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PAPBRS ANn PROCJ<JElDINGS OF THE .ROYAL S•tGU<il'Y 01" TASMANIA, VOLUME 93 SEA LEVEL CHANGE AND SHORELINE DEVELOPMENT IN SOUTHuEASTERN TASMANIA By J. L. DAVIES Department of Geography, University of Tasmania (With 5 Text Figures) ABSTRACT this event which initiated the present cycle of The existence of shore features associated with marine erosion. Since that time the sea has two older, high-er sea levels has been an important modified the initial coastline by the cutting back factor in the development of the present coast of of headlands and the building of spits and tidal south-eastern Tasmania. The Llanherne level lies marshes, and, simultaneously, smaller changes of b-etween 12 and 15 feet above present sea level sea level have occurred. It is with the inter­ and is probably related to the last interglacial relationship between these marine processes and or an interstadial time. The Milford level is two these later and smaller sea level changes that the to three feet above present sea level and is almost present account is concerned. eertainly post-glacial in date. The nature and effect of marine action at the two lev-els is discussed in relation to Seven Mile Beach spit, Ralph Bay THE LLANHERNE AND MILFORD SEA LEVELS Neck and Marion Bay spit. Two recent higher sea levels have played a large part in determining the morphology of the present coast. Features resulting from these two INTRODUCTION levels are well ctaveloped in the vicinity of Hobart The major outline of the coast of south-eastern airport and this provides a convenient " type Tasmania results directly from the world-wide, locality". At the higher level, high-water mark post-glacial rise of sea level, which drowned the stood at between 12 and 15 feet above that at Derwent and Pittwater valleys and flooded the present and it is proposed to designate this the lowlands of Ralph, Frederick Henry, Norfolk, and Llanherne level, since the small hill of that name, Blackman Bays. That such a submerg-ence took on which the airport control tower now stands, place is generally accepted, although its exact then formed an island. To the east of here, at extent has yet to be determined. Sections across Milford, the lower level is represented along the the Derwent near Hobart, published by Lewis shores of Pittwater by old vegetated cliffs fronted (1935), show a mud-filled channel cut in bedrock by narrow raised beaches. A part of these can be to about 150 feet below present sea level, thus imply­ seen from the old road on the Bluff at the western ing a strandline at least about 200 feet lower than end of the Sorell cau3eways. It is proposed to call that which exists today. A clearly-defined, sub­ this the Milford level and high-water mark at merged, mud-filled and apparently river-cut chan­ this time was between two and three feet higher nel runs from east to west from Norfolk into Fred­ than at present. These two old shorelines can erick Henry Bay, reaching a maximum depth of 132 be detected throughout south-eastern Tasmania. feet immediately to the north of Slopen Island. Where they occur their height has been checked Since it contain3 an unknown amount of mud, its using a level and staff and relating present and original depth can be assumed to be something in past high-water marks. Tidal range along these excess of this (Admiralty Chart No. 809, Frederick coasts is about three to four feet, ecept in Norfolk Henry and Norfolk Bays). Bay where it rises to six f-eet. Such evid•cnce for previous low sea levels prob­ Lewis 0935) discussed Tasmanian raised beaches ably relates in the main to a general average and river terraces and attempted correlations of Pleistocene low sea level. which approximately may each with his scheme of Tasmanian glacial stages. have been attained during each glacial. Detailed His lowest raised beach at 5 to 15 feet above high­ stratigraphic investigation in suitable localities wlll water mark was thought to date from the last be necessary before the full sequence can be worked interglacial. Later (Lewis, 1946J he named this out and this has not y•et been attempted. However, the Ralph's Bay stage and referred the next for the purpose of interpreting the present shore­ level at 50 to 60 feet to his Cambridge stage. Lewis's line, it is sufficient to appreciate that the last Ralph's Bay stage terraces seem to correlate major shift of sea level was an upward one, that generally with the Llanherne level and he does the main coastal outline results from the drowning not seem to have recognised the lower level as dis­ of a former sub-aerial landscape