Rosengren (2009, 2010) provides a detailed overview of the geology and geomorphology of the Bellarine Peninsula.

Deep drilling and geophysical surveys show that the Bellarine Peninsula has a basement of sedimentary rocks but also extensive areas of volcanic rocks which are buried by younger sediments. Sediments were deposited in river channels, floodplains and freshwater lakes about 100 million years ago and occur at the surface along the coastal slopes between Clifton Springs and Portarlington (Rosengren, 2010).

Over the last 20 million years the Bellarine has undergone complete or partial marine submergence on several occasions. The entire peninsula was inundated by the mid Miocene (23 – 5.3 million years ago) and the coastline was then situated as far inland as Meredith. The sea retreated during the Pliocene (5.3 – 2.5 million years ago) and left a thin deposit of sand behind. Following the tectonic uplift in the Pliocene, the coast between Torquay and Ocean Grove comprised a large funnel-shaped embayment, narrowing towards a wide channel that ran to Corio Bay. The embayment was shallow, with a floor of Tertiary (65 – 2.6 million years ago, mostly Miocene) sediments. Changing sea levels associated with the continued retreat of the sea constructed a series of small barriers and lakes throughout the embayment, resulting in the deposition of a series of dense shell beds and minor sands, which now underlie the Connewarre Lake system. During the last inter-glacial period, about 125,000 years ago, sea levels were generally about 6 m higher than present (Dahlhaus, 2010).

As a result of local tectonic uplift, the Bellarine Peninsula is an area of higher relief inside the Port Phillip Bay Sunkland. This uplifted area is dominated by a well-defined undulating low plateau and hills (less than 150 m elevation) of broad crests and gentle convex slopes with steeper slopes above some drainage lines (Rosengren 2009, 2011).

Rosengren differentiates three main terrain units, shown in Figure 3-1. These terrain units include:

The development of the open coast side of the peninsula has seen periods of volcanic activity and deposition. Lava flows from Mt. Dundeed between 1 and 2 million years ago left a base formation of volcanic material that has underlain the present geomorphology near the Breamlea and Barwon areas. Fluvial processes subsequently shaped the lowland areas during low sea-level periods. During this time the Barwon River possibly met the ocean further east towards Ocean Grove. During the Holocene when sea-levels were higher, the ocean and Corio Bay re-established a seaway across the Moolap Lowland, first established during the previous interglacial period (Rosengren, 2009). Falling sea levels and sedimentation closed the seaway within approximately the last 5,000 year, and gave rise to the Barwon estuary, Reedy Lake and Lake Connewarre. The alternating sea levels formed natural levee systems, and has allowed Reedy Lake to develop as a freshwater wetland.

Figure 3-1     The geology of the Bellarine Peninsula, over a 3D digital elevation model (adapted from Dahlhaus, 2010)

Along the coast, the lowering of sea-levels and aeolian sedimentation led to the formation of the high calcarenite cliffs and lower ridges. Over time these were overlain by sand to form the dune formations that are present today. The calcarenite ridge would have restricted the flow of the Barwon River, until such time that the river was able to breach the ridge and form the estuary entrance closer to the current location. Refraction and sedimentation processes then began shaping the curved spit on the Ocean Grove side of the Barwon River. This allowed the formation of the large deltaic complex beyond the river mouth, which was subsequently colonised by mangrove and saltmarsh species. Evidence suggests that there has been some rapid sedimentation within Lake Connewarre within the last 150 years which has shallowed the basin significantly (Rosengren, 2009).

The dominant sediment transport direction along the Bass Strait coast is from the south-west to the north-east. This trend has shaped the shoreline into characteristic crescent shapes between ‘hard points’ (i.e. rocky outcrops, headlands) in some locations. The beaches are generally low angle sandy beaches backed by dunes which advance and recede in response to natural processes. During the site visit in April 2013, the foredunes appeared to be in an erosional trend, with scarped frontal dunes due to recent storm activity. The dunes have a calcarenite core in many locations; however, the depth at which it occurs within the dune is uncertain except where dune blow-outs have left the core exposed. Depending on location, the height of the dunes/cliffs can be greater than 20 m, and wider than 100 m.

Within Port Phillip Bay the study area coastline extends from east of Point Lonsdale to Point Wilson. The shoreline consists largely of soft rock shorelines, bluffs and cliffs lined with narrow sand and rocky beaches. The bathymetry is largely very shallow, especially in the vicinity of The Sands. This is the flood-tidal delta of the Port Phillip Bay entrance. The shoreline is largely modified and developed, and in some locations very low lying.