Cenozoic eruptive rocks and terrestrial sediments were deposited on a structurally complex basement of Precambrian, Paleozoic, and Mesozoic rocks in the Yellowstone Park region. Eruption of lower Cenozoic andesitic breccias and tuffs appears o t have been cyclic, for the pyroclastic rocks become more mafic up-section with an increase in the relative abundance of breccia beds and decrease in tuff units. As each cycle proceeded, less silicic and more mafic material was ejected. The writer suggests that the cycles represent periodic tapping and drawing off of an upper, volatile-rich fraction of the source magma. A major hiatus separates the lower Cenozoic rocks from upper Cenozoic rhyolites and associated rocks in northern Yellowstone Park which are believed to be Pliocene and younger.

At least five phases of deformation can be recognized in the map area: (1) Precambrian metamorphism, (2) Lancian to early Eocene Laramide folding and faulting, (3) middle Eocene fracturing and normal faulting, (4) post-middle Eocene prelate(?) Pliocene normal faulting, and (5) post-late(?) Pliocene normal faulting of rhyolitic welded tuffs.

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