320 - 286 Million Years Ago
Highlights of the Pennsylvanian
- Wide spread swamps
- Many coal beds were laid down at this time
- The first reptiles evolved. They were small (about a foot long) and outnumbered by the amphibians
- Many insects
During the Pennsylvanian period (286-320 million years ago), extensive forests grew in swamps on river deltas. The coal swamps of eastern North America occupied a basin bordered on the south and east by the Appalachian Highlands and on the north by the low swell of the Canadian Shield. Trees over 30 metres high grew on a mush of organic material. In death they made their own contribution to the peaty layer.
Bacterial decay was limited by a lack of oxygen, allowing delicate fossil leaves to be preserved. As the area cycled from swamp to a shallow marine environment, the organic material was covered by layers of sediment. Sand formed sandstone, mud formed shale and siltstone, marine creatures with calcium carbonate skeletons formed limestone. The swamp deposit sank deep under the weight and the water was squeezed out. Ten meters of wet, porous material was converted to one meter of coal.