(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!
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.
Where sediment is neither being collected nor eroded, soils necessarily take their mineral components from the underlying parent rock.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.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.
Geologically speaking, any given patch of land is seldom in equilibrium for long.