Located within the Blue Joint Wilderness Study Area, Castle Rock is easily one of the most recognizable features in the Magruder Corridor. The bare prominence stands boldly above the ridge that stems southward from Nez Perce Pass. The actual “castle” of Castle Rock is a mass of porphyritic andesite, a highly durable volcanic stone. So, the core of Castle Rock is a long extinct volcano from which the outer layers have long since eroded away leaving the andesite core.
The hike goes generally upwards until you reach the base of Castle Rock with first 1/2 mile steeply ascending to a subtle junction with the Castle Rock trail, which is to the left. A small sign several yards up this trail simply states “Castle Rock”. The path cuts through a rather pleasant meadow and along a ridge. At the base of Castle Rock, several widely-spaced cairns mark the path to the summit (look for the arrow cairn). From here, we picked our way on the talus and climbed a couple of small cliffs. Once on top, a sizable cairn marks the summit. A simple strand of prayer flags had been placed on top of the cairn.
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Porphyritic andesite
Andesites are fine-grained, extrusive igneous rocks that are usually light to dark gray in color. Porphyritic refers to andesites that have two crystal sizes. When the magma erupts, large crystals form slowly at depth (known as “phenocrysts”), and small crystals form quickly at the surface. Geek moment over.
Magruder Corridor
