Oolithic limestones are composed of calcium carbonate in concentric spheres that were produced by high wave energy. Each layer represents an individual flood event in the coal swamp. On the right, a scientist observes the evidence of glacial and interglacial strata in Kansas. Glacial periods result in lowered ocean levels, while interglacial periods result in a rise in ocean levels, covering the continental shelf with shallow seas. Coal beds , which can be up to eleven to twelve meters thick, characterize the Upper Carboniferous.
Deposits reflect the transgression and regression of the seas over the continents.
The forests of seedless vascular plants that existed in the tropical swamp forests of Europe and North America provided the organic material that became coal. Dead plants did not completely decay and was turned to peat in these swamp forests. When the sea covered these swamps, marine sediments covered the peat. Eventually, heat and pressure transformed these organic remains into coal. Coal balls found in sites contain these plant remains.