This is the interpretion of the geological history of the basalt ridge at Dunkheld.

A basalt lava flow flooded into the Macquarie River in the Miocene, overwhelming the river. Something dammed the basalt flow such that it ponded to thickness in excess of 25 metres. Water e.g. channel and groundwater, that was trapped beneath the flow vented out through the thick ponded lava at the sites denoted by the yellow stars (the circular features on the aerial photograph) which I have callled steam vents. Steam vents 1 - 5 have been exposed during the removal of the basalt in the quarrying operation.

The Macquarie River channel was diverted by the ponded basalt and it would have first channelled at the northern edge of the flow and as down cutting of the country rock was easier, than the crystalline basalt, the channel has moved northwards as the downcutting has continued.

Note that the ridge (and the paleo-channel) shape is similar to the shape of the present river channel. The old Macqurie River sediments are in a channel structure below the basalt, which is exposed in the hillside at each end of the ridge, and when I was doing this research, the bottom level of the quarry had exposed some of these old river sediments.

Where the lava cooled unaffected by the steam vents, cooling was much slower and magamatic differentiation occurred with olivine sinking to the lower parts of the flow and lighter plagioclase dominating the upper parts of the flow. Columnar jointing was spectacularly developed thoughout the cooling flow and when unaffected by the steam vent the columns are 20 plus metres tall and grandiose in size. However, the columnar jointing was severely disrupted by the cooling surfaces that the steam vents caused and you will see that columns, which typically propogate perpendicular to a cooling surface, in this basalt flow radiate inwards towards the perpendicularly orientated steam vents.

The processes involved in the steam vents, hydrothermally altered funnel - shaped (inverted cone) zones in the basalt with the top of these funnels indicated by the circular features on the ground. The common alteration product is a sticky montmorillic clay and there is also some prenhite zeolitisation.

The following images illustrate the concepts and theory expounded here.