Halliday & Matthiessen (2014: 414n):
Note that a number of the items functioning as spatial adverbs can also serve as prepositions; cf. they clambered down and the[y] clambered down the hill. These combinations of verb of motion + adverb of place are less like prototypical phrasal verbs in that the adverb can be treated textually in the same way as a circumstance of Place: down they clambered, it was down that they clambered.