Coal ball
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A coal ball is a type of concretion, varying in shape from an imperfect sphere to a flat-lying, irregular slab. Coal balls were formed in Carboniferous Period swamps and mires, when peat was prevented from being turned into coal by the high amount of calcite surrounding the peat; the calcite caused it to be turned into stone instead. As such, despite not actually being made of coal, the coal ball owes its name to its similar origins as well as its similar shape with actual coal.
A coal ball
Sir Joseph Dalton Hooker, who along with Edward William Binney was the first to report on coal balls
Calcite (middle) and dolomite (top and bottom) are common materials found in coal balls.
A coal ball from southern Illinois
Carboniferous
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The Carboniferous is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period 358.9 Ma to the beginning of the Permian Period, 298.9 Ma. In North America, the Carboniferous is often treated as two separate geological periods, the earlier Mississippian and the later Pennsylvanian.
Cliff section through the Serpukhovian Red Wharf Limestone Formation, Wales. A marine limestone at the base of the cliff is overlain by an orange-coloured fluvial sandstone. Subaerial exposure of the limestone during a period of falling sea level resulted in the formation of a karstic surface, which has then been infilled by the river sands. A thin, estuarine silty mudstone overlays the sandstone, which in turn is overlain by a second marine limestone.
Hyden Formation over Pikeville Formation in the Pennsylvanian of Kentucky, US. The exposure has Pennsylvanian-aged cyclothemic sedimentary rocks of the Breathitt Group. The upper part of the roadcut is Hyden Formation, consisting of mixed siliciclastics and coal. The lower part is Pikeville Formation, also having mixed siliciclastics and coal.
A reconstruction of life on a forest floor during the Pennsylvanian Period. The reptile is Edaphosaurus, a pelycosaur. Plants are the seed ferns Neuropteris and Pectopteris, the club mosses Lepidodendron and Sigillaria, other plants include Cordaites, Calamites, climbing ferns, pond algae, and Sphenophyllum.
Etching depicting some of the most significant plants of the Carboniferous