SANDSTONE ISLAND: GEOLOGY, Geomorpiiology, Vegeration
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~/ WESTERIPORT BAY ENVIRONMENTAL STUDY 1173 - 1174 Project Report 4.3.3.1 SANDS'IDNE ISLAND: GEDIDGY, GEX)M)RPHOLCGY, VEGETATION E.C.F. BIRD ...... ',' ,', ..... .... .,' . -: ,- : . ..... <.<.. :/ '.' :<::.:::<.':<.~::.':/:./ :::>: .<::::>: .... ::.:::: .... ::'::> .... ..... .... , ... .. .' .......... ..... ........ .................................................. i ........ .' . ..' ., :::::.:-......................... , ... " ..... ,,-:<,' :-:-: :-','-:.:-:-:- .. .. : ': '::: :: ~:::::::::::::::!::::::: ~:::::' :: ." .~ .......•.••.•••••• -••••••••••.••••.••• :.••••.••..•••. -:: -::- ... :-<:::::::','::::.::::. ,1:" • ..... ...··ll:,::,.'} .. : . : :: : : :: : :: :::::::.::::::: ~ :: :: : ~: ::: : : : :: .: :: :: . ........................., ................ ' .. ... ·······:i~:·····/ ~~... .......... '.'::::)::::::::\)\>:-' UIIISTAY Foa COilE OVAl 11811 VICTOllA $51- ___Lt~2. 1511\ lfJ - " PIDJECr REPORT 4.3.3.1 -; SANDSTONE ISLAND: GEOLOGY, GEOMORPIiOLOGY, VEGErATION E.C.F. BIRD REPORT ro THE WESTERNPORT BAY ENVIRONMEN'.mL STUDY Prepared by E.C.F. BIRD GEDGRAPHY DEPARIMENT, UNIVERSITY OF MELBOURNE DECEMBER, 1974 NOTE • TR€ investigation reported herein was carried out with the support of the Westernport Bay Environmental Study, Ministry for Conservation, Victoria, Australia. This report is one of the various project reports received by the Study from participants. The contents do not necessarily represent the official view of the Study. Copies are available from: Environmental Studies Program Ministry for Conservation 240 Victoria Parade East Melbourne 3002 Australia SANDSTONE ISLA}ID --- Geology, geomorphology, vegetation E. C. F. Bit'd Sandstone Island lies in the southern part of Hastings Bight, about 3 kilometres SSE of the town of Hastings. It has an area of about 22 hectares, and is bounded by steep grassy bluffs up to 5 metres high, rising to the margins of a broad, gently-undulating surface with a maximum elevation of 14 metres above state datum and a gradual southward declination. Keble-(1950) described the geology of Sandstone Island as an anticline of Silurian sandstone, with associated quartzites and bands of shale. The anticline has an axis running NNE to SSW through the island, pitching southwards. The Silurian rocks outcrop in a few local exposures that interrupt the grassy bluffs on the eastern and western coast of the island, and more extensively in the shore platform that borders much of the island COgst, .exhibiting steeply-dipping, thinly bedded strata that have been planed across by marine ergsion. On the vlest coast the dip is typically about 45 , westerly and south-westerly, while on the east 0 coast it steepen& to more than 65 , and on the north eastern shore there is a sector with vertical strata; the anticline is thus markedly asymmetrical (Keble 1950, Fig. 10). Because of these geological·features, Sandstone Island has been included in the list of Sites of Scientific Interest published by the Town and Country Planning Board (Bird 1974). On the surface of the island the Silurian.rocks have been weathered to form podzolic soils, and on parts of the bordering scarps they have been weakened by weathering to such an extent that slope failure has ensued. At the northern end of the island the grassy bluff shows landslip scars and terracettes. The bordering bluffs are throught to have bean form?d originally as cliffs cut by marine erosion during a past phase when stronger wave action reached these island shores. This may have been prior to the accumulation of the extensive depositional mudflats that now surround the island, and diminish wave energy even when stormy weather produces rough seas out in Westernport Bay. Alternatively the bluffs could have been cut as cliffs when the sea stood higher, relative to the land, than it does now in Westernport Bay. Similar bluffs occur on other sectors of the Bay shore, including parts of French Island and Phillip Island, and they could have been shaped as cliffs during a Pleistocene (or possibly a Holocene) phase of higher - 1 - sea level. Whichever explanation is correct, they are now degraded cliff features, reduced to vegetated slopes by weathering processes in a subsequent period of diminished wave attack. The shore platform of planed-off Silurian strata 'was also formed during this phase of cliff recession, and is now subject to only minor and occasional m6dification (e.g. surficial disintegration to yield gravel) by the present wave regime. In some places it has been blanketed by the deposition of sand and mud. Wave energy is generally low on the western coast but waves.up .to 20 ~entimetres high were generated across a fetch of about one kilometres on a day when a strong westerly wind