We present chemical data for ash beds interbedded within mid-Miocene lacustrine sediment in the vicinity of Clarkia, north Idaho. The Clarkia lake beds are well known for their exceptional preservation of leaf and other fossils. The ash beds have a range of chemical compositions, indicating eruption from diverse sources located within different tectonic environments in western North America: the Yellowstone hotspot province, the Cascades volcanic arc, and potentially the Southwest Nevada Volcanic Field. A few ash beds can be correlated with tephra deposits of known age. These correlations indicate that the age of the Miocene Lake Clarkia deposits lies between 15.4 and 16.0 Ma. The lake formed when the ancestral St. Maries River was dammed by a flow of the Priest Rapids Member of the Wanapum Basalt (Columbia River Basalt Group), previously assigned an age of ∼ 15 Ma.