and that it was tinct, although he doe-s appear to have noticed its 89 90 SEA LEVEL CHANGE AND SHORELINE DgVELOPMENT IN S.K TASMANIA. FIG. L··--Location of the three areas discussed. existence. For instance, in referring to the South at.ed into a peninsula, along both northern and Arm peninsula, he mentions (1946, 154) that "a southern sides of which the sea cut low cliff:.; raised beach generally five feet skirts the (Figure 2Bl. On the southern shore, frontal dunes whoie coast, but at Maria Point this is 15 feet above were built up, although subsequently these have sea level." On the other hand, he later (p. 209) be-en much eroded by the small creek which now referred to his Cambridge stage the raised beaches runs through from south-west to north-east. This tying Llanherne island. In view of this confusion creel;;:, in addition to carrying tidal ftood and ebb, and uncertainty, Lewis's nomenclature is not used drains a large, shallow pool, which forms on the here. Llanherne surface south-west of the airport at The general nature of the features associated times of heavy rain. with the Llanherne and Milford levels and their Between the Milford shoreline and the present­ influence upon the development of the present day shoreline has grown up the Seven Mile Beach shoreline can best be discussed by reference to spit and factors influencing its alignment have three major structures, Seven Mile Beach spit, been discussed elsewhere (Davies, in press). That Ralph Bay Neck and Marion Bay spit, the location the spit appears to have grown from west to of which is shown in F'igure 1. east seems to be due to three main factors: the presence of the initial peninsula on the western SEVEN MILE :BEACH SPIT shore of Pittwat·er the prevalence of beach drifting The large spit of Seven Mile Beach is a ,.w,~··~a·.> from west to under the infiuence of local at the head of Frederiek waves generated by south-westerly and the the Pil,twater ria. decrease in wav•e height and shoreline ran the east·ern end of the from west to east caused bridge airfield to the eastern foot of swell. This last (Figure 2Al. The Bluff and Llanherne formed two demonstrated the construction of tied to this shorelin·e by a complex diagrams and inspection of berm heights and low frontal dunes beach cusp dimensions on the beaeh, is At the eastern, Je;>s sheltered end of the probably ~he most beach drifting blow-outs occurred and dunes moved appears to be and western peninsula over the southern slopes of Bluff. The plattorm is there because the Llanherne tombolo also now extending seaward from the Llanherne shore­ grew the west, presumably under the influence line. and on · whJch the new Hobart airport has of the other two factors. Bascom ( 1954 > has been built. in the main to be a depositional discussed the control of stream outlets by wave one of and shell. Both marine and aeolian refraction. sands belonging to this level have been well podzol­ The present is composed of a series of ised and Loveday ( 1957) in a soil survey of the between 40 and sand beach ridges, region, classified them as "podzols on cover sands". ally parallel with the present shore. At the time of the Milford shoreline, the former analysis of a cross profile of these islanc~s of Llanherm; and the Bluff were incorpor- suggests that, while they were being J. L. DAVJES 91 LLANHERNE MILFORD LEVEL LEVEL c 0 Pittwat<H P ESE o t,J!ILES 2 L_ l SE Beach PeniTJsula" The ·(dash-dot" indieatc of fo-redunes and beach on the present-day :-;pit only Pv<::~ry ridge 92 SEA LEVEL CHANGE AND SHORB~LIN.E DEVELOPMENT IN S.K TASMANIA I I I I I I I FREDERICK HENRY BAY RALPH BAY OUTER LIMIT OF PRESENT SALT MARSH. / Q-- PRESENT H.W.M - ·- MILFORD H.W.M. --- LLANHERNE H.W.M. • _.,_ SANDRlDGES AND FORE­ 0 DUNES. 0 ··-··- SHELL RIDGES 1 --------- HALF MILE ~'IG. 3.--li.Jvolut-i.on of Ralph Bay Ne·ck. Shingle structures are shown in blae-k; shell structures stippled. 93 sea level, or· at least high-water mark, fell by part way across the strait and its distal end about three feet. It seems clear that they were became recurved and complex. The two spits formed as the sea fell from the M.ilford level ap­ must have grown up simultaneously and afforded proximately to where it lies today and the field each other mutual protection. Wave action in the evidence is con:sistent with this conclusion. Loveday sheltered north-eastern corner of Ralph Bay is (1957) grouped Milford dunes, beach ridges and very limited and the shingle spit rose very little present-day dunes under the heading of " recent above the Milford high-water mark.

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