 About west of Thorp, the first glimpses of the Columbia River Plateau are seen where the Yakima River has cut into the westernmost edge of the basalt plateau.The Columbia Plateau basalt formed when lava poured out of fissures in the ground across eastern Washington during the Miocene era, 17 to 20 million years ago, erupting intermittently for over 10 million years.Many layers of basalt successively flowed over one another, back when the area was still flat.