Geologic studies indicate that the earliest rocks in this part of the High Cascades began forming when a chain of volcanoes erupted between five and three million years ago. During the last glacial period, the composite volcanoes of Mount Mazama and Mount McLoughlin began their initial build-up. On their north and east slopes, Mount McLoughlin and other peaks bear scars caused by glaciers. Like most other major drainages within the Sky Lakes Wilderness, Seven Lakes Basin and the deep canyon of the Middle Fork of the Rogue River were carved by the massive ice fields that covered the highest elevations of the Cascades. Approximately 6,700 years ago, when Mount Mazama exploded, collapsed, and formed the caldera of Crater Lake, some of the vast amount of rock and ash thrown into the air landed in the northern portion of the Sky Lakes Wilderness, creating the pumice-covered Oregon Desert.