlands of South Dakota, consist of erosional debris from the Rocky Mountains and windblown dust from the volcanic regions of the West; among the later were the San Juan volcanoes of southwest Colorado, the volcanic fields of northern New Mexico/Arizona and the Cascades of the Pacific Northwest. West of the Rockies, Great Lakes covered the tristate region of Wyoming, Utah and Colorado early in the Tertiary, depositing the famous Green River formation; uplifted later in the Period, this formation has been sculpted into the high mesas of that region. Some of these mesas, capped by basalt flows, have resisted erosion and retain high elevations (Grand Mesa and Battlement Mesa, in western Colorado, are fine examples).
Many of the intermountain basins and valleys are filled with thick deposits of erosional and volcanic debris from the Rockies, Sierra Nevada and volcanic ranges. Gravel beds on the west flank of the Sierra, deposited by Tertiary Rivers, were a prime source of gold during the California Gold Rush. In southern Colorado, the San Luis Valley formed as the Rio Grande rift developed late in the Period; today, it is filled with more than 10,000 feet of erosional and volcanic Tertiary sediments. Finally, volcanic laccoliths, intruded beneath the surface, were raised with the surrounding bedrock during the Miocene-Pliocene Uplift; freed from the encasing sediments by erosion, they now stand as prominant mountain clusters across the Colorado Plateau (the LaSal, Abajo and Henry Mountains of Utah are prime examples).
