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FIELD MUSEUM OF NATURAL HISTORY A forest of the Great Coal Age How Coal Was Formed     Let us take another imaginary journey, this time back through the ages-- millions and millions of years back to the Great Coal Age. There have been a number of little coal ages, but most of the worlds best coal was formed in the Great Coal Age.     In the Great Coal Age there were no people on earth. There were no animals that fed their babies on mother's milk. There were no birds. In the sea, however, there were fishes. On the land there were little lizard-like animals, huge spiders, and giant insects such as six-inch cockroaches and dragonflies with wings nearly a yard across.     The air was warm and moist. Much of the land was swampy forest. The forest trees were not oaks or maples or pines or any other trees that we know. They were mostly huge ferns, great mosses, and giant horsetails. They grew fast. They died and fell into the water. More trees grew.     Under the water the dead trees did not rot as they would in the open air. Thick layers of dead plant material grew thicker year by year. They formed peat much like the peat which even today forms under water in swampy forest and other swampy lands.     In the coal ages some of the forest land was slowly sinking. Many forests sank into the sea. There they became covered with sand and mud. Then slowly the land rose again. Once more forests grew on it. In some places this happened many times.