Halliday & Matthiessen (2014: 338n):
Some linguists have in fact thought that English is only lexically ergative. But this is not a tenable position once we realise that lexis and grammar are not separate modules or components, but merely zones within a continuum: ‘lexical ergativity’ in English is an extension in delicacy of ‘grammatical ergativity’ within the experiential clause grammar; and the explanation for the evolution of ergative patterning is grammatical in the first instance rather than lexical.