A thick sequence of intercalated andesite and basalt flows, tuffs, and volcanic sediments crop out in the Fly Creek Quadrangle and the north half of the Round Butte Dam Quadrangle. This sequence was deposited in the ancestral Deschutes River Valley along the east flank of the Cascade Range during the Pliocene and the Recent. These rocks are relative flat lying and appear to have been undisturbed
since the time of their implacement. An older (pre-Pliocene) andesite characterized by phenocrysts of basaltic hornblende crops out in the northeast corner of the area and serves as a basement rock. Younger basaltic andesites which are mineralogically similar to the hypersthene andesites exposed in the High Cascade Range to the west interfinger with the olivine basalt flows, tuffs, and sediments of the Madras Formation along the western edge of the Fly Creek Quadrangle. Olivine basalt flows similar to those intercalated in the Madras Formation overlie this sequence with no apparent unconformity in the eastern two-thirds of the area. Younger olivine basalts characterized by a diktytaxtic texture are intracanyon to the Madras Formation in the Deschutes, Crooked, and Metolius River canyons.
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