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 to 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.