3.3.2 Hills and low hills (Barwon Downs, French Island)

3. Southern Uplands (SU)
3.3 Very low relief (very low elevation, generally less than 100 m)

West of Port Phillip Bay

South of Colac, low hills formed on the uplifted Palaeogene sediments south of the Colac Monocline constitute the foothills to the north of the Otway Ranges. The gently undulating hills referred to as the Barwon Downs, are generally rounded, although some have broader planar crests, and may be described as undulating plains. The dendritic drainage network forms tributaries to the Barwon and Gellibrand rivers. Further east (south of the Barrabool Hills) the rolling hills are less well defined and are transitional to the sedimentary plains (Geomorphological Unit 6.2).

North east of Princetown, the dissection of the mostly Palaeogene sediments which drape the western edge of the Otway Ranges have formed rolling hills. The drainage patterns change from dendritic along the western fringe of Otway Ranges to arcuate and vaguely rectilinear as the unit merges with the dissected sedimentary plains (Geomorphological Unit 6.2.2).

Associated soil types are mottled and non-mottled gradational soils (Dermosols) with a sandy component, particularly in the upper soil. It is likely that the original vegetation comprised Lowland Forest.

East of Port Phillip Bay

Most of French Island is quite different in lithology to Phillip Island (see 3.3.3) being mostly comprised of early Neogene sediments, most of which are overlain by later sand sheets, though its southern margin is comprised of Older basalt (Palaeogene). Apart from its southern part, the drainage pattern is less well defined than Phillip Island. This is largely due to the more permeable nature of the underlying sediments. The north-western part of French Island is topographically lower and comprises dunefields (See 7.1.3). The soils are either yellow and brown texture contrast soils (Chromosols, Kurosols) or, on the deep sands, acidic bleached sands with “coffee rock” (Podosols). Much of French Island is a State Park with intact native vegetation (includes Sand Heathland, Plains Woodland/Sedgy Woodland/Damp Sands mosaic).

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