The channel narrowed in the particular way things narrow when nobody actually designed the narrowing, but it happens anyway after two different systems spend a long time pressing against each other and eventually reach a compromise they never formally agreed to.
It didn't arrive as a doorway or a clean threshold. It arrived as a gradual change in quality over roughly twenty feet. The walls edged inward by degrees. The ceiling decided it preferred a lower position. The water adjusted its run slightly as the channel width changed.
I had the bracket notation on my map in my coat pocket.
I didn't need to consult it.
The bracket section introduced itself through exactly the kind of structural evidence that brackets in a map annotation are meant to warn you about in advance.
I put my hand on the right wall.
The mortar was present for about the first six feet of the narrowing.
Then it wasn't.
