1.General Description

As a result of post-depositional warping, probably 60 million years ago, the Sydney Basin, of which the Upper Parramatta River Catchment is part, has become separated into a number of distinct geomorphic units.

·           Cumberland Plain. The cities of Sydney and Parramatta are located on the Cumberland Plain, which is a low lying, generally flat area less than 100 metres above sea level. The Cumberland Plain is bounded by the:-

·           Hornsby Plateau in the north, with the highest peak in the catchment at Castle Hill, (190 metres above sea level)

·           Blue Mountains Plateau to the west. The land surface rises from sea level at Penrith, through the Lapstone Monocline complex to reach a maximum of 1109 metres at Mt Victoria.

·           Woronora Plateau to the south which rises gradually to 850 metres at Moss Vale.

The geomorphology relfects the differences between the gently undulating Cumberland Plain (right) and the steep sided gullies of the Hornby Plateau (left)

The Upper Parramatta River Catchment has an area of 107 square kilometres. It is drained by the Parramatta River and its tributaries, (Hunt’s Creek, Darling Mills Creek, Quarry Creek, Grantham Creek, Toongabbie Creek, Lalor Creek, Blacktown Creek, Greystanes Creek, Coopers Creek, Finlaysons Creek and Pendle Hill Creek).

2.Description of Types

The geomorphology of the catchment reflects the underlying geology.

·           Darling Mills Creek and its tributaries in the north are deeply incised into the Hawkesbury Sandstone with steep V-shaped valleys and sandstone cliff faces. Little alluvium flows out of this system. The Hawkesbury Sandstone was laid down as a very thick sheet of sand by energetic, braided streams flowing from the south west during the mid Triassic (225 Ma)

·          Toongabbie Creek and its tributaries, flowing from the south, drain across the gently undulating Cumberland Plain with its foundation of siltstones, mudstones and shales. Extensive deposits of alluvium and the absence of rocky slopes are characteristic of this sub-catchment. The Cumberland Plain is a low, undulating plain underlain by horizontally bedded sediments of the Wianamatta Group, mainly Bringelly Shales. These shales are generally composed of claystones, siltstones and carbonaceous shales with sparse sandstone lenses. The sediments were laid down in a coastal alluvial plain during the middle Triassic, (200Ma).

Channel Geomorphology

Creekbank erosion and sedimentation are natural processes resulting from changes in creek channels and periodic flow regimes. The changed runoff and infiltration characteristics of an urbanised catchment, (see hydrology and flooding information sheets) such as the Upper Parramatta River Catchment means that erosion on land and in creeks occurs at an accelerated rate. Increased erosion from building sites and other exposed surfaces means that higher sediment loads are carried by streams. This combination of increased runoff, accelerated erosion and increased siltation causes unstable creek systems. Unstable creeks can undermine property and affect water quality and aquatic fauna through increased turbidity.

Examples of Streambank erosion at Grantham Creek (right) and Blacktown Creek (left).

3.Reasons for Distribution

The two main geomorphic units in the Upper Parramatta River catchment – the Hornsby Plateau in the north east and the Cumberland Plain in the south and west were formed by uplift and possible down warping in the Tertiary period probably 60 million years ago. These tectonic movements were related to the opening of the Tasman Sea and the uplift of the Kosciusko Mountain Range. Uplift of the Hornsby Plateau meant that the older rock units, namely the Hawkesbury Sandstones were exposed on the surface as the Wianamatta Shales eroded away. The more energetic streams flowing off the Hornsby Plateau, Darling Mills Creek and Hunts Creek are now cutting into the sandstones to form the steep sided gulleys typical of this type of terrain. On the other hand Toongabbie Creek and it tributaries are less energetic as they flow across the gently undulating Cumberland Plain therefore forming shallower, wider, meandering river channels.

4. Other Resources and Links

·           Upper Parramatta River Catchment Trust Stormwater Management Plan

·           Upper Parramatta River Catchment Trust www.uprct.nsw.gov.au

·           Department of Land & Water Conservation www.dlwc.nsw.gov.au

·           National Parks & Wildlife Service www.npws.nsw.gov.au

·           Canri www.canri.nsw.gov.au