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It is no longer a paradise for bacteria, and fungi.What organic material it did have is often lost by decay and slow oxidation.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!Forget about billions of years of soil accumulation!Where sediment is neither being collected nor eroded, soils necessarily take their mineral components from the underlying parent rock.
After about 18 inches the soil grades into a two-foot matrix of solid, smooth clay mixed with boulders.
Whatever damage is done to the clay by the few penetrating roots may, for all I know, be patched up by clay particles sifting down through the soil.
The yard is located, along with much of San Diego, on a plateau, and meandering streams over thousands or millions of years have brought rocks down from the hills and rounded them into boulders.
Thus, topsoil does not accumulate like most sediment, by simply piling up.
In the case of erosion, the topsoil, of course, is removed.
The geologic history of the strata making up the Grand Canyon is as much a history of erosion as it is of deposition!