QUARRY

open excavation in the Earth's crust from which stone is obtained. Quarried stone is classed either as dimension stone, meaning blocks or slabs cut to size, or as crushed and broken stone. Older quarrying was concerned almost exclusively with dimension stone for use as building material and therefore was limited to areas in which rock of uniform coloration and texture, such as granite or limestone, was available. Rock deposits intersected by numerous joints, or natural partings, are valueless as sources of dimension stone. After the surface has been cleared, a cut, or channel, is made to separate the rock from the solid bed. For softer rocks, such as limestone and sandstone, a power-driven cutter called a channeling machine makes a slot about 5 cm (2 inches) wide and several metres deep. Harder rock, such as granite, may be channeled by drilling closely spaced holes and cutting away the rock remaining between them. If floor seams are absent, the mass of rock is separated from the quarry floor by drilling horizontal holes beneath the blocks and driving wedges into the holes. In a limestone quarry the masses of rock freed may be in the form of long rectangles weighing several tons, which are cut into more convenient blocks. The separated blocks are conveyed to mills, where they are sawed into slabs, shaped or turned on lathes into columns, and finished by rubbing or polishing. Crushed stone is used for concrete aggregate, for road building, and, in the case of limestone, as flux in blast furnaces and for chemical applications. The quarrying technique consists of deep drilling and blasting to fragment the rock. A large number of charges are fired simultaneously, producing up to 20,000 tons of broken stone in one blast. The broken stone is crushed into smaller pieces that are separated into uniform classes by screening.