Northwest of Thorp at the junction of SR 10 and Thorp Highway, the Yakima River emerges from a canyon parallel to a basalt flow, the uppermost layers of which have been dated to 10.5 million years. The Thorp Prairie sits atop the basalt flows and ends at a deep canyon of Miocene columnar basalt structures carved by Swauk Creek whose headwaters are at Blewett Pass along US 97 to the north. The Thorp Prairie deposits were also delivered by the Thorp Glacial episode.