From Miller and Richter (1994) 
: "The Skookum Creek eruptive center is an erosionally dissected complex that may, in part, be as young as Quaternary. It consists principally of a series of rhyolite, rhyodacite, and andesite domes and their associated pyroclastic deposits, with an age of about 3.7 Ma, and an extensive sequence of relatively flat-lying andesite flows, some of which have been dated at 2.8 Ma 
. Both the domes and the flows are intruded by a few rhyodacite and andesitic dikes that appear to originate from a rhyodacite dome near the center of the complex. Relations between the volcanic units of the complex suggest that the flat-lying flows fill a caldera that is defined by the crude arcuate alignment of the domes."