Forest Preserve District of Cook County, Illinois
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"Modern" Coal Plants
Nature Bulletin No. 331-A   February 7, 1969
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

The Age of Cycads, when those strange tree-like plants predominated, began during the Triassic Period of the earth's geological history, reached its peak during the 60 million years of the Jurassic Period which followed, and ended during the first part of the Cretaceous Period that began about 95 million years ago. During the Jurassic, in addition to Cycades, there were also many species of ginkgos, and conifers which were the ancestors of our modern sequoias and pines. The ginkgo or "Maidenhair Tree", which we have imported from China and Japan, is the only one remaining of that tribe -- "a living fossil".

Although enormous reptiles then dominated the land, the waters and the air Jurassic rocks contain fossils of the first bird-like creature, many insects, and a few mammals something like the living duck-billed platypus. Also for the first time, annual growth rings appear in fossil wood although the climate seems to have been growing warmer and more humid over most of the earth.

During the Cretaceous Period the climate became uniformly warm and humid from pole to pole and then, all over the earth, an entirely new type of plants appeared. Previously, plants had reproduced by spores (like ferns) or by naked seeds (like conifers). Some of these, like the tree ferns, giant horsetails and cycads, had reached their climax and virtually disappeared; many others, like the ferns and club mosses, had dwindled in size and numbers to the minor importance they have today. Suddenly, great numbers of flowering plants -- plants in which the seeds are produced in a closed capsule or ovary -- appear as fossils in Cretaceous rocks. No one knows how they began or why they spread so rapidly and developed into so many thousands of species -- although the great numbers of insects may have helped -- but, with an almost explosive expansion, they took possession and dominated the plant world, as they still do.

There were probably grasses, sedges, cattails, and the like, but such plants, naturally, decayed quickly and there is little evidence of them. There were Mary of our modern trees and shrubs, or their close relatives. In sandstones formed during that period, now beneath Kansas and neighboring states, are fossils of forests native today in the subtropics. As far north as Greenland and Spitzbergen there were mixed forests of ginkgos, conifers -- sequoias, junipers, pines and cypress -- and hardwoods including maples, oaks, walnuts, persimmons, sycamores, tulip trees, eucalyptus, and even the breadfruit tree.

During the later part of the Cretaceous Period and the early part of the succeeding Tertiary Period, the earth' s land surface was blanketed with an amazingly luxuriant vegetation, just as it had been during the much earlier Pennsylvanian Period, and there were tremendous deposits of material which later became coal: especially lignite or brown coal.

During the middle Tertiary there was great volcanic activity. The Alps and Himalayas were formed, and the Rockies were thrust up to a higher level. Then the earth's climate gradually cooled. New plants appeared -- plants with soft non-woody stems that die clown after bearing seed but send up new stems each year or grow from seeds -- plants that had learned how to "dig in" and survive cold winters. Finally, about one million years ago, the Ice Age began and there have been at least four major advances and many retreats of great ice sheets from the polar regions. In the warm intervals between glaciers, large beds of peat were formed in many parts of the world, just as they are being formed today.

We may be living in one of those warm spells.

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