Paleontology and geology
During the Late Quaternary (Pleistocene), glaciation reached its maximum and large ice caps covered the mountainous regions of the state, while most of the lowland and coastal regions remained unglaciated and were covered by tundra and boreal forests. Arid sagebrush plains were widespread in eastern Oregon, while oak savanna typified the areas of the Willamette and other western Oregon valleys. These varied environments provided a rich habitat for mammoths, ground sloths, and bighorn sheep. The volcanoes of the Cascades, located in the central portion of the state, erupted intermittently.