Part 6

A “wet pit” operation produces sand and gravel from an artificial pond or lake. In some operations the sand and gravel is mined by a dredge that floats on the water (fig. 19). In some pits, a stream of water is directed from the dredge against the bank of gravel to wash the gravel into the lake. A large metal pipe at the front of the dredge slants down into the water and sucks up the sand and gravel from the underwater part of the deposit. The gravel, sand, and water is then pumped through a pipe at the rear of the dredge to the processing plant on the shore.

In some types of wet pit operations a large scoop or bucket operated from a crane on the shore, or by cables fastened to the shore, is used to dig sand and gravel from beneath the water.

Dredges are used to pump up sand and gravel from the beds of Illinois rivers, especially the Mississippi, Ohio, and Wabash.

The sand and gravel resources of many parts of the state have been mapped and studied by the Illinois Geological Survey, and work of this kind is continuing.

SILICA (TRIPOLI) AND OTHER MINERAL MATERIALS OF EXTREME SOUTHERN ILLINOIS

The hills of extreme southern Illinois contain several mineral materials that are entirely or largely restricted, in important quantities, to that part of the state. Some of them, such as silica and novaculite, come directly from bedrock deposits of great age; others, such as some of the sands and gravels, are of more recent origin.

Silica (Tripoli)

A mineral material unique in Illinois is the amorphous silica, or tripoli, mined in the hills of southern Illinois about 20 miles north of Cairo.

Most of the silica mines are found at the heads of valleys or in tree-covered hill slopes along the less traveled roads. An arched opening (fig. 20) with a road leading into it may be all that is visible of the mine from the outside. Inside are many rooms, some 30 feet high, separated by rounded pillars about 20 feet thick that have been left to support the arched mine roof.

Most of the silica in the mines is white, and this is the part that is mined. A silica deposit is made up of layers—some of white, powdery rock material, others that look chalky but are firm or hard.

In the mines the silica is blasted loose from the natural deposit and loaded into trucks that take it to the processing mills at Elco and Tamms, where it is crushed and then transferred to huge grinding mills that pulverize it to a very fine powder. Air currents of different velocities separate the powder into various grades of fineness, and the finished silica goes into large paper bags for shipment.

Origin of Silica.—Some of the history of the formation of silica deposits is not fully known, but it seems probable that their original character was quite different from what it is today. Investigations by Survey geologists suggest that the parent rock formation was limestone and chert. The limestone was composed of the mineral calcite, but scattered through it were myriads of small particles of quartz. Interlayered with it were bands and beds of chert that contained varying amounts of calcite.

As the rocks above them were worn away by the ceaseless erosion of streams and rivers, these original deposits were uncovered. Rain and snow-water then worked down into the limestone and chert deposits through cracks and crevices and dissolved the calcite in the rock. The quartz remained because it is much less soluble than the calcite. After many thousands of years all the calcite had been removed, leaving behind a “skeleton” of quartz—the silica deposits of today.

Uses of Silica.-The silica mined and milled in southern Illinois has many uses. A superfine grade, known as white rouge, is used to polish optical lenses. Other grades are used in scouring compounds, metal polishes, paints, electrical resistors, high-temperature pipe coverings, fiberglass manufacture, plastics, silicone rubber, wood filler, caulking compounds, ceramic products, floor tile, billiard cue chalk, as foundry parting or facing, concrete admixture, in the manufacture of buffing compounds that are used to polish metal objects, and for other purposes. Industry uses many thousands of tons of silica each year.

Chert and Chert Gravel

Deposits of chert, chert gravel, and ganister also are among the variety of mineral materials found in extreme southern Illinois. Chert consists principally of minutely crystalline particles of quartz. Some chert is popularly called flint. In southern Illinois chert occurs in two principal kinds of deposits, those composed of solid ledges and those consisting of gravel. The term novaculite is used in southern Illinois for those solid deposits that are white, comparatively thick, and free of other interlayered materials. No novaculite is mined at present, but it is said to have been sold in past years for making sodium silicate and silica brick.

The chert gravels of southern Illinois are of three kinds—novaculite gravel, Elco gravel, and “Lafayette” Gravel. The novaculite and Elco gravels consist of fragments of chert plus lesser amounts of fine silica particles and clay. The chert fragments of the novaculite gravel are angular, but the Elco gravel includes both angular and rounded fragments. These gravels are white, yellow, brown, or reddish brown, the novaculite gravel usually being the more highly colored. Deposits in Union and Alexander Counties have been used for road surfacing and other purposes. Deposits of chert gravel also occur in Hardin and Saline Counties, and some stream valleys in southern and western Illinois also contain such gravel.

“Lafayette” Gravel consists principally of brown chert pebbles. Most of the pebbles are rounded and have a smooth, semi-polished surface. The sand and clay occurring with the gravel are brown or dark red. In some places there are deposits of coarse, red quartz sand. The gravel is most abundant in the four southernmost counties of the state, and deposits may be as much as 65 feet thick. It is used principally as a road-surfacing material.

Ganister

Ganister occurs in the hills of Alexander and Union Counties and is a loosely consolidated, granular material consisting of irregular particles up to about an inch in diameter or of masses of material readily disintegrated into such particles. The particles are composed mainly of fine crystalline quartz. Ganister is white, cream, light yellow, or red and occurs in deposits up to 25 feet or more thick. Relatively small tonnages of the light colored ganister are now used, principally in making refractories, but ganister is said to have been more widely used in that field in the past. It is produced from underground mines. Some red ganister, or ganister like material, mined from open pits has been used for road surfacing.

Studies of Southern Illinois Materials

Survey geologists have mapped the chert- and silica-bearing formations of Alexander and Union Counties and also many of the various kinds of gravel deposits. Samples have been tested to determine their chemical composition and the size of the particles composing them. Laboratory studies by ceramists indicate that novaculite and novaculite gravel, when suitably processed, can be used for making silica brick, which withstands great heat. Canister and the gravels of southern Illinois offer a variety of raw materials awaiting increased industrial use.

Sands of Extreme Southern Illinois

In southern Illinois deposits of sand laid down in an arm of the ocean that once extended northward into Illinois from the Gulf of Mexico are found in Alexander, Union, Pulaski, Pope, and Massac Counties. The deposits are commonly a light color—white, cream, yellow, or gray.

The grains of the sand are almost all quartz and generally are angular. Some of the sands are of almost powder-like fineness, others are fine or medium grained. Many of the sands contain flakes of white mica, a glistening, silvery-looking mineral often mistaken for silver or platinum. Unlike these metals, however, mica is comparatively light in weight and is not metallic. Also present in some sands are small flakes of the mineral graphite.

The southern Illinois sands have not been widely used, but some of them have been employed in making concrete. They also may have possibilities for molding and core sand.

As a result of work by Survey geologists, the location and properties of many of the southern Illinois sand deposits are known.

CLAY AND SHALE