Tertiary Volcanic Rocks (older than 5 m.y.) of the Beaver Creek Watershed are mainly basalt and andesite flows. The volcanism in the area is related to crustal extension that occurred as the Verde Valley subsided relative to the Colorado Plateau (hence the formation of a lake in which the Verde Formation was deposited). The volcanic rocks arefaulted and fractured to some extent, allowing some water to infiltrate to lower sedimentary aquifer units; however, in some places they contain mudstone or ash beds that inhibit the downward movement of water and result in a locally perched aquifer.

 

Reference:

Owen-Joyce, S.J. and Bell, C.K., 1983 Appraisal of Water Resources in the Upper Verde River Area, Yavapai County, Arizona, Arizona Department of Water Resources Bulletin 2 (Prepared by the U.S. Geological Survey), available online at: http://www.verde.org/usgs/usgsstud.html.