Mt Napier erupted over several decades about 8000 years ago. The lava flowed into valleys and over the landscape forming a shield around the volcano itself. Scientists can date the lava flow that filled Harman's Valley with carbon 14 isotopes of the peat that formed in a swamp after the lava filled a valley.
Harman's Valley can be easily seen from a lookout on the road near Byaduk. And there are excellent information boards there as well. How else would I have known about isotopes!!