Halliday & Matthiessen (2014: 292):
Verbs serving in clauses with a circumstantial process are often derived from a basic use in ‘material’ clauses of motion … The unmarked present tense is the simple present … rather than the present in present of ‘material’ clauses … The Carrier is typically some immobile physical feature, whereas the Actor of a ‘material’ clause of motion is typically an animate being or a mobile entity. Because of the overlap of a large set of verbs, there will of course be cases that are indeterminate …