This photo was taken on the Missouri River in central Montana. It shows the Eagle Sandstone beneath the Claggett Shale. Both were formed during the late Cretaceous Period when the Western Interior Seaway covered much of eastern Montana. As the sea fluctuated, the type of sediment deposited in this place changed. Sand was deposited when this location was near the shore and silt (shale) was deposited when this location was deeper (farther from the shore). To understand how the depth at this location changed, watch this ANIMATION. Be sure to watch both parts.