 In southeastern Washington, eastern Oregon, and southern Idaho, flood basalt flows of extremely fluid basaltic lava spread in all directions from long fissures, building broad fields of gently sloping lava that spread widely over great distances.Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows have been extensively exposed by erosion (with extensive displays of columnar basalt), and measure almost 2 km (1.24 mi) in total thickness.The basalts flows which are visible in the Wallula Gap were laid down during the Miocene Epoch, about 17 million years ago.