The Coal Age
Approximately 323.2 to 298.9 million years ago years ago, the Earth was a much different place than it is today. The continents that we are familiar with now did not exist then. What we know today as the Anthracite Region of Pennsylvania was located on what was then known as Laurasia (Laurentia (the ancient geological core of North America), Baltica, and Siberia)) forming the northern part of the supercontinent Pangaea, parts of which (ancient Pennsylvania included) were located below the equator in the region of present day Brazil. Then, like today the temperatures of the area were consistently warm and humid, conducive for rampant plant growth.
Characteristic of the early part of the Carboniferous period (from about 360 million to 300 million years ago) were its dense and swampy forests of prehistoric vegetation, which contributed to large deposits of peat. Over millions of years the peat underwent transformation into thick coal seams in Western Europe and North America. The name “Carboniferous” refers to this coal. The Pennsylvanian period is named after the U.S. state of Pennsylvania, where thick widespread beds of coal formed during this time.1
How Coal Formed
Carboniferous coal was produced by prehistoric giant club moss or scale trees (Lepidodendron), seed ferns (Neuropteris), tree ferns (Pecopteris, and Sphenopteris), giant horsetails (Calamites), and enormous trees with strap-shaped leaves (Sigillaria). Over millions of years, the organic deposits of these plant debris formed the world’s first extensive coal deposits. Source Essays on the Coal-Formation and Its Fossils by Leo Lesquereux
The growth of these forests removed huge amounts of carbon dioxide from the atmosphere and stored it in the plants’ roots, branches, stems, bark, and leaves, leading to a surplus of oxygen in the atmosphere. Atmospheric oxygen levels peaked at around 35%, compared to 21% today.2
Through the ages the swamps and forests where these prehistoric plants grew were flooded, destroying the forests and covering the masses of plants with sand and fine sediment. Layers of tree stumps, spores, branches, and leaves were continuously slowly built up as the dead plants did not completely decay and were turned to peat in these swamp forests. Over time, heat and pressure converted these carbon rich peat layers into the coal beds of Pennsylvania.
The progression of living plants to coal is in four stages: peat, lignite, bituminous, and anthracite, the final product becoming dryer, less gaseous, harder and more dense as the stages progress. Heat is considered to be the primary influence changing peat to coal. “Heating causes hydrocarbon compounds (compounds composed of hydrogen, carbon, and oxygen) in the peat to break down and alter in a variety of ways, resulting in coal. In general, moisture and gases (for example, methane, carbon dioxide) are systematically expelled from the peat and resulting coal with increasing burial and heat.” Source Geological Survey, University of Kentucky
Centralia, Pennsylvania — world famous for being the town where in May 1962 the city council proposed cleaning up the local landfill by setting it on fire. Brilliant! Except that either they did not think it through or they forgot that the city dump was nestled in an abandoned coal pit with seams of coal radiating out from it in all directions including under the Town of Centralia. When the manager of the dump set the garbage on fire the burning trash set the exposed coal seams on fire. The dump fire followed the coal seams and eventually met up with the coal deposits under the town which caught fire and collapsed causing the earth above it to subside and carbon dioxide, carbon monoxide and other foul fumes to leak upward into basements and living spaces of homes and businesses in the town. However, it was not until 1992 that Pennsylvania took action to finally condemn all of Centralia’s buildings, evict the last citizens living there and eliminate the town’s ZIP code.3
Different nearshore conditions led to different types of sedimentary rocks being formed which is evidenced by the numerous types of fossil rocks found in the abandoned mining areas near Centralia. “Multiple transgressions and regressions of the Pennsylvanian seas across the continent can be seen in the rocks, and even counted, because they leave a telltale sequence of layers. As sea levels rise, the layers may go from sandstone (beach), to silty shale or siltstone (tidal), to freshwater limestone (lagoon), to underclay (terrestrial), to coal (terrestrial swampy forest). Then as sea levels fall, one may see a shale (nearshore tidal) grade to limestone (shallow marine) and finally to black shale (deep marine).” 4
Plant fossils can be found just east of Centralia along the northern flank of a former strip mine with a very broad exposure of dark-gray shale of the Pennsylvanian Llewellyn Formation. In this area the Llewellyn has many fossils of the stems and branches of plants, and occasionally fossil leaves can be found. The rocks are relatively easy to break apart with a hammer, and a screwdriver can then be used as a wedge to break open the shale. You have to be careful with the screwdriver to avoid scratching the fossils. The exposure of the shale also faces south, which means that you will get plenty of sun and lots of good light for finding and photographing fossils.ROCKHOUNDING PENNSYLVANIA AND NEW JERSEY, ROBERT BEARD, Page 119
If you go to hunt for leaf fossils here, bring plenty of water to stay hydrated. I went during the summer and the black shale absorbs heat and radiates it back — it tends to be hot on the hill, Be sure to wear work boots that you are not afraid to get dirty. The slopes of the shale outcropping are slick and you will sink down into to some areas of it which could result in shale dust and pieces going into your shoes if you are not wearing boots. Bring a 5 gallon bucket so you can carry fossil rock pieces back to your car in it. I brought a tool bag that I was able to sling over my shoulder to carry my tools in. Be aware that when you walk back to the shale outcropping from the Odd Fellows Cemetery parking area you will be going up and down some steep and rocky hills. Take your time and be careful.
That is a nice piece of fern fossil above. I bought this from the gift shop of the Lackawanna Coal Mine Tour, it only cost $7.95. You are probably not going to find a fossil this nice at the Centralia site. I do not know for sure, my wife only allowed me two hours to hunt for fossils (we were on vacation from Florida, after all and had other things to do). Maybe if I had more time, I could have found some fancier pieces, I don’t know. Depending on where you park, it will take about 10 to 15 minutes to walk back to the shale outcropping so watch your time and pace yourself.
Dark shale is a black, thinly laminated carbonaceous shale, exceptionally rich in organic matter (5% or more carbon content) and sulfide (especially iron sulfide, usually pyrite), and often containing unusual concentrations of certain trace elements (U (uranium), V (vanadium), Cu (copper), Ni (nickel)). It is formed by partial anaerobic decay of buried organic matter in a quiet-water, reducing environment (such as in a stagnant marine basin) characterized by restricted circulation and very slow deposition of clastic material. Fossil organisms are preserved as a graphitic or carbonaceous film or as pyrite replacements.5
The black rock is anchimetamorphosed (“a metamorphic grade in sedimentary rocks where changes due to diagenesis are overtaken by the very earliest phases of metamorphism“) fossiliferous carbonaceous shale (very slightly metamorphosed). Most plant fossils in shale are preserved as black, carbonized compressions. The whitish surface film of each plant is a silvery gray film of graphite (carbon). The graphite is covered with pyrophyllite (Al2Si4O10(OH)2 – aluminum hydroxy-silicate).6