21.6.2 Victoria: the Port Campbell Coast (Mepunga to Princetown)
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21.6.2 Victoria: The Port Campbell Coast (Mepunga to Princetown) Introduction downwashed sediment, with an intervening residual cliff face. If these processes continue the former vertical cliff The 50-km coastline between Childers Cove 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 Bay of Islands 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 shore 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, silts and thin sandy limestones (>Fig. 21.6.2.1). East of Mepunga the dune 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 headlands and intervening coves 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 Nullawarre plateau is dotted with numerous sinkholes (swampy depressions). Near Stanhope Bay the cliff crest shows a calcrete horizon exposed where overlying dune sands have been removed, and solution processes have weathered it in- to rugged, knobbly, karstic (produced by solution pro- cesses) topography. Behind Three Mile Beach a formerly vertical cliff has been cut off from marine erosion by the accumulation of beach and dune sand. 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 Landforms, 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 Inlet, a shallow lagoon 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 channel. 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 coasts bordering Bass Strait. 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 natural arch 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 dunes 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 reef, 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 landslide 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).