Halliday & Matthiessen (2014: 565):
As with parataxis, adverbial groups and prepositional phrases can be linked hypotactically: the tactic relationship is based on identity in function rather than difference in internal structure. Hypotaxis is used to construe spatial and temporal paths and to construe gradual narrowing of the specification of a location. It combines with (i) elaborating, (ii) extending and (iii) enhancing relations.