Outgrowth Surface Workings at www.japanese-home-gardens.com

Many are an outgrowth surface workings of surface workings into bank deposits because overburden became excessive as the quarry face progressed into the bank. Other mines are in areas where the topography will not permit surface workings or where it is desirable to excavate relatively thin strata of rock of special quality. Underground mining is more expensive per unit of output than surface quarrying but is often justified. Among advantages are extension of the operating- season and freedom from contamination of the stone. Sometimes underground mining enables unwashed stone to pass specifications.

Types of Quarries.—The greatest tonnage of stone is from surface quarries but there is an increasing number of underground mining operations. Surface workings are of the bank and pit types. Quarries developed below grade require the removal of overburden to expose the stone as do most bank deposits. Overburden may vary in thickness from a foot or two to 40 feet or more. Mining operations are scattered geographically and are mainly in limestone deposits, notably in Kentucky, Pennsylvania, Missouri and Tennessee.

Conventional practice in underground mining is an outgrowth surface workings of metal mining procedure, namely that of developing a number of working faces while leaving pillars of stone to support the roof. Where there are differences in the properties of the stone within an open quarry, there is a trend toward more selective surface quarrying. For example, a stratum of high-grade limestone suitable for flux may be overlaid with one that is too high in silica and the two may be drilled, blasted and processed separately.