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Illinois? Coral Reefs? No Way!
Producing Aggregate in the Quarries of Illinois

Guest Column by GeoT
Dateline: 07/13/01

The caller to the radio talk show said he "creates wealth" for a living - he runs a rock crusher! Yes, he does create wealth. The only place wealth is created is from the earth (one way or another) and crushing rock is one of the ways! Wealth to the tune of many millions dollars a year statewide!

425 million years ago found a very different Illinois than that of today. This was the Silurian Period of geologic time. Rather than being positioned at around 40°N and 90°W ­ the land that would become Illinois was south of the equator! Warm, clear, shallow seas covered the region. In those seas grew corals, calcareous algae, and lime secreting sponges. The corals were building immense reefs to be of great economic value, but the sponges would contribute problems. All this in the distant future.

The coral reefs belong to the middle Silurian,­ the Niagaran Series,­ part of a geologic complex surrounding the Michigan Basin, and taking its name from the famous waterfall between Lake Erie and Lake Ontario. It's amazing to think that same rock underlies parts of Illinois. But it does!

Following the Chicago fire of 1871 the need for more substantial fireproof buildings was obvious. Stone and brick this time! Much of the stone used was a dolomite belonging to the Silurian Period. Quarries near Joliet and Lemont produced great quantities ­ calling it 'Joliet Marble' or 'Athens marble.' (The famous surviving Chicago Water Tower was made of it prior to the fire). It weathers to a pleasing yellowish color, but in the process, has also been known to exfoliate ­ not desired by most architects! Limestone from Indiana soon replaced the local Silurian dolomite as a building stone. No peeling from it.

Aggregate is another matter. Aggregate is what the radio caller produces. Rock is quarried and then crushed into various sizes according to how the aggregate will be used. The primary use is in concrete. The construction industry uses concrete in many ways from foundations of buildings to sidewalks. The frame of a skyscraper is of steel, but the walls and floors are commonly made of concrete. Roads, whether of concrete or asphalt, use aggregate as the road metal. Aggregate in greater sizes is used inside sheet pile for shoreline breakwaters, and as rip rap: lining streams, ditches, lake shorelines, and spillways for erosion control.

Two of the largest quarries in Illinois are the Thornton Quarry (photos) and the McCook Quarry. Both are located in the Silurian coral reef complex of northeastern Illinois. Thornton Quarry covers some 550 acres divided into 6 working pits, and is as much as 200 feet deep. These, plus 20 other quarries in the Chicago area, feed the hungry construction industry with aggregate. The best dolomite lies near the heart of the ancient coral reef and this is where the center of each quarry is located. As quarrying moves outward from this center, the rock enters what is known as an inter-reef zone, and problems can result. Here is where the sponges lived. Back in the Silurian they secreted lime, buy they also made their hard parts of silica. The silica became a variety of quartz known as chert. This hard chert is not only bad news for the rock crushers (if there is too much of it), it is also bad news for concrete. Cryptocrystalline chert is chemically reactive to the Portland cement and causes 'pop outs' and weakness in the concrete. So, who would have thought a 425 million year old sponge might be causing problems in a concrete road being built for 21st century transportation?

Click here for a topographic map.

Thornton Quarry is said to be among the world's largest rock quarries. Look how Interstate 80/294, Illinois Route 1, and the railroad cross the quarry on narrow zones of rock left for that purpose. Arched underpasses allow quarry equipment to move from one pit to another. Operators plan to quarry to the 400-foot level ­ then, quarry an underlying dolomite unit via underground techniques.

Click here for a topographic map.

McCook and Thornton Quarries will find an additional use ­ as storage for runoff and sewage! At a price. Half of the potential rock for quarrying will then be inaccessible. Chicago's Deep Tunnel is being constructed for drainage purposes and planned storage of the water will be in parts of the quarries. The runoff/waste water comes from older combined sewers in the city. It will be treated before release.

Another factor in the quarrying of rock for aggregate is the cost of transportation. This is the reason quarries need to be located within an economic range of where the aggregate will be used. 30 to 50 miles is usually the limit. Inquiry at a local construction company found the aggregate they use comes from a quarry in west central Indiana. Kentland, Indiana. Interestingly, the dolomite in this quarry shows shatter cone fractures that may indicate the site of a meteor impact! A similar situation occurs in the Chicago area in what is known as the Des Plaines Disturbance.

So, we owe a debt to the corals and continental drift for our rock quarries in northeastern Illinois. Next time we see one we'll have a little better idea of what goes on there and why.

It also explains that pinkish colored highway I drove on in Wyoming: granite with large pink feldspar crystals was used as the aggregate!

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GeoT is a long-time high school geography teacher from Illinois. In addition to geography, he enjoys railroads and model railroads, old Oldsmobiles, and gardening.

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