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(Peat bogs and coal-forming swamps are an exception, but we would not count them as topsoils.

Under unusual conditions a layer of topsoil can be "fossilized," even to the point of preserving the three-dimensional shape of tree leaves, as is the case at Yellowstone National Park.) In the long run, buried sediments are usually cemented into sedimentary rock, which brings us back to the beginning of this cycle.

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At about the three-foot level (in the center of the yard) the red-brown clay is abruptly terminated by a reddish conglomerate we call hardpan.

A few sickly-looking roots, long dead for all I can tell, do penetrate the clay, usually by hugging the surfaces of the boulders, before being stopped cold by the hardpan.

This is usually the fate of every plot of land which remains above sea level long enough.

Large areas of Canada, for instance, have been eroded down to the Precambrian basement rock!

Take the soil in my mother's backyard, for example.

After about 18 inches the soil grades into a two-foot matrix of solid, smooth clay mixed with boulders.

The sediment added to our patch of land may be great for building new soil, but if it accumulates too quickly it will merely bury the existing soil. In any case, the old topsoil, now compressed and deeply buried by sediment and soil, is no longer turned over by earthworms or small animals.

It is deprived of oxygen and fresh organic material, such as rotting leaves.

Consequently, there is a practical limit to how deep the soil can get even if erosion never occurs.

The accumulating humus will also reach an equilibrium, when new material balances that lost by decay and oxidation.

Geologically speaking, any given patch of land is seldom in equilibrium for long.

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