The Woodstock Quartz Monzonite is a Silurian or Ordovician quartz monzonite pluton in Baltimore County, Maryland. It is described as a massive biotite-quartz monzonite which intrudes through the Baltimore Gneiss at a single locality surrounding the town of Granite, Maryland.
The extent of this intrusion was originally mapped in 1892 as the "Woodstock granite". It was given its current name in 1964 by C. A. Hopson. Hopson grouped the Woodstock Quartz Monzonite with the Ellicott City Granodiorite and the Guilford Quartz Monzonite as "Late-kinematic intrusive masses."
Woodstock granite has been used in the Capitol Building, the Library of Congress, and in buildings in Baltimore.
Description
The Woodstock Quartz Monzonite was described in 1898 as "perhaps the best granite in Maryland for general building purposes" by Edward B. Mathews of the Maryland Geological Survey. Mathews described the mapped extent of the granite and inclusions of gneiss within it as follows: The granite mass as indicated by the map forms a more or less oval, isolated area of granite extending scarcely two miles northeast and southwest and a mile northwest and southeast. Although so small, it is one of the most important economic areas within the state. This mass of granite, which is evidently intruded into the gneisses, is entirely enveloped by them and sends no dikes or apophyses into the surrounding rock. That the gneiss is really older than the granite is shown by the great number of inclusions found within the latter. These are chiefly of gneiss, and they occur often in huge irregular blocks six to eight or even ten feet in size, showing narrow rims due to contact metamorphism. They are beautifully puckered and wrinkled and being much richer in ferro-magnesian silicates than the granite itself, their irregular outlines contrast sharply with the lighter background. Mathews described the granite itself as follows: The appearance of the Woodstock granite is well represented in (the polished slab shown at right) which reproduces the polished surface in natural size. The color of the rock is bright gray, with something of a luster imparted by the quartz and the unaltered feldspars, the latter often giving an additional faint pink tone. The mica occurs in evenly disseminated fine black flakes which emphasize the grain of the rock and only slightly subdue the bright fresh aspect of the stone. The size of the constituent grains which varies from 0.05-0.2 inches in length, and from 0.01-0.10 inches in breadth, for quartz and feldspar, is little marred by the less resistant mica wearing away and leaving small depressions, that are scarcely discernible to the naked eye. The polished surfaces, such as are represented in (the same figure), are darker than the rough or ashlar finished stone.
Hopson reported the chemical composition (by %) of the Woodstock Quartz Diorite from the "Sylvan Dell Quarry" (probably the Waltersville Quarry) in Granite, Maryland, as follows:
Early quarrying
The Walters/Blunt family of Waltersville (later Granite, Maryland) "founded the local quarrying industry circa 1820."