One of the most humorous landmarks encountered by our riders is a unique formation near Beaumont known locally as "Butt Rock." I had seen this rock many times before, but had totally forgotten about it and that it would be visible on this particular ride, which was a loop through Idyllwild. Butt Rock is located in Lamb Canyon near the top of a ridge on the west side of CA-79 (Beaumont Avenue), about 4½ miles south of the City of Beaumont, and can be seen when driving south from Beaumont at the last major curve in the road before Gillman Springs Road. It brings an immediate smile to everyone's face as they look up to see this huge rock mooning them from above.
Butt Rock is part of the Mount Eden Formation of upper Miocene geologic age. More specifically, it is a large boulder that has become exposed due to erosion of a boulder conglomerate lens within the formation located along the top of the ridge, and is composed of light gray quartz diorite, a rock type that is very resistant to the effects of weathering. The boulder conglomerate lens is, actually, an elevated erosional remnant of an ancient alluvial fan deposit derived from the San Jacinto Mountains to the east. The boulders, and Butt Rock, are the same type of rock that comprises Mount San Jacinto.
Butt Rock is not officially designated on any maps and is not easy to find. The inset map shows the approximate location. Every time I see Butt Rock, it seems to me that it would be more apropos to officially name it "Plumber's Rock."