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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.
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.
) Just because a patch of topsoil takes x centuries to build up doesn't mean that the land is x centuries old.
Most likely, that topsoil began to build up only recently, geologically speaking, and has either reached a practical limit to its depth or has been subject to erosion.
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.
That means less chemical weathering from bacteria and fungi.
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.
(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!
(Peat bogs and coal-forming swamps are an exception, but we would not count them as topsoils.