The best known American variety of bluestone is a feldspathic sandstone, which is produced in hundreds of small quarries in adjacent areas of Pennsylvania and New York. The other, lesser known, type of American bluestone is formed from a different sedimentary rock, limestone.
Bluestone from Pennsylvania and New York is commercially known as bluestone or Pennsylvania Bluestone. These are a group of sandstones defined as feldspathic greywacke. The sand-sized grains from which bluestone is constituted were deposited in the Catskill Delta during the Middle to Upper Devonian Period of the Paleozoic Era, approximately 370 to 345 million years ago. If the initial deposit was made under slow moving water the ripples of the water action on the sand or mud will be revealed. This deposition process may be seen today at any ocean beach in shallow water or in a stream bed where conditions allow it to be observed. The term “bluestone” is derived from a deep-blue-colored sandstone.
The Catskill Delta was created from run off from the Acadian Mountains (“Ancestral Appalachians”) which covered the area where New York City now exists. This Delta ran in a narrow band from southwest to northeast and today provides the base material for the high-quality bluestone which is quarried from the Catskill Mountains and Northeast Pennsylvania.
Bluestone from the Shenandoah Valley is regionally known as bluestone but a less ambiguous name is Shenandoah Valley Bluestone. The limestone formed during the Ordovician Period approximately 450 to 500 million years ago, but it was formed at the bottom of a relatively shallow ocean that then covered what is today Rockingham County, Virginia. However, the limestone that accumulated in Rockingham County was darker in color than most other limestone deposits because it was in deeper waters exposed to less light. The darker blue color resulted in limestone from this region to be dubbed bluestone and with two sequences measuring about 10,000 ft, it gives the area one of the largest limestone deposits in the world. The stone eventually fades from a deep blue to a light grey after prolonged exposure to the sun and rain.
See Some of Our Work: