The modern Macquarie River immediately north of the Dunkheld ridge.

Imagine this as the equivalent setting, at a time (in the Miocene), when the Dunkheld lava flowed down and into this Macquarie River valley. Thereafter the surrounding land is removed by erosion and this channel sediment topped by the basalt subsequently becomes the highest elevation.

The sediments of this channel are derived from the granite country rock and are little different from the old Macquarie River sediments exposed in the quarry floor, some 50 plus metres above.