The Lower Verde River Valley is a discrete groundwater basin separated from the Phoenix basin by the McDowell Mountains and bounded on all sides by bedrock, (see figure 1). The basin formed during the late Miocene to Pliocene in response to crustal extension during the Basin and Range orogeny. The perennial Verde River flows southward across the valley from Bartlett Dam in the north to its confluence with the Salt River on the east side of Mount McDowell in the south. Two major ephemeral streams join the Verde River within the valley. Camp Creek drains the northwest part of the basin near Humbolt Mesa and enters the Verde River at Needle Rock. Sycamore Creek drains the northeast part of the basin, on the west side of the Mazatzal Mountains, and enters the Verde just north of Fort McDowell. Both streams typically show no surface flow except during periods of extended and heavy rainfall. The purpose of this study is to try to delineate the subsurface geology beneath the Town of Fountain Hills and the Lower Verde River Valley in order to place constraints on the shape and distribution of the groundwater reservoir(s). The surface geology was compiled mostly from recent geologic mapping by workers at the Arizona Geological Survey (see previous work section below) and by students from Arizona State University. The subsurface geology was interpreted from examination of over two hundred well logs obtained from the Arizona Department of Water Resources, combined with structural interpretations based on observed surface geology and geophysical data.