The Painted Hills preserve a record of long-term climate change and shorter-term fluctuations, with the hills’ stripes showing warmer, wetter conditions in the more frequent red bands near the bottom and increasingly drier conditions in the yellow soils that gradually replace them, until higher rock layers lose the reds completely and leave the yellows and tans that dominate Carroll Rim. The red hill layers reflect a paleoenvironment very different from the semitropical jungle of deeper and older rock layers, as temperature and rainfall became less constant and more seasonal in a local shift that was part of a worldwide change at the Eocene-Oligocene boundary. The resulting forest was largely deciduous and included many extinct yet familiar kinds of trees from the aspen, birch, walnut, apple, and maple families, while conifers also appeared and the deciduous dawn redwood dominated the forest and fossil record. The yellow layers reflect a significantly cooler and drier paleoenvironment with even greater seasonal variation in temperature and rainfall, as deciduous forests began to open during a slow transition toward today’s open rangeland, standing water became less common, forests still covered the area with more space between trees, and shrubs and other smaller plants became more common in the increasingly open understory.