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As more and more of that rock is weathered by the mechanical effects of freezing and thawing, the chemical and mechanical action of roots, or by other means, the soil is deepened.

However, the deeper that soil gets, the more insulated the parent rock becomes to weathering.

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.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.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!(Topsoil is full of microbes that love to munch away on organic material, and don't forget the earthworms.Those earthworms don't get their calories from rock and clay!Such sediment, even if from nearby hills, would normally carry very little organic material as the weathering slopes, themselves, would not have much to begin with.