Tomstown Dolomite

The Tomstown Dolomite or Tomstown Formation is a geologic formation in Maryland, Pennsylvania, Virginia and West Virginia. It preserves fossils dating to the Cambrian Period.

In Maryland it is described as "Interbedded light gray to yellowish-gray, thin- to thick-bedded dolomite and limestone; some shale layers; gradational contact with Antietam; thickness 200 to 1,000 feet."