The Dalles Formation is a geologic unit in Oregon around The Dalles and the nearby Columbia River region. Dating from the Miocene to lower Pliocene and overlying the older Columbia River Basalt Group, it consists mainly of weakly consolidated river and volcaniclastic deposits. Its interbedded sandstone, shale, conglomerate, and tuffaceous rocks record the combined effects of sediment laid down by rivers and material derived from volcanic activity. Some areas contain coarse conglomerates with angular volcanic fragments, while finer tuffaceous sandstones and silts are more widespread. Neogene fossils preserved within the formation help confirm its age and add paleontological significance to Oregon’s geologic history.