The Picture Gorge exposure lies east of the Blue Mountain uplift, which cuts southwest–northeast through the Horse Heaven mining district northeast of Madras.

The rapid deposition of the ash preserved the remains of plants and animals living in the region.

Because ash and other debris fell during varied climatic and volcanic conditions and accumulated from many eruptions extending into the early Miocene (about 20 million years ago), the sediment layers in the fossil beds vary in their substance, chemical composition and color.

[3] The lowermost layer contains red ash, such as that exposed in the Painted Hills Unit of the national monument.

[5] Among the notable plant fossils are Metasequoia (Dawn Redwood), a genus thought to have gone extinct worldwide until it was discovered alive in China in the early twentieth century.