coincided with a high tide. The east coast faces a longer fetch, and is subject to occasional moderate wave action when south-easterly winds accompany high tides. Under these conditions, wa,ves wash on to, and sort, the sands and ferruginous gravels that form the beach on the south-eastern sector of the island coast. Although there are patches of sand and gravel on the upper shore, the zone exposed at low tide consists of extensive mudflats, partly concealing rocky outcrops and some gravelYy patches and shell beds. The soft mud carries only a patchy and variable growth of Zostera. Above mid-tide level, and especially in the zone close to high neap tide level, there are stands of mangroves, the most extensive being off the north western sector of the island coast. Scattered mangroves also occur along the west coast, where they are backed by an irregular zone of salt marsh, and on the south-eastern coast in front of part of the sand .and gravel beach. The salt marsh (including tall Arthrocnemum scrub) is best developed in the lee of the extensive mangroves off the north-western shore; elsewhere it has been either eroded, or partly buried by sandy drift. These features are common on many sectors of the shoreline of westernport Bay, where there has been local die-back or disappearance of the mangrove fringe since it was first mapped by Smythe in 1842 (Bird and Barson 1974). On Sandstone Island Smythe mapped a broad and continuous mangrove fringe extending out from th~ north and west coasts, where now it is reduced and fragmented, with some dead mangroves in areas that have become sandy. On the east coast Smythe showed no mangroves, even in the sector where they now exist patchily. It is possible that there were mangroves here in 1842, but that Smythe considered them too sparse to be worth mapping. - 2 - Under natural conditions Sandstone Island was proba.bly covered by scrub and woodland similar to that still seen on the mainland at Golden Point. On Smythe's map areas of scrub and woodland were stippled, and the same shading was applied to Sandstone Island as to the hinterland of Golden Point, which now has manna gum woodland«Eucal!;tus viminalis) with an understory of shrubs, notab y Acacia armata, Le£tospermum ¥uniPerinum, Bursaria spino~a, and Mela euca erici olia. Grazing stock were introduced to Sandstone Island in the eighteen-forties, and grazing has proceeded intermittently until about seven years ago. The original scrub woodland has been cleared' and re\>laced by tussocky grassland, ~'li thin which areas of blackberry (Rubus sp.) are now spreading •. A few sawn-off Eucalypt trunks persist near the middle of the island, .but most of the trees now growing are introductions, notably pines, elms, oaks, and a fig tree. It is possible that a house or hut once stood near the northern end of the island, where there are extensive areas overrun by garden plants, including arum lilies, and some old waterholes. A few rabbits persist, and there are indications (abandoned burrows, networks of rabbit runs) that they were formerly abundant, but have been reduced by myxomatosis and shooting. 'llhe vegetation of the island is thus that of abandoned farmland, and very little of the native flora remains. The following plant list was compiled by Mrs. M. Barson during a visit to the island in September 1974. FILICINAE GRAM I NAE Pteridium esculentum *Bro~us diandrus *Dactylis glomerata fvIonocot;y,ledoneae Distichlis distichophylla AGAVACEAE *Erharta erecta *Holcus lanatus *Agave americana *Lolium perennis Poa sp. AI"1ARYLLIDACEAE Spinifex hirsutus *Narcissus sp. IRIDACEAE ARACEAE "'Iris germanica *Zantedeschia aethiopica JUNCACEAE CYPERACEAE Juneus maritimus Scirpus nodosus Scirpus sp .. *denotes naturalised alien - 3 - Dicotyledoneae r"IALVACEAE AIZOACEAE *Lavatera' arborea Disphyma australe Malva sp. Tetragonia tetragonioides I'-HMOSACEAE APOCYNACEAE Acacia mearnsii "'Vinca major *Albizia lophantha CAPRIFOLIACEAE MORACEAE Sambuctis gaudichaudiana *Ficus carica CARYOPHYLLACEAE MYRTACEAE *Cerastium glomeratum Eucalyptus sp. Spergularia media Melailieuca ericifolia CASUARINACEAE OXALIDACEAE Cas.t18rine sp. Oxalis co~niculata CHENOPODIACEAE PAPILIONACEAE Arthrocnemum arbusculum *Genista linifolia Atriplex paludosa *r1edicago truncatula Chenmpodium spo Hemichroa pentandra PINACEAE Salicornia quinqueflora *Pinus radiata Suaeda australis PLANTAGINACEAE CONPOSITAE *Plantago coronopus *Arctotheca calendula Plantage varia *Sonchus oleraceus PLmmAG INACEAE CONVOLVULACEAE Limonium australe *Convolvulus arvensis POLYGONACEAE Dichondra repens *Rumex acetosella CRUCIFERAE *Rumex crispus Cakile maritima PRIMULACEAE EUPHORBIACEAE Samolus rep ens Adriana sp. RHANNACEAE FAGACEAE Pomaderris oraria "'Quercus robur ROSACEAE FMlIIlARIACEAE......... *Rubus sp .. lltFumaria sp. RUBIACEAE GERANIACEAE