HISTORY AND GEOLOGY OF THE SHAWANGUNK MILLSTONE INDUSTRY, ULSTER COUNTY, NEW YORK

The Shawangunk Formation is a Lower to Middle Silurian orthoquartzitic conglomerate and sandstone unit with shale interbeds. The formation is interpreted to be a molasse sequence of fluviatile sediments deposited within the foreland basin of the Taconic Orogen and later deformed during the Alleghanian Orogeny. The Shawangunk Formation crops out from the NJ/PA Delaware Water Gap area north-northeastward for 150 km to where it pinches out in Ulster County forming the prominent Shawangunk Ridge in the western Hudson Valley region of NY State.

From the mid-1700s to mid-1900s, millstones were mined from the Shawangunk Formation in Ulster County and marketed under the trade name Esopus millstones (after a Hudson River town from which they were shipped). Esopus millstones were often considered to be comparable in quality to the well-known French buhr millstones but domestically available at less cost. While early stones were produced for the milling of local grain, later uses included powdering local limestone for natural cement, milling mustard seeds, and grinding industrial minerals such as barite, gypsum, and talc. Millstones were manufactured in a variety of shapes and sizes, typically ranging from 15 to 90 inches (38 cm to 2.29 m) in diameter, depending upon intended usage. Millstone quarries in the Shawangunks were small-scale operations dominated by a few families who passed on their skills from father to son. While the heyday of the industry was in the late 1800s, by the turn of the 20^thcentury the industry still reported over $20,000 each year in sales.

Dozens of abandoned millstone quarries may be found spread over a distance of approximately 16 km on the western side of the Shawangunk Ridge extending through several Ulster County townships. Examination of quarries reveals a common geologic setting – they are confined to conglomeritic horizontal to sub-horizontal planar bedded sections within the formation and often exhibit suitably-spaced orthogonal joint sets generally parallel and perpendicular to the regional NNE-SSW strike of the Shawangunk Ridge. In addition to geologic considerations, land access and ease of transport to local shipping routes (canal and later railroad) also undoubtedly played a role in the siting of the quarries.