Depth means how settled a thought is
Sky is where half-formed things drift. Bedrock is where a settled one comes to rest and stops moving.
Four strata needed to mean something before any content could go in them. Candidates were topic, age, and maturity.
The site had already answered it, though not on purpose. From the motion script, written months before there was anything to put in these zones:
sky — slow, perpetual drift, never settles surface — quick, springy rise, things pushing up out of the ground soil — the same, slower and heavier, pushing through resistance bedrock — settles once, firmly, then does not move again
That is not a description of subject matter or of publication dates. It is a description of how sure of something you are.
So: sky is fluff — questions and hunches. Surface is what I’m actively turning over. Soil has been worked a few times and holds its shape. Bedrock is settled, and other notes stand on it.
Two things follow. Notes move: a thought sinks as it firms up, which is why zone is a field and not a folder, and why a note’s URL has no zone in it. And a note’s sprite is the form of its stratum — a half-formed thought is drawn as a cloud, a settled one as a stone. You can see how much to trust a note before reading a word of it.
The honest test of this scheme is whether bedrock stays nearly empty. If everything I write ends up down there within a month, I’m not using the scale, I’m just flattering myself.