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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.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.Sediment, in the form of dust, would normally come from very dry areas where organic material would be quickly oxidized.(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.Geologically speaking, any given patch of land is seldom in equilibrium for long.Either it is collecting sediment or being eroded away, usually the latter. Water-borne sediment will be washed in from higher ground, perhaps hills and mountains hundreds of miles away.
However, the deeper that soil gets, the more insulated the parent rock becomes to weathering.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.The geologic history of the strata making up the Grand Canyon is as much a history of erosion as it is of deposition!Consequently, a patch of soil cannot be older than the last local erosion--whenever that might have been.