Quick viewing(Text Mode)

21.6.2 Victoria: the Port Campbell Coast (Mepunga to Princetown)

21.6.2 Victoria: the Port Campbell Coast (Mepunga to Princetown)

21.6.2 : The ( to Princetown)

Introduction downwashed sediment, with an intervening residual face. If these processes continue the former vertical cliff The 50-km coastline between Childers and Glenam- will be re-shaped into a convex-above-concave slope ple consists of steep, often vertical and sometimes over- profile similar to that of valley sides inland (Bird 1977). hanging cliffs of up to 70 m high, cut into soft Port Campbell In the of marine dissection of the receding Limestone. It is exposed to high wave energy from the pre- cliffs has isolated numerous scattered stacks, which persist vailing SW ocean swell and storm waves arriving through deep water over a narrow section of the Australian conti- ⊡⊡ Fig. 21.6.2.1 nental shelf, here only about 60 km wide. The sea is very The cliff in Port Campbell Limestone at Goudies Lookout rarely calm. Even on windless days, a long, even ocean swell has a series of ledges on the outcrops of hard horizontal generated out in the Southern Ocean rolls in, waves break- strata. The scale is given by the man (arrowed) on the ing on the every 10–16 s. In a SW gale huge storm cliff-top, who has a fishing line down into the surf. waves crash against the cliffs, sometimes surging up more (Courtesy Geostudies.) than 30 m to their crests. Mean spring tide range is small (about 1.2 m at Port Campbell), so that the large waves consistently attack the base of the cliffs. The Port Campbell coast can be treated as a unit devel- oped on a single geological formation (Baker 1943), the Port Campbell Limestone, of Miocene age, consisting of stratified yellow-brown calcareous clays, and thin sandy limestones (>Fig. 21.6.2.1). East of Mepunga the calcarenite capping fades out along the cliffs, which are backed by a gently undulating treeless healthy plateau that in places rises more than 30 m above sea level. Generally the cliff crest is even, except where it declines towards the incised river valleys, but in the vicinity of Childers Cove, where the ground slopes inland, there are high and intervening with lower bay- head cliffs. Because of the landward decline from the cliff crests, these cliffs are diminishing in altitude as they retreat. Inland, the wide plateau is dotted with numerous sinkholes (swampy depressions). Near Stanhope Bay the cliff crest shows a calcrete horizon exposed where overlying dune have been removed, and solution processes have weathered it in- to rugged, knobbly, karstic (produced by solution pro- cesses) topography. Behind Three Mile a formerly vertical cliff has been cut off from marine erosion by the accumulation of beach and dune . It has been modi- fied by subaerial degradation, and shows grassy bevels and ­convexities on the crest and concave basal fans of

Eric C.F. Bird (ed.), Encyclopedia of the World’s Coastal , DOI 10.1007/ 978-1-4020-8639-7_21.6.2, © Springer Science+Business Media B.V. 2010 (Dordrecht) 1320 21.6.2 Victoria: The Port Campbell Coast (Mepunga to Princetown)

