The fossil beds were laid down in a lake roughly 15 million years ago, when a drainage basin was dammed by the flood basalts of the Columbia River Plateau. Narrow and deep, the lake's cold, anoxic water and rapid sedimentation created perfect fossil conditions. Furthermore, the basin itself has remained tectonically stable, with little deformation since then. The fossils indicate that the region's climate was much warmer and wetter than today's, and similar to that of southern Florida. ,

Though a number of animal species have been found, the site is best known for its fossil leaves. Their preservation is exquisite; fresh leaves are not only unfossilized, but sometimes retain their fall colors before rapidly oxidizing in air. Indeed, scientists have managed to isolate small amounts of DNA from certain specimens. Not surprisingly, the good condition of the fossils has prompted them to be used as evidence by young Earth creationists.