Geology and Geomorphology of the Portland Shire

Coastal Geomorphology

Seen in the contents of Victoria, the Shire of Portland includes parts of three main provinces - the Follett Coastal Plains, the Western District Volcanic Plains, and the Merino Tablelands. A major geological and topographic structure - the Kanawinka escarpment and its southern extensions - separates the low Follett Plains from the more elevated and dissected terrain of the Merino Tablelands and western limits of the Western District Volcanic Plains. The prominent escarpments which extend south and south-east from Winnap are intepreted as initially fault scarps or monoclinal flexures which have been modified by marine and sub-aerial erosion since early Pleistocene times.

West of the escarpments, the surface materials are almost entirely Pleistocene and Holocene sediments, mainly swamp deposits and calcareous and siliceous sands. No Cretaceous rocks outcrop, and outcrops of Tertiary sediments are restricted to the valleys of the Glenelg, Crawford and Stokes rivers. These valleys provide the main relief in an otherwise flat to undulating landscape interspersed with irregular and elongate dune ridges and numerous swampy or dry shallow depressions. These depressions are in part deflation hollows - mainly on the areas of siliceous sands - and in part are solution features on the areas of calcareous sands.

A prominent escarpment extends north-west from Bridgewater to Kentbruck and then trends north to north-easterly at Jones Ridge adn Drik Drik. This escarpment is a result of movement of the fault system and has been modified by later erosion. The escarpment is marked by an abrupt steeping of slopes and is incised by several small streams such as Johnstone, Moleside and Glenaulin Creeks and the Crawford River. Incision by these streams has exposed a variety of volcanic and sedimentary rocks and initiated lanslips in several places, notably on the Crawford River near Greenwald East.

To the east of the escarpment, the surface geology and relief is much more varied than in the western part of the Portland Shire. Much of the landscape is dominated by the effects of Pleistocene volcanic activity which are evident in the forms of several eruption point features - low volcanic domes and cones of basalt and pyroclastics (tuff and ash) - and extensive sheets and restricted flows of lava. The youngest volcanic activity is in the extreme east of the Shire in the form of the clearly defined Tyrendarra and Harman Valley lava flows. These are some of the best preserved Late Quaternary volcanic features in Australia.

The volcanic eruptions have had a profound effect on the drainage systems of the eastern part of the Portland Shire. Large lakes and swamps (now artificially drained and reclaimed) resulted from the damming and diversion of the Surrey, Fitzroy and Crawford rivers and Darlot Creek. Drainage interruption has occurred on the Crawford River due to both tectonic uplift, which has caused extensive alluvium to be deposited near Hotspur, and to landslips which have temporarily dammed the Crawford River near Greenwald East. (Douglas and Ferguson, 1976).

The north-eastern part of the Portland Shire, i.e. the upper reaches of the Stokes River and the area north of Grassdale, is a dissected tableland (Merino Tableland). Here, the Tertiary and Cretaceous rocks are deeply weathered and there are numerous landslips on the steep valley slopes.

The coastline of the Portland Shire displays considerable geomorphological variation. Transgressive dune sheets and ridges of Pleistocene and Holocen age are found between the Glenelg River estuary and Cape Nelson. Several large lakes and swamps occur where streams have been blocked by transgressive dunes or where groundwater collects in interdune hollows or corridors, e.g. Malseed Lake and Swan Lake.

The coast is cliffed from Cape Nelson to Dutton Bluff and around the Bridgewater Promontory. These cliffs are cut in late Cainozoic sediments and volcanic rocks. Cliff top parabolic dunes occur on Cape Bridgewater and Cape Nelson. The coastline west of Portland consists of the low parallel Pleistocene and Holocene dune ridges and swales back by Dutton Bluff - a former marine cliff - and the Mount Clay escaprment. The Fitzroy and Surrey rivers have their outlets across these dunes.

Summary of main geological materials in the Portland Shire.

Epoch	Marine and Coastal	Non-Marine	Volcanics
Holecene	Beach and dune sand	Swamp and river alluvium
Pleistocene	Cliff top and older dune ridges Bridgewater Fmn	River terraces and alluvium Malanganee Sands	Tyrendarra and Harman Valley lava flows
Pliocene	Whalers Blurr Fmn		Greenwald - Cobboboonee Basalt Hamilton Basalt
Miocene	Dorodong Sands H Port Campbell E Limestone Y T G Wataepoolan Limestone Member E R
Oligocene	S O Koonalunda B U Lens U P R Glenaulin Clay Member Y
Eocene	Dilwyn Fmn
Palaeocene	Pebble Point Fmn
Cretaceous	Otway Group