Halliday (2008: 19-20):
The point to be made is that one aspect of the complementarity of spoken and written language lies in their different ways of achieving and managing complexity. The complexity of written language resides in its density: the way it packages its meanings into highly condensed, mainly noun-based structures which combine into rather simple clausal configurations. The complexity of spoken language resides in its intricacy: the way it knots together long strands of quite sparsely loaded clauses into intricate patterns of logical-semantic relationships. So the complementarity of speaking and writing is not simply that of their different modes of being and of happening, but rather in the different strategies for the organisation of meaning — as “spoken language” and “written language” — that have evolved to match, and to exploit, these two different modes of existence.