Mythical geology at the mouth of the Tweed River

My knowledge of Gaelic mythology is a bit limited but it is interesting to see where geology, Gaelic mythology, Captain Cook and Tweed heads have something in common. I’ve not been to Ireland or Scotland but I’ve experienced a feature that is quite famous in these countries that is also present on the northern rivers.

Fingal Head, clearly showing the basalt columns

Just to the south of the Tweed River mouth lies Fingal Head and Cook Island. Cook Island, is of course named after then-Lieutenant Cook who sailed along the section of coast in 1770. Fingal Head, however, is named after Fingal, a mythological Gaelic hero from Scotland, who never came to this part of Australia! So why is it named so?
To understand the name of Fingal Head you need to know about the story of the Giants Causeway in Ireland and Fingal’s Cave in Scotland. I’m not a good story teller so here is a link (if this link is still not working try this one instead). My summing up of the story is that one of the two warring giants built a causeway to the other side of the Irish Sea so that he could fight the other. The other giant tore it down so that only each side of the causeway remains, one in Northern Ireland the other, Western Scotland. Local tourist information says that Fingal Head is named after the Irish hero. This is actually incorrect, the Irish hero is named Finn MacCool. The name Tweed River should hint that it is actually the Scottish hero that Fingal Head is named after. So, where does the geology come in?

The giants causeway is made from basalt. The volume and thickness of the basalt lava flows means that different parts of the lava flow usually cool at different rates (though, as pointed out by Goehring et al. 2006, the actual mechanism is completely unknown). However, the general idea is that in the case of Fingal Head the lava flow has cooled quite quickly, resulting in contraction of the rock and cooling joints being formed. The incredible thing about nature is that these cooling joints forms columns of rock that are of similar thickness and cross sectional shape, usually hexagons. This formation style is called columnar basalt. Indeed the rock that makes up the causeway has been shown to extend under the sea all the way from Ireland to Scotland. While the scale is not as great as in the British Isles, Cook Island just a short distance off the coast is part of the same lava flow at Fingal Head. This area, therefore has very similar features as the Giants Causeway and in my opinion the name Fingal Head is very appropriate.

The lava at Fingal Head is apparently derived from the Tweed Volcano (classified as Lismore or Beechmont Basalt, depending on what side you are of the Queensland border). Whether it is a lava flow erupted from the original central vent or vents on the northern flank of the volcano is not known. It is worth knowing that columnar volcanic rock is actually fairly common. Indeed, even better columnar formations can be seen elsewhere in the region. If you travel inland from Bellingen up to Ebor and visit the waterfall there (Ebor Falls) you will be able to see some spectacular formations. Columnar jointing is not restricted to basalt lavas either, some rhyolite cliffs around the Tweed Volcano also show this feature too.

References/Bibliography:

*Goehring, L, Morris, S.W. &  Lin, Z. 2006. Experimental investigation of the scaling of columnar joints. Physical Review. V64.
*Stevens, N.C., Knutson, J., Ewart, A. & Duggan, M.B. 1989. Tweed. In Johnson, R.W. (ed). Intraplate Volcanism in Eastern Australia and New Zealand. Cambridge University Press.

Looking out from the lookout at Point Lookout

The view of the National Park from Point Lookout

One of my favourite places is Point Lookout at the New England National Park between Ebor and Dorrigo. Point Lookout is spectacular for it scenery and feel. On most days you can see the pacific ocean while looking over rugged hills and valleys and I particularly like going there during winter where icicles hang from trees and the waterfalls below the peak are frozen. Point Lookout is nearly 1560metres high which I understand makes it the highest point in northern New South Wales. Like the beauty of the Mount Warning area and Tweed and Brunswick River region, Point lookout owes its attractiveness to the erosion of a large shield volcano.

