Wheeler Geologic Monument Wheeler Geologic Monument (Half Moon Pass quadrangle) provides exceptional exposures of the three outflow tuff sheets erupted from the San Luis caldera complex. Lowest sheet is the Rat Creek Tuff, which is nonwelded throughout, but grades upward from light tan rhyolite (approx. 74% SiO2) into pale brown dacite (approx. 66% SiO2) that contains sparse dark brown andesite scoria. The distinctive hornblende-rich middle Cebolla Creek Tuff contains basal surge beds, overlain by vitrophyre of uniform mafic dacite that becomes less welded upward. The uppermost Nelson Mountain Tuff consists of nonwelded to weakly welded crystal-poor rhyolite, which grades upward to a densely welded caprock of crystal-rich dacite (approx. 68% SiO2). White arrows show contacts between outflow units. Photograph by P.W. Lipman.