Halliday & Matthiessen (2014: 729):
Nominalising is the single most powerful resource for creating grammatical metaphor. By this device, processes (congruently worded as verbs) and properties (congruently worded as adjectives) are reworded metaphorically as nouns; instead of functioning in the clause, as Process or Attribute, they function as Thing in the nominal group. Thus, for example:
is impaired by alcohol alcohol impairmentthey allocate an extra packer the allocation of an extra packersome shorter, some longer of varying lengththey were able to reach the computer their access to the computertechnology is getting better advances in technology
What then happens to the original ‘things’? They get displaced by the metaphoric ones, and so are reduced to modifying these: alcohol becomes a Classifier of impairment; the computer, one extra packer and technology go into prepositional phrases functioning as Qualifier to, respectively, access, allocation and advances.