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Geology and soils

The geology of the Shoalhaven is dominated by Permian age sandstones and siltstones. Older Ordovician age slates and shales make up the basement in the Clyde Valley with volcanic intrusions evident in the north of the City and at Milton and Bawley Point. The area is predominantly hilly or mountainous country to the west with a narrow coastal strip to the east. Based on geology and the geomorphology of the region, a number of land units were identified by National Parks and Wildlife Service. 

The following table represents Land units from the Shoalhaven showing surface rock type, age of rock type and resulting topography (Source: NPWS).

Table 3 - Land Units

Unit

Surface Rock Type

Age of Rock

Topography

Clyde River Valley

Metamorphosed slates and phyllites;

Some shales intensely folded with a north/south strike;

Ordovician (marine)

Hilly - ridge and valley systems

Mt Budawang Ranges

Devonian quartzites and sandstones;

Some ryolite folded north/south strike

Devonian (marine)

Mountainous ridge valley system; North/south strike ridge and drainage systems

Upper Clyde Plateau

Massive sandstones, siltstones and conglomerates horizontally bedded; Gorges are Ordovician sediments

Permian (marine)

Deeply dissected sandstone plateau and mesa

Sassafras

Tertiary basalt and silcrete contact zone

Tertiary

Undulating low hills above sandstone plateau

Shoalhaven Plateaus

Nowra sandstone; Some Berry siltstone residuals;

Ordovician and Devonian shales and phyllites;

Limestone pocket;

Permian plateau; Ordovician and Devonian Gorges

Broad plateau surfaces deeply dissected by north/south streams and Shoalhaven River

Kangaroo Valley

Narrabeen shales, Illawarra coal measures;

Gerringong Volcanics; Latite/tuff;

Berry and Nowra sandstones

Late Permian (marine)

Broad dissected valley of Kangaroo and Yarrunga Rivers; Steep cliffs and latite beaches

Budderoo Plateau

Massive Hawkesbury sandstone

Triassic (fresh)

Broad plateau Waterfalls on cliff line to Kangaroo Valley

Cambewarra Range extends into Kiama Shire to coastlines and onto Mt Coolangatta

Gerringong volcanics capped by Hawkesbury residual mesas; Latite/tuff sandstone; Illawarra coal measures

Late Permian (marine)

Narrow low range parallel to coastline cliffed and beached at most levels

Shoalhaven Floodplain

Sands and silts of marine and fluvial origin

Quaternary

Low lying floodplain; Coastal and sandridge complexes;

Delta complexes; Large depressions

Yalwal Plateau

Sandstones;

Siltstones;

Rhyolite;

Granite

Permian plateau; Devonian valleys

Undulating plateau with some dissection by steams extending to coastal unit by way of a large ramp

Jervis Bay

Sandstones of Schnapper Point formation

Permian

Elevated coastal plateau dissected by large estuarine embayments

Coastal Hills and Ranges (extensive unit)

Sandstones of Schnapper Point formation;

Siltstones of Wandandian formation; some basalt and silcrete;

some granite

Permian (marine)

Low hills, ridges and valleys between the escarpment and coastline;

Coastal lakes and estuaries;

Sand ridge systems and headlands

Milton Intrusion

Monzonite intrusion

Mesozoic

Undulating hills with colluvial flats

Durras Range

Basalt capped Schnapper Point sandstone;

Monzonite (essexite); small limestone bed

Permian sandstone; Tertiary basalt

High coastal range and foothills to coastline; Offshore islands

 

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