Chadron State College
Chadron State College


Stratigraphy is the science of naming and relating the layers of rock that form the earth's crust. You might compare it to the study of geography and geographical place names. Continents are divided into countries, countries into states or provinces, states into counties, etc.

The rock layers of the earth's crust are classified in a similar way. A "group" is made up of "formations," and formations are comprised of "members." Found within the members are volcanic ashbeds that can be dated using mass spectrometry. When fossils are discovered, their distance from the nearest ash layer is noted and their approximate age can be determined.

  • In this diagram, a composite chart of the stratigraphy of the Toadstool park area has been superimposed over a photograph taken near the park's campground.
  • Around 10 million years ago, sedimentation ceased and erosion began in this location.
  • Since that time, roughly 150 meters of sediment has been completely eroded away.
  • The "brown siltstone," the Whitney, and most of the Orella members of the Brule formation are gone. The Chadron formation is partially exposed. The Chamberlain Pass formation (CP) and the Lower purplish white ash layer have not yet been exposed here.

Composite chart adapted from:
Terry and LaGarry (1998)
Geological Society of America Paper #325 p. 117-141