Oregon was never under a continuous coat of ice during the Pleistocene epoch, as was much of continental North America. At this time glaciers formed on Mount Hood, Mount Jefferson, the Three Sisters, and sky-piercing Mount Mazama in southern Oregon, and were scattered through eastern Oregon and along the Columbia River gorge. Among the largest moraines is a lateral one on the east side of Wallowa Lake, in extreme northeastern Oregon. It is approximately six miles long, one-fourth of a mile wide, and between six hundred and seven hundred feet high.
An event of importance at the close of the ice age was the violent eruption of Mount Mazama, which either blew up, scattering its substance over the surrounding countryside, or collapsed and fell into its own crater. Perhaps both explosion and collapse occurred. This cataclysm resulted in the formation of the huge caldera now occupied by Crater Lake.
Another period of land depression followed, during which Oregon lost still more of its western coastal area. The Willamette Valley became a sound or fresh-water lake formed by the damming of the Columbia by ice, at which time water flowed 300 feet above the present level of Portland, 165 feet above that of Salem, and 115 feet above that of Albany. An important development of this period was the fault ing in the Great Basin area of southeastern Oregon, when the impos ing Steens and Abert Rim Mountains were formed.
During recent time, deposits found in Oregon have included stream gravels, silt washed from the valley sides, dunes along the coast and in the lake region of eastern Oregon, peat bogs in the coastal dune area, volcanic deposits in the Cascade Mountains, shore deposits along the beaches, and many others. The shifting dune sands damming sluggish streams have created a chain of beautiful fresh-water lakes along the ocean shore.
In many parts of the Cascade Mountains there are cinder cones that have the appearance of recent origin. Some of them may be not more than a hundred years old. The Portland Oregonian reported an eruption of Mount Hood as late as 1865.
Since 1862, when Dr. Thomas Condon, Oregon's noted pioneer geologist, discovered and made known to the world the now famous fossil beds of the John Day Valley, Oregon has been an important center for paleontological research. Exploration has been rewarded by yields of a number of the most highly prized specimens of prehistoric plant and animal life uncovered in the United States, and has revealed
