Halliday (2008: 167-8):
So the written language tends to be configurational in its grammar: it displays the classic “constituent structure” on which formal linguistic categories are modelled — again, like writing itself; while the spoken language is organised like speech, with experience modelled as a flow of events where entities — persons and objects — are given value by their rôles as participants in these events. And since writing is, both as it evolved in the species and as it is developed by each individual, subsequent to and derived from speaking, the written sentence is itself a semantic junction — between its original state of being as a “flow of meaning”, in the spoken mode, and its adopted state of being as inscription, or documentation, in the written.