Volcanic ash in Indiana: Tioga and Bentonite(SG-07)
Just as Mount St. Helens recently covered some western states with volcanic ash, a volcano covered Indiana with ash long ago. About 380 million years ago, violent volcanic eruptions in what is now central Virginia threw a great amount of ash into the air. This ash covered much of the present eastern United States. Near the vents, ash beds are about 200 feet thick. The beds decrease in thickness westward, and only a bed a few inches thick marks the event in Indiana. This ash bed, deposited in the Middle Devonian Period, is the Tioga Bentonite.

The Tioga Bentonite can be distinguished from the surrounding rocks, and it can be traced on geophysical well logs throughout the eastern United States. Analysis shows the presence of the clay mineral potassium bentonite, along with angular quartz fragments and well-formed crystals of feldspar, mica, and zircon, which had begun to form deep in the earth before the ancient eruption.

Our Hoosier State Beneath Us: Structural Geology