THE MAIN RANGE by Neville Stevens and Warwick Willmott
Total Page：16
File Type：pdf, Size：1020Kb
THE MAIN RANGE by Neville Stevens and Warwick Willmott The steep eastern escarpment of the Spicers Gap, partly because of the For instance, very gassy lavas often Main Range, stretching from the New presence of the trachytes. The maximum solidify before gases can completely South Wales border at Wilsons Peak to total thickness now exposed is about 900 escape, resulting in a rock perforated by north of Cunninghams Gap, acts as a m and it may have been originally as small holes (or vesicles). In some cases natural barrier between the Moreton much as 1000 m. It is uncertain how far the holes may be filled with minerals Region and the Darling Downs, and north it extended; basalts around such as zeolites deposited from late forms part of southeast Queensland’s Toowoomba may have come from fluids and gases. When in-filled, the mountainous Scenic Rim. separate fissure vents. vesicles are referred to as amygdules. How did these mountains get there in the The volcano is believed to have Lava cooling conditions can also result first place, and why is there such an originally extended much farther in a variation in the extent of jointing obvious escarpment? This leaflet eastward. Whilst the remaining lavas of (fracturing). Some basalts may have provides some of the answers, as well as the Main Range have a gentle slope to very few, while others may be highly describing the rocks and landscapes that the west from the escarpment crest, it is fractured, and hence extremely can be seen. assumed that they originally also sloped susceptible to weathering. Thick flows down to the east of the escarpment, e.g. cool slowly and commonly develop a Origin to the Kalbar-Boonah district, and to the regular pattern of cooling fractures northeast towards Rosewood, where called columnar jointing. The Main Range is composed of they have since been removed by numerous, nearly horizontal lava flows, erosion. Evidence for this comes from The trachytes are mainly light purple- mainly of basalt, which were erupted the elevation of the base of some grey with numerous white to pink between 25 and 22 million years ago in remnant basalts which are left at a fairly feldspars throughout, although some the Tertiary period, to gradually build low level around Perrys Knob near dark brown to grey varieties also occur. up a complex and elongate volcano. Rosewood and other places west of Such lavas are more viscous (sticky) There are also some prominent flows of Ipswich. than basalt and remnants of flow trachyte in the sequence. banding commonly can be seen. Again, Lavas of the volcano gas bubbles are common, but these are Basalt lavas have a low viscosity, generally larger than in the basalts. The tending to flow large distances, and to Although most of the lavas were of flows are thicker and with fewer joints form gently sloping "shield" volcanoes basalt, some trachyte and local rhyolite (fractures) than the basalts, and this rather than classical steep-sided peaks, flows were erupted towards the south, combined with a greater chemical such as Mount Fuji in Japan (the name where there are also fragmental rocks stability, results in a greater resistance to "shield" comes from the shape of an from explosive eruptions. In other places erosion and the formation of cliff lines. upturned warrior’s shield). in the south there are lava flows intermediate between basalt and Source of the lavas The Main Range shield volcano formed trachyte. The basalt flows tend to be part of a wider area of volcanic activity between 15 m and 25 m thick, whereas Unlike Mount Warning of the Tweed which extended northwards past the trachytes may be up to 80 m thick. Volcano, there is no well marked centre Toowoomba to Kingaroy and beyond. of eruption for the basalt lavas, and it is Other major volcanoes of similar age in The basalts are black to dark grey rocks, likely that in the southern part at least, the region include the Focal Peak generally very fine grained, but with they came from numerous basalt dykes Volcano near Mount Barney, and the some light coloured rectangular crystals (dyke swarms) which fed small vents; well known and much larger Tweed of plagioclase feldspar. Several factors examples of dykes are exposed around Volcano whose remains form the have resulted in considerable variation Mount Alford to the east, and along the Lamington Plateau and the Border in appearance between individual flows, highway to the east of Cunninghams Ranges (see sketch). including the cooling conditions of the Gap; others may lie beneath the area at lava, the amount of dissolved gases, and present covered by lavas. The Main Range volcano appears to the time interval between successive have been highest southwards