ABSTRACT The John Day Formation of central and eastern Oregon, contains a widespread assemblage of both ash-flow and airfall tuffs, yet only a few corresponding caldera sources have been identified in the region. Investigators have long speculated on the sources of tuffs in the John Day Formation and have suggested that these pyroclastic rocks were vented from now buried eruptive centers in or marginal to a nascent Cascade Range. Recent detailed geologic mapping in the John Day and Clarno Formations, however, indicates the presence of at least three large-scale rhyolite caldera complexes centered along the northeast-trending axis of the Blue Mountains. This field guide describes a three-day geologic transect, from the scenic high desert of central Oregon eastward across the axis of the Blue Mountains, that will examine the physical volcanology and geologic setting of the 41.50-39.35 Ma Wildcat Mountain caldera exposed along the crest of the Ochoco Mountains, the 29.56 Ma Crooked River caldera at Prineville, and the 29.8 to 28.1 Ma Tower Mountain caldera near Ukiah.

Abstract The Painted Hills unit, formerly Painted Hills State Park, lies along Bridge Creek about 9.5 mi (16 km)northwest of Mitchell, Wheeler County, Oregon, near the intersection of T.1O and 11S. and R.20 and 21E. Itcan be reached by taking the Bridge Creek road north from U.S. 26, which runs between Prineville and Mitchell (Fig. 1). The turnoff is located 4 mi (6.5 km) west of Mitchell and is marked by a sign directing travelers to the Painted Hills. The Bridge Creek road is paved and is passable in all weather by passenger cars.

Abstract The Painted Hills unit, formerly Painted Hills State Park, lies along Bridge Creek about 9.5 mi (16 km)northwest of Mitchell, Wheeler County, Oregon, near the intersection of T.1O and 11S. and R.20 and 21E. Itcan be reached by taking the Bridge Creek road north from U.S. 26, which runs between Prineville and Mitchell (Fig. 1). The turnoff is located 4 mi (6.5 km) west of Mitchell and is marked by a sign directing travelers to the Painted Hills. The Bridge Creek road is paved and is passable in all weather by passenger cars.

ABSTRACT This fi eld trip guide explores the interactions among the geologic evolution, hydrology, and fluvial geomorphology of the central Oregon Cascade Range. Key topics include the geologic control of hydrologic regimes on both the wet and dry sides of the Cascade Range crest, groundwater dynamics and interaction between surface and groundwater in young volcanic arcs, and interactions between rivers and lava flows. As we trace the Willamette and McKenzie Rivers back to source springs high in the young volcanic rocks of the Cascade Range, there is abundant evidence for the large permeability of young lava flows, as manifested in streams that dewater into lava flows, lava-dammed lakes in closed basins, and rivers that emerge from single springs. These dynamics contrast sharply with the older, lower permeability Western Cascades terrane and associated runoff-dominated fluvial systems. On the east side of the Cascades we encounter similar hydrologic characteristics resulting in complex interactions between surface water and groundwater as we follow the Deschutes River downstream to its confluence with the Crooked River. Here, deep canyons have cut through most of the permeable part of the geologic section, have been invaded by multiple large intracanyon lava flows, and are the locus of substantial regional groundwater discharge. The groundwater and surface-water interaction in the Deschutes Basin is further complicated by surface-water diversions and an extensive network of leaking irrigation canals. Our west-to-east transect offers an unparalleled opportunity to examine the co-evolution of the geology and hydrology of an active volcanic arc.