After study, it appears that this was one a series of fills of a drainage channel in the area.

SU552 fills the cut SU553 which is a channel in the bedrock; they are a continuation of the same channel as SU563 and SU564. This channel is interrupted by SU478 but it is unclear which feature is cutting which. This channel runs underneath the wall SU420. The formation of the channel fill is unclear. The blocks at its east end look intentionally deposited. The rest of the fill (small, unjoining pottery fragments and file) may be accumulation, but probably are intentional construction fill from before the building of the structure.