Halliday & Matthiessen (1999: 178):
Circumstances of the “macro” type are realised by prepositional phrases, which as we have seen are like miniaturised clauses; their components are thus of different types — a (minor) process plus a participant. Circumstances of the simple type, on the other hand, are realised by adverbial groups; these are groups of adverbs, like more soundly, not so very fast, with the single adverb again as the limiting case.