Point lookout is located on the rim of an escarpment which formed through the erosion of the Cenozoic aged (in this case 19-18 million years) Ebor Volcano and the much older Devonian to Carboniferous (up to ~416Ma) accretionary complex rocks that make up the balance of the New England tablelands. Today, only the north western portions of the lavas (called the Ebor Volcanics) and the central weathered volcanic plug from the Ebor Volcano remain. Research by Ollier (1982) suggested that the central volcanic plug of Ebor Volcano was centred on what is called the crescent which is actually a fairly insignificant looking feature when compared with the rugged valleys today.

It is interesting to note that even though the nearby 23 Million year old Mount Warning (located near and over the Queensland border) is regarded as one of the biggest shield volcanoes in the southern hemisphere, having a height of around 2000 metres before it was eroded, the Ebor volcano was probably a similar size or bigger at its greatest too. It is a bit of a mystery why so little is left of Ebor Volcano when so much remains of the Tweed Volcano/Mount Warning.

The Crescent complex once thought to be Permian (290Ma-250Ma) as recently at the 1970's and was considered part of the intrusives that constitute the New England Batholith. In fact most of the most 'current' geological maps of the area were drawn at this time and so they are incorrect. But since investigations on the radial drainage patterns and geological features by Ollier in the late 1980s followed by dating by Gleadow and Ollier (1987) (which is difficult due to how weathered the Crescent is) and more recent work by Ashley et al (1995) now it is known to be the centre of the Ebor Volcano and aged around 19 Million Years. Ashley et al (1995) also discovered that a nearby basalt called the Doughboy Basalt was around 46 Million years old which is clearly not related to the Ebor volcano but is consistent with other locations where an older Cenozoic basalt is present before the hot spot volcanism that formed the Ebor, Mount Warning and other volcanoes existed.

When I was last at Point Lookout there were several bush walks from long and difficult to short and easy. The most difficult ones take you into the valleys where the rock has been eroded into the older accretionary complex. But even on the short one you can see some interesting 'recent' volcanic rocks. On a section of the walk around the top of the cliffs where security fences are necessary (lest you plummet away!) there is cuttings through the rock. In this rock look closely and you'll see some big crystals within a fine groundmass. This rock is a type of basalt called tholeiite (which means that it has crystalised with a certain geochemical signature) and the crystals are feldspars which is a common rock forming mineral. The feldspars here quite obvious and seem to catch the light at two angles, this feature is called twinning and is characteristic of the calcium rich variety of feldspar called plagioclase. Along the bigger walks below the point dacite can be found as well as basaltic and dacitic breccias at the stunningly beautiful during winter, weeping rock and numerous palaeosols.

The remnant of the shield volcano shows the characteristic radial drainage pattern for volcanic shields but the eroded central areas of the volcano (including the caldera if there was one) drains fairly directly to the east via the Nambucca River. The radially draining creeks and rivers are well known for their waterfalls such as Dangar Falls and Ebor Falls.

The road from Dorrigo to Armidale is not a busy route, it is often missed by many people but I always recommend people visit the New England tablelands because of its beauty and uniqueness in Australia. Point lookout is just off the Waterfall Way which name probably gives you an indication of many of the other attractions. In my opinion, the depths of winter are the best times to visit to get the mood and subtle beauty of the area. I should get back there myself... it has been too long since I was last there.

You may be interested in a self-guided geological tour. Bob and Nancy from Armidale have a wonderful site which includes an excellent (and expanding) range of geological tours including ones of the Northern Rivers Area. Their tour guide on Point Lookout can be accessed from their webpage (very much worth the look) or  directly linked from here.   

References/bibliography:

*Ashley, P.M., Duncan, R.A. & Freebrey, C.A. 1995 Ebor Volcano and the Crescent Complex, northeastern New South Wales: age and geological development. Australian Journal of Earth Sciences V42.
*Gleadow, A.J.W. & Ollier C.D. 1987 The age of gabbro at the Crescent, New South Wales. Australian Journal of Earth Sciences V34.
*Ollier, C.D. 1982 Geomorphology and tectonics of the Dorrigo Plateau, NSW. Australian Journal of Earth Sciences V29.