State Route 36 passes through Fredonyer Pass. The pass is west of Susanville, California and southeast of Mount Lassen and is the route's highest point. The pass is an approximate northern boundary between the Sierra Nevada and the Cascade Range. This irregular boundary is sometimes defined as the southern extent of Cenozoic igneous surface rock from the Cascade range This boundary roughly follows the drainage of the North fork of the Feather River southwest from Fredonyer Pass. Note that there are other Cenozoic igneous rocks in the Sierra (e.g., near Lake Tahoe), but there is a clear geological division near Fredonyer Pass.