Sandstone-coal-tonsteins in the Cretaceous of Wyoming, USA.

The outcrop seen here consists of Upper Cretaceous sedimentary rocks near the town of Superior, Wyoming. The unit at top is a quartzose sandstone of the basal Ericson Sandstone. Below the Ericson Sandstone is the uppermost Rock Springs Formation. The black layers are coals (hand samples indicate that these are apparently sub-bituminous coals) - this is the Rock Springs No. 5 Coal Bed. The thin, whitish-colored beds in the coal interval are soft claystones that were originally volcanic ash beds. They have been chemically altered as a result of deposition and burial in the acidic, reducing conditions of a coal swamp environment. Such altered volcanic ash beds are called tonsteins.

Stratigraphy: lower Ericson Sandstone over upper Rock Springs Formation, Upper Cretaceous