⊡⊡ Fig. 21.6.2.2 offshore in various stages of reduction to flat platforms, Subsiding columns of joint-bounded rock on usually just below low tide level. At Peterborough the cliffs the cliff near London Bridge. (Courtesy Geostudies.) are interrupted by the mouth of Curdies , a shallow bordered seaward by the dune-capped sand barrier behind Newfield Bay. The lagoon is fringed by reeds and rushes, which form an extensive swamp at the northern end, where the Curdies River flows in from a wide valley incised into the Port Campbell Limestone plateau. The out- let from Curdies Inlet is often sealed off by the accretion of beach sand, but after rainy weather the lagoon level rises, and overspill cuts a new outflow . In some years this process has been hastened by digging a trench through the sand barrier to initiate outflow. When the outlet is open the lagoon level falls, and a sandy threshold bank is exposed. The Grotto occurs where one of the sinkholes on the coastal plateau has been intersected by cliff recession to form a hole floored with fallen boulders and opening sea- ward through an archway to a ledge with a marine pool, a few metres above sea level. The pattern of cliff recession is much influenced by vertical joining, mainly NW–SE and NE–SW. The cliff face shows breakaways along joint planes, and in the bay near London Bridge high rectilinear columns of horizontally bedded strata have broken away from the cliffs, leaving a serrated cliff edge >( Fig. 21.6.2.2). London Bridge used to be a fine example of a double natural archway formed on an elongated promontory where the sea had cut out two caves between bordering joint planes (>Fig. 21.6.2.3). On 15 January 1990 the inner arch collapsed into the sea, leaving a heap of blocks and boulders that has subsequently diminished. The outer arch persists, standing offshore. the bordering Bass . During the Last Glacial Much of the coastal outcrop of Port Campbell Limes­ ­low-sea level phase bluffs of this kind must have extended tone is capped by a red-brown clay varying in thickness of along the whole of the formerly cliffed coastline. Aborigines up to 6 m, and containing small iron concretions known who came here about 40,000 years ago would have seen as buckshot gravel. The clay outcrop sometimes forms a these bluffs as an inland slope, descending to the wide bevel above the vertical limestone cliffs, as at Point Hesse. plains that led them across to Tasmania: a landscape prob- This post-Miocene deposit was described as the Hesse ably largely covered by forest, scrub and heath. The bluffs Clay by Gill (1976). Its contact with the underlying Port have not yet been rejuvenated by marine erosion, which Campbell Limestone is often uneven, lowered into depres- has cut cliffs along the coastline to the east and west. sions that have formed where percolating groundwater The bluff behind Two Mile Bay declines to a narrow has dissolved the limestone. Some of the smaller streams swampy lowland terrace bordered by a dune ridge on its on the coastal plateau disappear down sinkholes near the seaward side, capping low cliffs cut in Port Campbell edge of this impervious mantle of clay. The Arch east of Limestone. There is a visor and notch overlooking­segments Point Hesse (also known as Marble Arch) is a ­ of beach, ramps of abraded limestone, and a modern sub- on a rock ledge 6 m above the sea. It is still much as it was horizontal shore platform with a veneer of beach sand. The when Baker (1943) photographed it 65 years ago, the rock are underlain by an emerged shore platform, and surface having been indurated by carbonate precipitation. the bluff to landward is a former sea cliff, degraded by sub- The coast at Two Mile Bay >( Fig. 21.6.2.4) is of great aerial processes that have formed a colluvial apron interest because it preserves a fragment of the Late (>Fig. 21.6.2.5). Its profile is similar to the slopes inland Pleistocene landscape and throws light on the evolution of on the sides of the incised Port Campbell valley. Victoria: The Port Campbell Coast (Mepunga to Princetown) 21.6.2 1321

⊡⊡ Fig. 21.6.2.3 London Bridge as it was in 1989, before the inner arch collapsed in 1990. (Courtesy Geostudies.)

⊡⊡ Fig. 21.6.2.4 An aerial view of Two Mile Bay, Port Campbell, showing the Pleistocene bluff running behind the dune-fringed coastal terrace. (Courtesy Neville Rosengren.)

East and west of Two Mile Bay the bluff has been reju- usually structural, in the sense of harder limestone payers venated as receding cliffs, and its preservation in Two Mile exposed by marine erosion in front of a receding cliff. Bay is because of the presence of a nearshore limestone Port Campbell is a valley-mouth inlet, bordered by , which diminishes wave attack on this sector. Because cliffs that decline landward into bluffs on either side of a of this reef, swell breaks into surf farther offshore than on small crescentic sandy bay-head beach. The cliffs are pen- other parts of the coast. The reef is thus a protective fea- etrated by caves at the mouths of underground tunnels. ture which has preserved a segment of Late Pleistocene There was a major east of Port Campbell in coastline that would have to be cut back about 300 m to 1939 when a section of cliff near Sentinel Rock 70 m long convert the bluffs into vertical cliffs. and up to 12 m wide suddenly fell into the sea. The scar of In general shore platforms are poorly developed on this fall can still be seen, and after 70 years only part of the the Port Campbell coast, and where they do occur they are tumbled rock has been consumed by marine erosion. 1322 21.6.2 Victoria: The Port Campbell Coast (Mepunga to Princetown)

⊡⊡ Fig. 21.6.2.5 Cross-section across the coastal terrace at Two Mile Bay, Port Campbell, based on diagrams by Baker and Gill (1957). (Courtesy Geostudies.)

⊡⊡ Fig. 21.6.2.6 A cliff collapse near The Amphitheatre in 1970. (Courtesy Geostudies.)

