Broken stone: the underground mining of crushed stone aggregates in the Chicago area

The Chicago region has long been one of the largest producers and consumers of crushed stone aggregates in the United States. Chicago’s “broken stone” production began in the mid-19th century to supply material for macadamizing city streets. One of the first patents for a “stone-breaking” machine to make macadam was issued to a Chicago group in 1859. From about 1820 to 1900, however, aggregates were only a minor stone product of the Silurian dolomite quarries in northeastern Illinois. During this time, huge quantities of building stone (“Athens Marble”) were quarried at Lemont and Joliet for the construction of building foundations, load-bearing walls, sidewalks, and many other structures. Chicago quarries were famous for their large-scale production of lime used for mortar. The stone industry changed rapidly during the 1890s following the introduction of inexpensive portland cement, and by 1900, aggregate had become the dominant stone product of the region. Marking the start of the modern stone industry was the replacement of building stone with concrete, which requires large volumes of aggregates. After 1900, the Good Roads Movement also increased the demand for aggregates as the old macadamized and paving block roads were replaced with pavements made with portland cement and asphalt. These changes caused a large number of quarries in the region to close, especially those producing building stone. For the next 70 years, the remaining quarries were able to meet most of the Chicago area’s greatly expanding aggregate needs. By the 1970s, however, many of these quarries had depleted their reserves of Silurian dolomite available via surface quarrying. Urban expansion already had covered most of the undeveloped aggregate sources located close to the city, forcing the industry to explore for new sites far from Chicago. Significantly, however, the potentially greater transportation costs of stone from these more distant sources made underground mining for aggregates locally an economically viable possibility. In the early 1980s, the Elmhurst-Chicago Stone Company decided to target Ordovician carbonates via underground mining beneath their depleted Silurian dolomite quarry in Elmhurst, Illinois. With the success of this first mine, other producers began to plan and open additional underground mines in the Ordovician rocks of the region. Currently, seven mines are operating and one is under construction in the Ordovician rocks of northeastern Illinois, although the original mine at Elmhurst has closed. Of the operating mines, five are located in depleted Silurian quarries, one is in a depleted gravel pit, and one is at a never-quarried old industrial site. Numerous other potential sites in the area, most already owned by aggregate producers, have been evaluated for future mining of Ordovician rocks. Ordovician strata are more uniform in thickness and character than local Silurian strata, which are depositionally complex because of numerous ancient reefs, and are increasingly truncated by erosion going from east to west through the region. Therefore, Ordovician carbonates are a more uniform target for mining throughout nearly the entire region, with the only significant variable being depth of burial. In addition, in areas near the eastern portion of the region, thick deposits of Silurian rocks have never been quarried because of thick deposits of overlying Quaternary sediments, but these could also be extracted by underground mining. Most operating mines have been developed on properties already owned by aggregate producers, which helps solve the problem of prospecting for and permitting new sites. However, important issues have emerged even with producer-owned potential sites for underground mining. Many depleted sand and gravel operations, as well as depleted stone quarries, have been sold off for other uses, and this practice was continued even after the opening of the Elmhurst mine. In many cases, the mineral rights needed for underground mining have not been retained and these potential reserves have been lost. In addition, other proposed uses have competed for the potential mine sites. In the 1980s to 1990s, the Ordovician reserves at the two largest quarries in Cook County, Vulcan’s McCook Quarry and Material Service’s Thornton Quarry, could have been lost because they faced at least partial condemnation for use as large public works sanitary reservoirs. The loss of Ordovician reserves at these sites could have exceeded 500 million tons, which at the time would have represented up to 60% of crushed stone reserves in the county. Eventually, this loss would have caused dramatic increases in transportation costs to bring aggregates from more distant areas, and taxpayers at all levels would have carried much of the burden of these increases. Fortunately, other solutions were found and the reserves have been preserved for the future.