Minto Crags (known by aboriginal residents as Whimpullin) is an impressive ring dyke of fine grained rhyolite. It is part of the innards of the Tertiary volcano known as the Main Range Volcano.
This ring dyke along with the other intrusion features in the area such as plugs, radial dykes, sills and domes are the more resistant innards of the complex shield volcano. These intrusions punctured the sedimentary coal measures of the Marburg and Walloon series.
Minto Crags/Whimpullin has been dated as being of Oligocene age.
I know of no other ring dyke system in Australia as spectacular and as easy to interpret as this one. There is a swarm of small ring dykes on the flanks of Mt Alford, about 10 kms north of Minto Crags / Whimpullin.