May 25. Devonian glass sands

Shales like the Marcellus Shale are usually deposited out in quiet, relatively deep water. Closer to shorelines, and on shorelines, you get coarser sediments including sand. Sands on beaches tend to get washed and winnowed by wave action, so unless there is some source for other materials than the quartz grains in the sand, you can end up with a rock, sandstone, that’s really pretty pure quartz, silicon dioxide.

Photo by Kevinaj, public domain via Wikipedia.
Caudy’s Castle (Oriskany Sandstone), West Virginia

In what is now Pennsylvania, Maryland, West Virginia, and Virginia, during the Devonian, such a sand was laid down early in the period, around 400 million years ago. The clean quartz sand forms a layer that is 100 feet or more thick, spread over a large area. The purest portions were mined historically for making glass, which requires quartz of high purity. Even a trace of iron, less than a fraction of a percent, will color glass.

The historical name for these glass sands was Oriskany, but that’s really a large group of related, similar, but not necessarily exactly equivalent sandstones. The glass sand miners didn’t care of course – they just went after the sand that gave them the best glass. These sandstones in the subsurface can also serve as excellent reservoirs for natural gas.

—Richard I. Gibson

Photo by Kevinaj, public domain via Wikipedia.  Caudy’s Castle (Oriskany Sandstone), West Virginia.