This is the basaltic rock that once filled the ancient Seven Creeks Valley, from Gooram down to Euroa (Miocene environment and climate described below). The lava flowed from one of the several volcanic vents in the district, about 7 million years ago. The quarry site where these specimens came from is likely the plug, or the base, of one of the lava vents. Somehow, this lava from deep below the earth’s crust, found its way up, through and between the granitic rocks of the Strathbogie Ranges that had already been there for 350+ my! Its probably not surprising that this happened near the edge of a batholith and where the granite may be relatively shallow and perhaps fractured and ‘porous’.
The rock is very hard and bluish-grey in colour. These specimens have been recently quarried and most surfaces have a black, speckled appearance, with the black specks up to 2 mm in diameter. These ‘deposits’ can be scraped off the surface (with a knife), but can also be found inside the rock matrix; perhaps they are associated with minute fractures, or vesicles. Other minerals, some gold-coloured, can also be seen. Where the rock is polished, tiny vesicles can be clearly seen.
Here’s the quarry site showing the vertically jointed, columnar basalt making up the volcanic ‘plug’. The joints/fractures (formed when the lava cooled and shrank), have allowed water to deeply infiltrate the rock and deep weathering is clearly visible. Rock close to the surface has weathered into ‘blocks’ and those at the very surface become completely detached from the rock-mass below and erode to form the characteristic rounded boulders of many volcanic landscapes.
Though this basaltic rock is much harder than the granitic rocks that make up the adjacent hills, this deep weathering allows more rapid denudation of the basalt. Much of the Gooram lava flow appears already to have been eroded away, but it is thought that the flow continues some distance north, beneath more recently deposited sediments, perhaps beyond present day Euroa.
This story was inspired by the recent Strathbogie Ranges Conservation Management Network field day with Dr. Neville Rosengren – many thanks!
And this was all happening when:
- Australia was isolated from other landmasses (recently detached from Antarctica allowing a cold, circumpolar current to develop) and its northern edge was colliding with groups of islands in South East Asia.
- From the middle Miocene, as polar ice sheets rapidly began to grow in Antarctica, Australia became progressively drier. But the earth was still several degrees C warmer than today.
- In the late Miocene (5 to 10 mya), sea-levels dropped. In southern Australia, the Nullarbor Plain – once the limestone bottom of an ancient sea – was exposed.
- Rainforests retreated to the wetter coastal areas of Australia. Open forests and woodlands were spreading in the drier inland areas.
- Great herds of large wombat-like marsupials roamed Australia – some had even developed trunks.
- A new group of kangaroos that hopped began to dominate over their slower four-footed walking relatives.
- Gigantic thunder birds ruled the Australian roost.
- The fiercest predators included powerful flesh-eating marsupials called thylacines and lioness-sized marsupial lions.
What was happening in the rest of the world
- Grasslands were spreading in Africa, Asia and North America.
- Ancestral forms of apes lived in Africa, Europe and Asia.
- Like Australia, South America had many marsupial groups, including the ancestors of sabre-toothed marsupials.