 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.North and northeast of the town of Thorp along the Yakima River channel is the gradual upward lift of the Thorp Drift, marked by an elevation change due to the incline onto the terminal moraine that marks the furthest advance of the Thorp Glacial stage.