2. Western Uplands (WU)
2.2 Strike ridges and valleys - Grampians Range
|The striking cuesta and vale topography of the Grampians Ranges occur within the western extents of the Midlands of Victoria. Extending from Mount William in the south to Mount Zero in the north, the Mt Difficult and Mount William ranges provide spectacular relief over surrounding landscapes of gentle to undulating topography that have defined drainage networks supplying the Wimmera River in the north. |
The cuesta landscapes of the Grampians Ranges general height varies between 450 m and 1 000 m AHD with significant peaks of the Mount William Range including Mt William (1 167 m) and the Major Mitchell Plateau (over 1 100 m). This series of strike ridge and valley terrain is repeated in a pattern running east west that extends for over 40 kilometres. Subsequent streams of these valley floors carry large volumes of water to reservoirs (Wartook Reservoir) and lakes from major tributaries including MacKenzie River and Mount William Creek.
There are two small areas of plateaux where the sandstones have very low angle dips including Major Mitchell Plateau and an area above Zumsteins in the Mount Victory Range. At the northern end of the Mt William Range near Halls Gap the landform converts from cuesta landscape to form a series of hogbacks where sandstone beds dip with very high angles and steep ridges (Sibley 1967). Dip slopes in the cuesta landforms vary according to degree of fracturing/jointing and dissection that are reflected in vegetation and faunal relationships (Sibley 1967). Slope classes defined include dissected dip slopes (sandstone strata fractured and dissected with steep buttresses separated by chasms – e.g. Wonderland Range slopes), smooth dip slope (extensive areas of bare dipping rock – e.g. "Elephants Hide") and colluvial dip slopes (smooth dip slopes that are unbroken with scree material common – e.g. Mirranatwa).
The Grampians Group (Spencer Jones 1965) comprises a rock sequence with a stratigraphic succession of quartzo-feldspathic to micaceous sandstone overlain by a micaceous mudstone and finally a quartzose sandstone package (Cayley & Taylor 2001). Latest interpretation of these sedimentary stacks has a number of thrust faults that have intersected this sequence and juxaposted the stratigraphy in a repetition of strike ridges and valleys through differential erosion. Formed during the Late Ordovician to Early Silurian, the sediments of the ranges belong to the Red Man Bluff Subgroup and Mount Difficult Subgroup with a cumulative stratigraphic thickness of 3 700 m.
Narrow intermontane valleys with subsequent streams occur between the classical cuesta landform patterns of the strike ridges that host significant deposits of colluvium and alluvium from dip slopes and scarps. Jointing and faulting has resulted in significant dissection of the stratigraphic sequence controlling stream pattern density across and along slopes. Stream pattern drainage of the tributaries is pinnate in nature with steep slopes supporting major stream courses running perpendicular to dip slope. Easterly facing cliffs and steep slopes, along with westerly dipping rock slopes have thin to non-existent regolith and soil development that are sandy. Flat to gently sloping topography has been found with pronounced weathering profiles. The Major Mitchell plateau and Mount William are possibly the only remnants of the once extensive Mesozoic palaeosurface (Hills 1975; Cayley & Taylor 1997). The ranges are likely to have been exposed in early Cainozoic times as a series of arcuate ridges with the topography more pronounced with further dissection and erosion of less resistant beds of the repeated sedimentary stack.
With the high proportion of rock outcrop, fresh and shallow soils (Rudosols) are common with sandy soils (Tenosols, Kandosols) occurring on mid to lower slopes and texture contrast soils, often acidic on the lower slopes (Kurosols), where there is some clay accumulation in the subsoil. All soils have high concentrations of coarse and fine sand that have extremely high rates of infiltration and allow rapid drainage.
A number of woodland, forest, shrubland and heathland vegetation communities have been recorded including Lowland Forest, Heathy Dry Forest, Rocky Outcrop Shrubland, Heathy Woodland, Rocky outcrop Shrubland/Rocky Outcrop Herbland Mosaic, Shallow Sands Woodland and Sedgy Riparian Woodland. Native vegetation is well preserved within these landscapes with wet and dry Sclerophyll forests dominated by Brown Stringybark (Eucalyptus baxteri), Messmate (Eucalyptus obliqua), Long leaf box (Eucalyptus goniocaylx) and heath understorey.
Fire is a rather unique feature that has been common throughout the European settlement of the ranges but also in pre-European times when the Jardwadjali tribe occupied these ranges. Erosional events often occur post fire events where understorey is removed leaving vulnerable sandy topsoils exposed to extreme rainfall events and significant wind from the south and west. Landslip scars on the south facing scarp of the Mount Difficult Range near Halls Gap also highlight failures of shallow soils that have been saturated. Rock falls are extremely infrequent and are mainly confined to scarps and cliffs of south and easterly facing rock exposures.