The largest open surface granite quarry in the world

Posted by | November 12, 2012

“The principal outcrops of granite in Surry County are found in the northern part of the county near the Virginia line in the vicinity of Mount Airy, the county seat. The granite is exposed in flat surfaced masses in rather an advanced stage of decay immediately to the north and south of Mount Airy where quarrying on an extensive scale has been conducted for some years.

“The North Carolina Granite Corporation’s Mount Airy quarries, located less than 1 mile northeast of Mount Airy, were opened in 1889, and the first shipment of stone from them was made in July 1890. The total shipment of granite from these quarries from 1890, when 135 carloads were shipped, to 1904 when 1,282 carloads were shipped, was 13,232 carloads.

North Carolina Granite Corporation, Mt Airy NC

“Quarrying is confined to a 40 acre tract of continuously exposed granite over the slope and top of a long hill which rises about 125 feet above the valley bottom. The company holds more than 1,200 acres additional of ground over which granite is exposed.

“Quarrying has extended over practically the entire 40 acre tract, the greatest depth of working being about 30 feet. The rock is a biotite granite of very light gray, nearly white color and medium grain. The biotite is not, except in one opening, equally distributed through the granite, but is entirely absent from some parts of it, is uniformly distributed through others, and shows a marked tendency to segregation in still other parts.

“Quartz feldspar areas of extreme whiteness, ranging from several inches to as many feet in diameter, in which biotite is entirely lacking or represented by only a few shreds, are common through the granite.

“This unequal distribution of the characterizing accessory (biotite) renders the granite in places less uniform in color than might be desirable for some purposes. The granite that has a uniform color is most pleasing in appearance and forms excellent and desirable stone for all uses except for monumental stock, for which the contrast of color between the cut and polished faces is not great enough.

“The company is adequately equipped with all the necessary machinery and appliances for quarrying and handling the stone. In 1905 a large stone cutting plant was erected. The stone is carried from the quarries to the railway cars by a system of inclined ways run by gravity. The limit in size of dimension stone is the capacity of the railroad cars. Blocks weighing 20 tons are reported to have been frequently shipped from the quarries.

North Carolina Granite Corporation, Mt Airy NC“The product is marketed over a large territory, chiefly in States south of New York. It is used for general building and paving purposes. The quarry waste is utilized for roofs on cotton mills, macadam on streets and roads, ballast along the railroads, and granolithic work.

“All the stone used in the dry dock at Newport News, VA and the concreting material used in the Fort Caswell fortifications, Cape Fear River, NC, came from the Mount Airy quarries.

“The method of quarrying the granite consists in drilling a hole about 3 inches in diameter perpendicular to the surface to a depth equal to the thickness of the stone desired, usually 5 to 7 feet, then firing a succession of light blasts.

“The operation is begun by discharging about one fourth of a pound of dynamite in the bottom of the hole; this small charge pulverizes the stone slightly and forms a small chamber. The tamping is then cleaned out and hole is recharged in the same manner; this time however, with about a handful of powder.

“Small charges of powder are exploded in the hole until a small seam has been started at the bottom extending parallel with the surface. To determine if this has been done a small steel rod bent at the lower end and sharpened to a point is passed up and down the hole until the crack is located. After the crack has once been started the charges are gradually increased until it extends a distance of 75 feet or more from the hole.

North Carolina Granite Corporation, Mt Airy NC

Aerial view of the North Carolina Granite Quarry, Mt Airy, NC.

“The use of explosives is then discontinued, and a watertight connection to the hole is made by fastening a piece of iron pipe in the hole with melted sulphur. To this connection is attached an ordinary force pump and water is pumped into the crevice formed by the explosives. The crevice is extended by continuous pumping for a few hours until finally it covers an area of perhaps 2 acres and the pressure finds vent by tearing the rock out to thin edges on the side of the hill.

“This method is used in the warmest weather when the surface of the rock is naturally somewhat expanded and more raised. It is very doubtful whether it could be employed during cold weather; experience shows that the hotter the weather the easier the work.

“Sheets of stone covering areas of 1 to 2 acres from 6 to 8 feet thick close to the hole are easily raised by this method. It is often found necessary to clean off a ledge of stone made in this manner before attempting to form or raise another sheet on the surface below. For this reason the quarry covers considerably more area than one having natural seams —horizontal sheeting.”

source: ‘Granites of Southeastern Atlantic States,’ in Bulletin – United States Geological Survey, Issue 426, 1910, pp. 148-151

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