Halliday & Matthiessen (2014: 425):
But note that prepositional phrases are phrases not groups; they have no logical structure as Head and Modifier, and cannot be reduced to a single element. In this respect, they are clause-like rather than group-like; hence when we interpret the preposition as ‘minor Predicator’ and ‘minor Process’ we are interpreting the prepositional phrase as a kind of ‘minor clause’ — which is what it is.