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.
Forget about billions of years of soil accumulation!
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.
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.
(Peat bogs and coal-forming swamps are an exception, but we would not count them as topsoils.