Coastal Geomorphology

The coastline of the Portland Shire consists essentially of three units. Between the South Australian border and Descartes Bay, the shoreline is broadly arcuate in plan and is backed by transgressive dune sheets and ridges. Several episodes of Pleistocene and Holocent transgressive dune activity are indicated by a series of partly consolidated and stabilised dune ridges. These are now being buried by active transgressive dunes. Lakes and swamps such as Swan Lake, Long Swamp and Lake Bong Bong have been formed by the blocking of small coastal streams by moving dunes or the impoundment of ground water in inter-dune hollows and corridors. The Glenelg River is the only stream to maintain a nearly continuous opening across the dune belt, although on rare occasions Swan Lake overflows to the sea through anarrow outlet channel. The Bridgewater Lakes are former coastal lagoons now cut off from the sea and impounded between the moving dunes and the Bridgewater scar. This transgressive dune unit is also represented in Bridgewater Bay.

The second coastal unit comprises the high cliffed coastline between Descartes Bay and Dutton Bluff. The cliffs west and south of Portland are of Pliocene/Pleistocene volcanic rocks overlain by Bridgewater Formation and younger cliff-top parabolic dunes. North of Portland Harbour, the cliff sections display Whalers Bluff Formation sediments and weathered later volcanic materials.

The third coastal unit extends east from Dutton Way to the boundary of the Shire. This consists of a belt of low parallel Pleistocene and Holocene dune ridges separated by narrow swamp dune swales. Inland is the former sea cliff of the Dutton Bluff and the escarpment of Mount Clay. The lower courses of the Surrey and Fitzroy rivers are deflected for several kilometres to the east by a zone of dune ridges and flow parallel to the coast before entering the sea through shallow, sand encumbered outlets.