Hi Ben, I heard this in Burma... so it could well have been from the commentaries. But I vaguely remember having subsequently come across it in the Sutta Pitaka. Could easily be mistaken though.
I think it refers to the strength and duration of underlying tendencies; the lightest likened to writing on water. Then (I think) writing on sand, followed by etching in tree bark, and finally carved in stone.
The memory of this simile came up in a sitting this morning. A pattern of constriction in the chest becoming painfully clear. A pattern which seems like it has been there forever. And, after almost three decades of "huffing and puffing" it seems like at least the etched in wood kind. Maybe the psychic equivalent of bonsai... or foot-binding. Not the kind of thing that a little crystal-waving can clear up. But then again, maybe I'm exaggerating the whole thing.
Dukkha patipada dandabhinna... meh?
So I thought it might be good to look up the teaching, to see what else might be included. Even rocks occasionally get annihilated by thunderbolts, no?
retrofuturist.. thanks for the link... I'll check it out.