Twenty-five million years ago, this area had meandering streams and lush, rolling hills until lava began flooding the land through long cracks or vents, cooling into basalt before lakes, swamps, and streams reformed on the new flat surface and were covered again by later flows. The intervals between eruptions ranged from months to hundreds or thousands of years, and this cycle continued for 15 to 20 million years, creating basalt layers that are part of one of the world’s largest lava fields, covering more than 200,000 square miles in Washington, Oregon, and Idaho and reaching more than 10,000 feet thick in places. Near the end of this cycle, uplift associated with the forming Cascade Mountains and pressure beneath the earth’s crust warped the once-flat basalt into hills, including the folded structure known as the Umtanum Anticline. The rise of the Cascades also changed the Columbia Basin into a semiarid plain by blocking moist ocean winds. Before that uplift, the Yakima River meandered across the landscape, and as the land slowly rose across its path, the river eroded downward at about the same rate, cutting a steep, narrow gorge while preserving its earlier bends and exposing basalt layers that record the lava floods that created the Columbia Basin.