Halliday & Matthiessen (2014: 726, 728-9):
Typically, written language becomes complex by being lexically dense: it packs a large number of lexical items into each clause; whereas spoken language becomes complex by being grammatically intricate: it builds up elaborate clause complexes out of parataxis and hypotaxis. …
The nominal group is the primary resource used by the grammar for packing in lexical items at high density. … In written language, the clausal patterns are typically rather simple; but the ideational content is densely packed in nominal constructions … And it is the written kind of complexity that involves grammatical metaphor.
