3.1.2 Deeply dissected upland ranges of the Southern Uplands

On both sides of the Otway Range divide rugged topography has developed comprising ridges and spurs separated by deeply dissected steep valleys. The drainage is strongly controlled by the geological structure. Parallel to the south-east coast, the drainage is generally south-east and north-west, whereas west of Mount Sabine, the drainage is generally north and south. The depth of dissection lessens towards the ridge, while the south-eastern flanks of the ranges have developed the steepest topography, probably in response to the fluctuations in sea level over the past million years that have changed the base level of the rivers and streams that drain the landscapes. The fluctuation in sea level and the associated uplift of the Otway Ranges has resulted in the rapid erosional processes that has transpired in numerous large landslides, with probably more activity in the past 6000 years since the slight drop in stream base levels (renewed erosion) and warmer (and therefore wetter) climates have prevailed (Dahlhaus & Miner 2002).

The streams flowing south are mostly short and enter the sea directly, whereas those to the north join the larger Barwon and Gellibrand river systems. The long profiles of the streams are mostly steep and stepped, each step representing a waterfall over a thicker, more resistant bed of sandstone. The steps may also represent knickpoints migrating upstream, corresponding to coastal terraces that are poorly developed in some areas, such as around Apollo Bay.

The Otways coast largely comprises spectacular cliffs and shore platforms, actively incised in the Early Cretaceous sandstones and shales by the swells and storms of the Southern Ocean coming predominantly from the west. The coastline has been eroded within the past 6000 years, and the recession has been estimated as 105 m on mudstones and 53 m on sandstones along the Otway coast (Bird 2000). Soils vary from shallow stony loams in the steep and actively eroding parts of the landscapes, to brown and yellow gradational soils with associated colluvium on the slopes. Thick deposits of colluvium and landslide debris are common.