Millions of years ago, this area lay on the edge of a huge plain bordered by distant mountains, with swamps of cypress trees, hills and mountains covered by maple, sycamore, ginkgo, sequoia, and fir, and ancestors of rhinoceroses and camels feeding on lush vegetation. That landscape was repeatedly transformed when lava welled up from within the earth and rushed across the land, covering tens of thousands of square miles and burying forests, rivers, and swamps before cooling into basalt. Scientific evidence indicates that such events occurred repeatedly between 14 and 18 million years ago. More than fifty lava flows lie beneath this place, each having gushed from cracks in the earth more than 100 miles to the east. The uppermost, the Kelly Hollow flow, is nearly 200 feet thick here and continues nearly 200 miles southwest to the Oregon Coast. Farther east, layers of basalt appear in road cuts and canyon walls, ancient trees entombed in basalt can be seen at Ginkgo Petrified Forest State Park, and beyond the park the basaltic cliffs of the Columbia River were carved by Ice Age floods.