Australia: The Land Where Time Began

A biography of the Australian continent 

The Warrumbungles "Little Broken Mountains"

The Warrumbungle Range in northern New South Wales, a spur of the Great Dividing Range,  is what remains of a period of volcanic activity in the area between about 17 and 13 Ma. The hills and jagged outcrops of this range differs in appearance from the rounded mountain ranges to the wast. To the west the vast plains stretch beyond the horizon. The Warrumbungles are low, even by Australian standards, at about 600 m, but there are many isolated spires, domes, bluffs and other shaped peaks that rise to about twice the height of the main range.

The number of volcanoes that erupted during the active phase of the Warrumbungles is unknown as the main craters were eroded away long ago. All that is known from what remains of the volcanoes is that there many volcanoes that erupted during a number of active phases over a period of about 4 million years. Between each active phase and the next the rocks from the previous active phase were eroded before the next phase.

Before the period of volcanic activity began the area was a sandstone and shale plateau that had been eroding for some time. The active phase began with a period of seismic activity during which lines of weakness in the floors of the valleys became fault lines. Vast amounts of lava spread from the fissures that formed along these fault line, so much that even deep valleys were filled and a thin cap was formed on the peaks, and rivers were dammed or diverted. The accumulating lava eventually formed a huge basalt mound. The character of the lava changed from this point, becoming thicker, and carried volcanic bombs, breccias and tuffs.

Many dykes were intruded in to the sandstone strata, and as the eruptions become explosive ash and debris were scattered far and wide around the area. As the explosive eruptions continued along the fissures the heavier material falling near the eruption formed cones along the fault line. Occasionally the viscous lava of some vents solidified in the opening and these plugs were later shattered and thrown out explosively. Much of the rocks of these cones were shattered. This resulted in more rapid erosion. Subsequent lava flow in some places cemented this volcanic debris together.

The lavas of the final stage of volcanic activity were more viscous, slow-moving trachytes that cooled rapidly, remaining close to the sited of eruption. Their main constituents were quartz and alkali feldspar, eventually congealing sufficiently to plug the vents. By this time sufficient steam had been vented to the atmosphere that there was not enough pressure to blast out the plugs, and the volcanoes slowly became dormant.

At the time of the cessation of the volcanic activity the mountains formed by the activity were much higher than they are at present. Weathering and erosion eventually reversed the landscape, the softer sedimentary material of sandstones and shales being removed to such an extent that the more resistant volcanic material that had filled the valleys and any other hollows grew in height above the surrounding plain as the surrounding rocks were worn away leaving the volcanic rocks as mountains. The trachyte pinnacles and domes were the dykes and volcanic plugs that formed before the volcanic activity subsided.

Most of the softer volcanic rocks, as wel as those that were shattered by the explosive eruptions, have been disintegrated to for rich soils on the plains. Around the valley of Wambelong Creek is a large rim in the shape of an amphitheatre. Most of the major peaks of the Warrumbungles are situated along this rim. Mt Wambelong is the highest peak on the rim, standing 1205 m above sea level.

The most spectacular structure is the Breadknife, a narrow trachyte wall that reaches 100 m on the western side that tapers to a crest that less than 2 m wide at the top. It was formed as a dyke in the same crater that was plugged by what is now the 1094 m high Crater Bluff. Belougery Spire is a 1061 m trachyte tower that originally plugged a subsidiary vent. Grand High Tops is a large ridge. Bluff Mountain is a 1200 m high massive dome. Tonduron Spire, 1130 m high, is a basaltic tower. To the north, Belougery Split Rock 770 m high, is a white trachyte dome that dominates the lower mountain country. Timor Rock, 730 m high, lies to the east is a bluish trachyte mound.

The Warrumbungles have been said to be a mixing place where the flora and fauna of the wetter coastal regions come into contact with those of the arid inland. It has been found that the species do not mix indiscriminately, the habitats are mostly defined, but there are cases where species from completely different environments can be found near each other. An example can be found in some places on the high ground where blackboys (aka grass trees) (Xanthorrhoea arboreas) can be growing near snow gums (Eucalyptus pauciflora).

before European settlement the Warrumbungles were part of the territory of the Kamilaroi tribe.

Sources & Further reading

Helen Grasswill & Reg Morrison, Australia, a Timeless Grandeur, Lansdowne, 1981

Last updated  27/03/2011



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                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading