Largest ‘fossil forest’ discovered in US

To better appreciate the discovery of the world’s largest fossil forest, one has to understand first how coal is formed because the remains of rainforests could likely be found only where there are coal seam deposits.

Coal is a black or brownish black combustible mineral formed about 300 million years ago when the earth was covered by swampy forests of scale tress (lycopods) giant ferns, horsetails, and club mosses. Layer upon layer of these plants died and were compressed and then covered with soil. As the layers were successively covered their access to the air was limited and this stopped the full decomposition process creating peat.

For the peat to become coal, it must be buried by sediment. Burial compacts the peat and, consequently, much water is squeezed out during the first stages of burial. Continued burial and the addition of heat and time causes the complex hydrocarbon compounds in the peat to break down and alter in a variety of ways.

The gaseous alteration products (methane is one) are typically expelled from the deposit, and the deposit becomes more and more carbon-rich as the other elements disperse. The stages of this trend proceed from plant debris through peat, lignite, sub-bituminous coal, bituminous coal, anthracite coal, to graphite (a pure carbon mineral).

          Fossilized fern

The recent discovery of the fossil forest in the underground coal mines in eastern Illinois only relates what the past was for the state. Long ago, it was the site of one of the world’s first rainforests dominated by tall, asparagus-like trees believed to extend for up to 100 miles and preserved in the ceiling of a coal mine some 250 feet below the surface.

The compacted remains of these forests are encased in shale above coal seams. After the coal is extracted the fossils can be seen in the mine ceiling.

According to scientists, the forest was preserved when an earthquake dropped the area a few feet allowing flooding from an adjacent river, which drowned the vegetation and buried it in sediment. The sudden flooding in the submerged block killed the rainforest. Mud and silt rushed into the depression, preserving the stumps and logs in a layer that eventually became shale.

Note fossilized tree trunk between shale  ceiling and coal seam

Researchers have traced a five-mile route through the forest, stopping to measure huge fossilized trees still stand rooted in their original but compacted soil, surrounded by fallen leaves.

One fallen scale tree, measuring more than 100ft long, was traced by researchers until it disappeared into an as yet unexploited coal seam.

"What's extraordinary about this discovery is that this forest has been preserved in its growth position," said one of the scientists who saw the site. "It's an upright forest with trees still standing upright."

It is said that the forest can be viewed only from below. The scientists who discovered it had to crane their necks, illuminating the ceiling with miners’ helmet lamps.