A fall of this magnitude probably occurs on each sector of the effects of waves and spray during storms, as well as these cliffs once in 1,000 years, but it can happen at any runoff during heavy rain (Baker 1958). The bald-topped time: there have been many smaller rock falls. Baker’s Oven Rock had soil and weathered material swept The present cliffed coast has a generally sinuous or away by wave overwash, and stripped zones are promi- crenulate outline, with numerous small bays and narrow nent on cliffs either side of Sherbrook River. Photographs gorges. Goudie’s Lookout (>Fig. 21.6.2.1) is a narrow published by Baker (1943) showed storm waves breaking ­promontory 128 m long, 6–12 m wide and up to 60 m over a 25-m high at Broken Head. The backwash high, with prominent layers of more resistant limestone following such giant waves sweeps loose material away etched in the cliff. from the cliff top and cliff face, and as a result there is a Most cliff falls go unrecorded, but the cliff collapsed at cliff crest ledge up to 50 m wide, backed by a low concave The Amphitheatre in 1970 >( Fig. 21.6.2.6) and Elephant bluff of brown clay up to 4 m high at the trimmed edge. Rock, the long, narrow promontory behind Mutton Bird By contrast, some slightly more sheltered sectors of the , lost its “trunk” during a storm in 1935 (Baker cliffed coast have a cliff-top deposit of yellow-brown calcar- 1943). eous marl (parna) as a wedge or levee up to 0.5 m thick and In many places the crests of the vertical cliffs have been 100–150 m wide, thinning and fining landward. Ashton stripped bare of soil and vegetation by wind erosion and et al. (2002) concluded that this marl was derived from the Victoria: The Port Campbell Coast (Mepunga to Princetown) 21.6.2 1323 cliff face by wind action, and carried up and over the cliff be retained on exposed to strong wave energy: it crest. It contains Miocene fossils derived from the cliff out- is silty, soon dispersed and carried well out to sea. Beaches crops. The marl has been deposited on trees in bark fur- do occur where there is, or has been, sandy dune ­calcarenite rows and branch angles. It rests upon a buried podzolic soil on the cliff crest, as in the Childers Cove area and near that emerges landward, and carries ­calcicolous vegetation . As the cliffs recede, quartzose and calcare- that passes into calcifugous heath on the podzolic soils. ous sand from this capping material falls to the shore, and East of Broken Head the cliff outlines consist of alter- has been incorporated in the beach. The general absence nations of deep narrow gorges, such as Survey Gorge, and of beaches has enabled storm waves to attack the base of long narrow promontories, both bordered by vertical the cliffs vigorously and consistently. Cliff recession would cliffs. Island Archway is an elongated penetrated by a have been slower had wider, more protective beaches been wide arch, the whole formation strongly influenced by able to accumulate along this coastline. erosion along joint planes. The arch collapsed into the sea Pleistocene dune calcarenite appears east of Gibson in June 2009. Mutton Bird Island is another block pene- Steps, where it is banked against an eroded bluff that trated by a natural arch and the Razorback is a long, high backs a scrubby landslide where Port Campbell ­Lime­stone and remarkably narrow ridge to the east of Lochard Gorge. has collapsed behind a re-entrant on Gellibrand Clay This is another steep-sided inlet between rectilinear prom- (>Fig. 21.6.2.9). ontories, with a beach of in-washed sand (>Fig. 21.6.2.7). The dune calcarenite forms a high and wide coastal East of The Razorback the vertical cliffed coastline ridge on the southern side of the Latrobe Creek valley. straightens, and is bordered by a group of tall stacks known It descends gradually SE to the shore to outcrop in the as The Twelve Apostles, with outlines related to patterns of cliffs towards Point Ronald, and is truncated by a steep jointing and stratification (>Fig. 21.6.2.8). In the lee of cliff 90 m high, alongside the mouth of Gellibrand River. these are minor protrusions, and it is possible that several It is possible to distinguish six successive dune formations, of these were formerly linked to the adjacent stacks by separated by wavy unconformities that represent episodes arches that have since collapsed. One of the stacks col- when dune accretion was interrupted by wind erosion, lapsed in 2006. truncating the earlier dune bedding. Three of the un-­ Beaches are poorly developed on the Port Campbell conformities include palaeosol horizons and associated coastline, due partly to a meagre sand supply and partly to underlying calcrete layers that mark stages when the land wave reflection from the vertical cliffs, which prevent surface became sufficiently stable for soils to form. beach accretion. The rivers that drain to this coast carry Gellibrand River forms a convenient boundary little sediment into the sea, and the material derived from between the Port Campbell coastline and the rising slopes cliff erosion is generally too soft and too fine in texture to of the Otway Ranges.

⊡⊡ Fig. 21.6.2.7 Lochard Gorge. (Courtesy Geostudies.) 1324 21.6.2 Victoria: The Port Campbell Coast (Mepunga to Princetown)

⊡⊡ Fig. 21.6.2.8 The 12 Apostles. (Courtesy Jenifer Bird.)

⊡⊡ Fig. 21.6.2.9 The Glenample Landslide, east of Gibson Steps. (Courtesy Geostudies.)

References Baker G, Gill ED (1957) Pleistocene emerged platform, Port Campbell, Victoria. Quaternaria 4:55–68 Bird ECF (1977) Cliffs and bluffs on the Victorian coast. Vic Nat Ashton DH, Williams RJ, McDonald M (2002) The ecology of cliff-top 94:4–9 heathlands at Port Campbell, Victoria. Proc R Soc Vic 114:22–41 Gill ED (1976) - district. In: Douglas JG, Fer- Baker G (1943) Features of a Victorian limestone coastline. J Geol guson JA (eds) The geology of Victoria, Vol 5. Geological Society of 51:359–386 Victoria, Special Publication, , , pp 299–304 Baker G (1958) Stripped zones at cliff edges along a high wave energy coast, Port Campbell, Victoria. Proc R Soc Vic 70:175–179

Top View