Halliday & Matthiessen (2014: 311-2):
But a prepositional phrase is an odd sort of hybrid construction. It has a nominal group inside it, as a constituent, so it looks bigger than a group; and yet it is still not quite a clause. In English, this nominal group inside a prepositional phrase is no different from a nominal group functioning directly as a participant in a clause, and in principle every nominal group can occur in either context; e.g. the mighty ocean, participant in little drops of water make the mighty ocean, circumstance in I’ll sail across the mighty ocean. And if we focus attention on the nominal group in its relation to the overall process, it still seems to be some kind of participant: even in the sailing, the mighty ocean does play some part. But it is allowed in, as it were, only indirectly — through the intermediary of a preposition …