Pennsylvanian in Illinois
The theory of Plate Tectonics has shown that our Earth's outer surface, or "plates", are constantly in motion. If you could rewind roughly 300 million years of plate motion you'd find that Illinois was located near the Earth's equator. The climate was hot and sticky and lush tropical forests of rushes, ferns, and scale trees (primitive tree-like plants) would have covered the landscape. You would be standing in a swampy environment sandwiched between the mainland and the ocean (like the Florida Everglades). We call this time period the Pennsylvanian Period (318-299 million years ago, also called the Upper Carboniferous) and the swampy environment that existed at this time is responsible for the extensive coal deposits present in Illinois.
Making Coal
When the vegetation in these Pennsylvanian age swamps died it fell into the water and a chain of events began that would slowly transform the material into coal. First the vegetation was converted to peat. Peat is mixture of sediment and partially decayed plant material and can be burned but doesn't produce much energy and gives off a lot of smoke. After about 30 feet of peat form we find that the lower portion of the peat forms a gel-like substance (anaerobic bacteria are responsible for this) that will eventually become coal. The gel-like material is called gytta and, after burial under thousands of feet of sediment and heating to at least 100 degrees Celsius, it turns into coal.
The Seas Come and Go and Come and Go
The rocks of the Pennsylvanian Period exhibit a very curious pattern. The types of rocks repeat themselves over and over and over. Repeated rising and falling of sea level is responsible for this repetition of rock types that geologists call cyclothems. An ideal cyclothem starts with rocks that formed above sea level (nonmarine) at the bottom and changes to rocks that formed in successively deeper sea water (marine) at the top. Then the whole process repeats itself. A coal seam is part of each cyclothem and we recognize over 75 coal seams in Illinois. That is a lot of changing sea level!
Why Do They Call it the Colchester Coal?
Naming coals is like real estate, it comes down to location, location, location. This coal seam was first named the Colchester Coal back in 1866 because it was mined and is exposed at the surface near Colchester, Illinois. Likewise we have coal seams named after such cities as Springfield, Rock Island, Danville, and Herrin. You may also hear this coal seam referred to as the Colchester No. 2 Coal because prominent coal seams in Illinois were numbered from oldest (Rock Island No. 1) to youngest (Chapel No. 8) in the past.
Coal Mining Near Colchester
Evidence of past coal mining in the area is visible as one drives along U.S. 136 between Colchester and Macomb. To the north of the highway numerous steep-sided hills covered in trees can be seen in the middle of farm fields. These hills are composed of the "spoil" (waste material) from underground coal mines. Each hill is near an old mine opening. The Colchester Coal is the fourth largest coal resource in the state (19 billion tons) but, since it is such a thin seam, only the 5% of it (1 billion tons) that can be cheaply surface-mined is an economically viable resource.
Coal Mining in the Park
The state of Illinois purchased this property in 1948 and built the dam to make Argyle Lake State Park. Prior to that time the Colchester Coal was mined in the park area. In fact, the Illinois State Geological Survey coal mine map of this area shows about 13 mines located within the park boundaries, including an underground mine operated by Amos Atkinson from 1925-1929 that is located just southwest of the earthcache location. I haven’t tried to find this particular old mine because it is off the park trails, but another old coal mine is easily accessible from the road and is labeled on the trail map for the park.
Questions to Answer for this earthcache
1. Estimate the thickness of the coal seam. The rock exposure is very steep so be careful if you choose to get up close and personal with the rock!
2. The layering (bedding) of the rock above the coal seam has a distinctive appearance . Describe the appearance of the layering and suggest a process that could explain its appearance.
3. There are three rock types at this exposure: coal, sandstone, and shale. Based on their position in the exposure, list these rocks from oldest to youngest. (Hint: Use a geology textbook or the web to look up the geologic dating principle known as the Law of Superposition.)
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