Immediately downstream of the Bacchus Marsh Basin, begins the Port Phillip Sunkland. This is a large area of land which comprises the Werribee plains and Port Phillip Bay. It is known as a fault trough. See our map, “Landforms of the Werribee Catchment.” The Sunkland formed eons ago, and then was subject to volcanic activity in the north west corner of it, which caused basalt flows known as the ‘Exford Volcanics’, impeding or altering the direction of flowing streams.1
The Werribee River had to cut down through some of the lava flows, which in did in what is now the volcanic gorge section of the river. Melton Reservoir now occupies the beginning of this gorge, and good views of the reservoir can be seen from Southern Rural Water’s public reserve accessible from their gate in Exford Rd. Views can also be taken seen from Hopetoun Park and other locations in Melton South.
For a description of the Melton reservoir, see visit the Southern Rural Water website.
At the highest points of the escarpments of the gorge can be seen remnant volcanic cap which is underlain by soft sediments which in turn sit above older volcanic basalt, and which are sometimes exposed on the bed of the river. Softer sediments such as these were eroded in the Rowsley Valley by the Parwan Creek, and have caused siltation of the Melton Reservoir.
This gorge country holds many beautiful views and some remnant flora and fauna no longer present in other areas of the river. For excellent descriptions and activities in that area see http://pinkertonforest.com/index.php/werrivee-river-gorge
Downstream of the Exford Weir, the river continues through its volcanic gorge until Cobbledick’s Ford.
1 Condon, M.A The Geology of the Lower Werribee River, Victoria
Proceedings of the Royal Society of Victoria, Vol 63 p. 1-24, 1951