Abundant late Oligocene paleosols in eastern Oregon compose a paleoclimatic archive rivaling the resolution of deep-sea cores, recording 105 Milankovitch-scale cycles over the 5.1 m.y. duration of the middle John Day Formation. Paleoclimatic cycles are apparent from the fossil record of snails, mammals, trace fossils, soil structure, depth to calcic horizon of paleosols, and carbon and oxygen isotopic composition of pedogenic carbonate. Interpreted Oligocene alternation between semiarid sagebrush steppe and subhumid wooded grassland has the same amplitude as that inferred during accumulation of the Quaternary Palouse Loess in Washington and Oregon. This similar amplitude is surprising because large ice caps like those of the Quaternary did not extend across North America or Europe during the Oligocene. Thus ice-albedo amplification of Milankovitch-scale insolation variation cannot explain the similar magnitude of Oligocene paleoclimatic fluctuation. Weak orbital signals were more likely amplified by greenhouse gases such as CO2 and CH4 due to changing carbon budgets in the sea and on land.