Aerial view of Hance rapid, approx. mile 77. A billion years of history are represented in this photograph. Three major mountain building episodes are illustrated, the earliest being the unconformity between the igneous and metamorphic rocks of the inner gorge (the jagged rocks at river level), and the overlying Late Precambrian sedimentary rocks. These rocks are overlain in turn with angular discordance by Paleozoic sedimentary rocks, representing the second mountain building episode. The Cenozoic and modern uplift and erosion cycle is the third mountain building episode represented. Not seen in this photograph is a fourth episode, called the Ancestral Rocky Mountains, that formed during Pennsylvanian time and were eroded by the Permian.