from flows. headwater retreat of the steep coastal streams. This has been favoured by close spacing of parallel streams, vertical jointing and horizontal layering in the basalts, and undercutting by the more easily eroded shales and sandstones beneath. The escarpment has gradually retreated westward past the centre crest of the volcano, as virtually all the lavas now remaining are sloping to the west. North of Cunninghams Gap the escarpment turns westward and forms an amphitheatre around the headwaters of the sizeable Lockyer Creek, with Toowoomba on its rim. However, the Mistake Range and Little Liverpool Range continue the northward trend of the Main Range into the Lockyer Valley. The western side of the range is characterised by flat-topped ridges, the summits of which approximate the original surface of the volcano, separated by west-trending valleys which have gradually deepened from streams that began on the western flank 20 million years ago. The headwaters of many have been removed as the escarpment has retreated westwards, leaving the prominent ’gaps’of the range (such as Cunninghams Gap and Spicers For the trachytes there are several since been removed by erosion along Gap). possible sources, such as vents east of with the vents. the escarpment which have since been The trachyte lavas are more resistant to removed by erosion. These may be Development of the landscape erosion than the basalts, and where they represented by deeper intrusive plugs of occur on the western side of the range trachyte that have remained as resistant The eastern part of the shield volcano they form cliff-fringed valleys such as peaks in the Fassifern Valley, such as once spread across the Fassifern Valley, Emu Creek (with the striking pinnacles Mount Fraser and Mount Edwards. probably as far as Mount Maroon, of The Steamers), and Condamine (Such vents could also have been a Boonah and even Ipswich. Other basalts Gorge. source of the basalts before they were fom this volcano or other vents probably plugged by the trachyte). continued northwards across the The escarpment is part of the Great Lockyer Valley as far as Ravensbourne. Escarpment of eastern Australia, which There is some evidence that the extends from Cape York Peninsula to prominent and extensive Steamers The relatively steep gradients and the Victoria - NSW border. Usually the Trachyte in the south may have been greater erosive power of the streams that Great Escarpment is well to the east of erupted from the Focal Peak Volcano to developed eastwards from the volcano the Main Divide, but here the two the southeast, which is roughly the same crest to the coast led to more rapid coincide. It is here too that the Great age. erosion on the east than on the west, Escarpment is highest and most where there was a lesser fall to the spectacular; this is because the Main Other intrusions east of the escarpment inland rivers. Consequently the Range lavas are of relatively recent include the rhyolites in the plug of prolonged erosion over 20 million years accumulation on the sedimentary shales Mount Greville, the sill of Mount French has removed mainly the eastern side of and sandstone beneath. and the ring dyke of Minto Crags. the volcano, exposing soft shale and Rhyolite lavas are not seen in the face of sandstone of the underlying Walloon The term Main Divide is preferred to the escarpment, and it is Coal Measures and Marburg Formation Great Dividing Range, as the latter is not sandstones (as well as the intrusive plugs a range (a chain of mountains) over most likely that this magma rose to the and dykes). of its length- in many places the country, surface, but being viscous (sticky) did On this eastern side a north-trending although elevated, is flat. not spread far from the vents, and has escarpment has been produced, from the farther to the west. At 17.8 km, at the 4WD turn-off to The Head locality, cliffs of Condamine Gorge can be seen down the valley. These are of a thick trachyte flow, which being resistant to erosion has delayed broadening of the valley as the river has cut downwards. Continue on the Killarney road which climbs over a break in this cliff line. At the Moss Gardens-White Swamp Lookout car park at 20.8 km, dark grey basalt with zeolite (chabazite) in gas bubbles occurs in a cutting a few metres below the car park. About 50 m up the road, massive, light purple-grey trachyte with feldspar crystals and flow banding, overlies the basalt; this is the trachyte of the major cliff line. The lower part of the flow has cooled quickly to a natural dark-coloured glass. At Carrs Lookout at 23.9 km, the vista to the north over the western side of Mount Suberbus shows the ridge crests sloping gradually to the west; these approximate the original surface of the western flank of the volcano.