| Summary: | The Agua Fria River area is part of a mass of crystalline Precambrian rocks covered, in several places, by Cenozoic volcanic and sedimentary rocks. The volcanic rocks and the sediments are largely the products of eruptions in middle (?) Cenozoic time. The rocks are sequences of volcanic flows of basaltic and andesitic composition, conglomerates, and interlayered tuffaceous sandstones. In places the cover of Cenozoic rocks was apparently in excess of 2000 feet, but erosion has removed much of this material. Pleistocene or Recent gravels, terrace deposits, and dissected alluvial fans are unconformable on the Cenozoic rocks throughout the area. The Cenozoic rocks are nearly horizontal; some have a slight dip to the northeast. At the southern end of the area are broad, open anticlines and syndines which are thought to be marginal features of a major shear zone which lies to the south of the area. The source of all the volcanic debris is unknown. An andesite plug and three basaltic dikes in the southern part of the area served as a source for at least some of the material. Some of the Cenozoic deposits seem to have been -deposited on an erosion surface that had hundreds of feet of local relief. Little is known of the time relationships of the various Cenozoic rocks in the area for none has yielded datable fossils. By lithologic and structural analogy, the Cenozoic rocks of the Agua Fria River area have been assigned tentatively to middle and/or upper Mioceno and lower Pliocene.
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