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.
) Just because a patch of topsoil takes x centuries to build up doesn't mean that the land is x centuries old.
The accumulating humus will also reach an equilibrium, when new material balances that lost by decay and oxidation.(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.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.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.I suspect that most of them belong to plants which were chopped down years ago.There's not much down there in that clay to completely rot them away.Thus, topsoil does not accumulate like most sediment, by simply piling up.In the case of erosion, the topsoil, of course, is removed.