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Tomahawk Creek Land System

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Tomahawk Creek and its tributaries have dissected out deep valleys with characteristic north-not’-west- and south-sou’-east-oriented parallel rides and spurs. Small remnants of lateritic plateaux on the high parts of the landscape are bounded by scarps on which ironstone outcrops. Tertiary sand is often exposed in a narrow band below these scarps, and springs are often present at this level. Silt and clay are the more common parent materials on long straight slopes leading down to the valley floor. Small dissected terraces are found along the wider valleys.

The terrain to the south and east of Tomahawk Creek shows a lower local relief than the area to the north and west. Areas of lateritic plateaux are often wider and surrounding slopes are shorter and more gentle. Site drainage is affected by the more subdued relief. Woodlands and lowlands appear to have been more common in this area, with open forests to the north and west.

Most of this land system has been cleared as part of the Heytesbury Settlement Scheme, and dairy farming is the main land use. Subsoils on many slopes are dispersible and gully and tunnel erosion are quite active. Some landslips have occurred, particularly below springs at the base of scarps.
A Study of land in the catchments of the Otway Range and adjacent plains - tomahawk creek

A Study of land in the catchments of the Otway Range and adjacent plains - tomahawk creek

Cattle graze the undulating plateau country.

A Study of land in the catchments of the Otway Range and adjacent plains - tomahawk creek


Area: 101 km
2
Component and its proportion of land system
1
15%
2
6%
3
10%
4
50%
5
9%
6
10%
CLIMATE
Rainfall, mm

Annual
: 850 – 1,050, lowest January (40), highest August (125)
Temperature, 0oC
Annual: 13, lowest July (8), highest February (19)
Temperature: less than 10oC (av.) June – August
Precipitation: less than potential evapotranspiration November – March
GEOLOGY
Age, lithology

Pliocene lateritized sand and clay

Miocene unconsolidated sand, silt and clay
TOPOGRAPHY
Landscape

Deep valleys dissected out from lateritic plateaux
Elevation, m
50 – 160
Local relief, m
70
Drainage pattern
Trellis predominantly, some dendritic areas
Drainage density, km/km2
2.9
Land form
Plateau remnants
Scarp
Valley floor
Land form element
-
Upper slope
Upper slope
Mid slope
Lower slope
-
Slope (and range), %
1 (0-3)
28 (13-40)
12 (8-18)
12 (8-20)
5 (1-8)
0 (0-2)
Slope shape
Straight
Concave
Straight
Straight
Straight
Concave
NATIVE VEGETATION
Structure

Open forest

Open forest

Woodland

Open forest

Low woodland

Woodland
Dominant species
E. obliqua, E. baxteri
E. obliqua, occasionally
E. viminalis
E. radiata, E. baxteri,
E. viminalis
E. ovata, E. obliqua,
E. radiata, E. baxteri
E. radiata, E. ovata
E. viminalis, E. ovata
SOIL
Parent material

Lateritic remains

Colluvial lateritic ironstone

Siliceous sand

Sandy clay (in-situ)

Colluvial/alluvial sand over sandy clay

Sand and clay alluvium
Description
Mottled yellow and red gradational soils with ironstone
Stony red gradational soils
Grey sand soils, uniform texture
Yellow-brown gradational soils, coarse structure
Grey sand soils, structured clay underlay
Grey gradational soils
Surface texture
Sandy loam
Gravelly sandy loam
Coarse sandy loam
Sandy loam
Sandy loam
Sandy loam
Permeability
Moderate
Very high
Very high
Low
Very low
Very low
Depth, m
1.6
1.0
>2
>2
>2
>2
LAND USE
Cleared areas: Mainly dairy farming; some beef cattle grazing.
Uncleared areas: Hardwood forestry for sawlogs, some posts and poles, gravel extraction; nature conservation.
SOIL DETERIORATION HAZARD
Critical land features, processes, forms
Low inherent fertility and phosphorus fixation lead to nutrient decline. Leaching of salts leads to increased salinity of drainage waters.
Steep slopes with weakly structured surfaces of low water-holding capacity are prone to sheet erosion. Low inherent fertility and high permeability lead to nutrient decline.
Emergence of springs from these permeable aquifers leads to seasonal waterlogging and soil compaction. Permeable soils of low inherent fertility are prone to nutrient decline.
Highly dispersible clay subsoils of low permeability receiving seepage water are prone to gully and tunnel erosion and to landslips and slumping.
Dispersible soils of low permeability receiving seepage water are prone to gully and tunnel erosion, waterlogging and surface compaction. Permeable surfaces of low inherent fertility are prone to nutrient decline.
Dispersible clay subsoils of low permeability receiving rapid run-off from surrounding hills are prone to gully erosion. Rising water tables and low permeabilities lead to seasonal waterlogging and soil compaction.
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