Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 130   November 8, 1947
Forest Preserve District of Cook County 
William N. Erickson, President
Roberts Mann, Supt. of Conservation

When the glaciers covered and then uncovered this Chicago region, as they did at least four times in the past million years, they left behind them raw ground-up rock. In places it was only a few feet thick; in others it was heaped up hundreds of feet deep. Some of this rock was in the form of rounded boulders, many of them huge; some was gravel; some was sand; but most of it was finely-ground rock "flour" which we call silt or clay.

Such an area of the earth's surface uncovered near an ice sheet becomes Trunda. The word is of Finnish origin, meaning a bare treeless stretch of country. Only primitive plants can grow on such raw earth -- lichens and mosses, with perhaps a little grass and small willows -- and the only common animals are the insects that live in the ponds, and the reindeer and lemmings that feed on the lichens and mosses.

Lemmings are small rodents that multiply in unbelievable numbers on the tundras that spread over vast areas of the arctic regions in Europe, Asia and North America.

A few predators -- foxes, wolves, hawks, owls and other birds that feed on these animals -- are found, but that is all.

The tundra is frozen year in and year out except for a thin surface layer that thaws during the brief arctic summer. Then the lichens and mosses grow rapidly.

In some places the ground is covered by frozen Sphagnum moss to a depth of twenty feet. Reindeer moss, which is not a moss but a lichen, grows in thick beds covering large areas. In this short summer, hordes of mosquitoes and other insects hatch, mate, lay their eggs, and prepare for the long arctic winter.

Miniature tundras are found occasionally in the Chicago region where the top soil has been removed artificially or by water erosion. In such places there may be found dozens of kinds of lichens, mosses, and other primitive plants crusting the otherwise barren earth. Trees and other plants, and the animal life that goes with them, must wait for the lichens and mosses to slowly convert the surface of the rock "flour" into top soil.

A lichen is the curious marriage of two plants: a fungus and an alga. Why, Alga!

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