Chicago's Coral Cities

A modern coral reef.

        Because I am a pebble with a view of Chicago's skyline, I have seen the city grow and, as I have watched, I thought of a city that I saw built in this very spot by creatures quite unlike the inhabitants of Chicago. Luckily, undersea currents had rolled me about and animals had scuffed me out of the mud so that I was not buried at that time. The land was again covered by a great warm sea; but in spite of this fact there were countless creatures living together here in huge clusters of buildings, cities that they themselves had built. These were the corals.
       You have surely heard about corals that build reefs today, and you probably know also that coral reefs are found in the warm tropical regions of the world. This kind of coral is an animal that can't survive in a region unless the climate is tropical. The reef-building corals of today are very much like those that lived in ancient times. And so, because we know the habits of corals that are living now, we know that Chicago was once covered by a shallow sea and that its climate was warm all the year around.
       Actually, the name of the coral animal isn't coral at all, but polyp. This polyp is able to take out of the water a substance called lime, of which certain rocks are made. This would appear to be a task harder than it sounds because the lime is completely dissolved in the water and can't be seen at all. After the polyp removes the lime from the sea, he uses it to build his little house. The house is a kind of tiny cup in which the polyp sits. It is this house, built by the polyp, that is known as the coral.
       The little polyp is just a hollow blob of soft fleshy tissue with an opening on top. This opening, the mouth of the polyp, is surrounded by tiny waving arms. The polyp builds his house not only for comfort but also for protection because he is so soft and almost too small to be seen. The limy coral serves as an outside skeleton or protective armor for the polyp. Most corals live together in groups or colonies, although each polyp stays by itself. It is the colonial corals that build the great coral reefs or "cities."  The coral cities of Chicagoland were much more beautiful than the cities built by men, and their inhabitants did not have a housing shortage. As a matter of fact, though the polyps built their houses skyscraper-high in proportion to their tiny bodies, they only lived in the penthouse apartments. The polyp just didn't know when to stop building.
       In order to eat, the little polyp sticks his tiny arms above the wall of his room. He waves them back and forth to sweep all sorts of microscopic plants and animals toward him. He sweeps these into the opening on his top side. In the meantime, however, he is still building the walls of his room higher and higher.
       After a while, the wall is so high that the polyp can't reach over the top for food. Then he pulls himself up and builds a floor underneath himself so that he can again reach over the side. The polyp does not learn from this experience; and so the great reef is built higher and higher, up to the ocean surface.
       Although the polyp himself does not become fossilized, the great reefs that he builds do. In many places around Chicago you can see the fossilized ancient reefs right in the rock that formed above them.