The Eocene Drum Mountains Rhyodacite is a heterogeneous assemblage of dark-colored flows, breccia, and tuff exposed at many places in the Drum Mountains, around Spor Mountain, and in the Black Rock Hills, located northwest of Spor Mountain. The rhyodacite is the oldest volcanic formation recognized in the vicinity of Spor Mountain.

SLIDE 18 shows vertical-layered flow rock (beneath hat) and breccia (to right) near the south end of The Dell.