Pennsylvanian strata of western Pennsylvania exhibit evidence of a hierarchy of paleoclimatic changes. Long-term (10⁷ years) climate trends reflect plate movement and tectonic events. These long-term trends are overprinted by changes of much shorter duration (100–400 k.y., and 10–20 k.y.). During deposition of the Pottsville and Allegheny formations (Bashkirian-Moscovian), the Appalachian climate exhibited perhumid to humid situations during periods of glacial advance, and humid to dry subhumid conditions during glacial retreats. Marine faunas and coal swamp floras during this interval of time exhibited a remarkably consistent taxonomic and ecological structure. Tetrapod amphibian faunas were highly aquatic. When the Cone-maugh Group was deposited, the ancient Appalachian climate became progressively drier. Glacial stages were dry subhumid and during deglaciation semiarid to arid. This reduction in precipitation produced changes in coal-forming floras, as lycopsid-dominated assemblages gave way to tree fern–dominated associations. Coincident with this climatic drying, tetrapod faunas became highly terrestrial in the basin. During the deposition of the Monongahela Group, the Appalachian climate returned to humid conditions during glacial periods. However, there is evidence of drier subhu-mid conditions during the intervening interglacial episodes as indicated by the pervasive presence of mudcracked nonmarine limestones. Nested lacustrine cycles within the Monongahela Group indicate short-term alternations between wet and dry periods that may have been driven by Earth’s precession. Coal-forming mires continued to be dominated by tree ferns, and vertebrate faunas tended to be found within fluvial lake environments. The latest Pennsylvanian and/or early Permian strata exhibit a return to Conemaugh-like deposition as evidenced by the pervasiveness of redbeds, dry climate floras, and highly terrestrial vertebrate faunas.