PAPBRS ANn PROCJ
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 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. On the sands," although he noted that podzolisation is pro·· Frederick Henry Bay side, wave action is much ceeding in those sections furthest inland which " it greater and the sand spit grew to about six feet can be assumed ... will eventually lead to formation abcm:: the Mil.ford high-water mark by dune accu of a profile similar to that described for the podzols mulation on the wave-bui.lt bar. In between the on cover sands" (under which heading the Llan two spits, salt marsh started to develop and to herne dunes and beaches are grouped) the south of the shingle spit is a group of banks A similar sequence of features can be seen on of shingle and shell also related to the Milford the north from the Hobart-Sorell road near the l!eveL They are now above high-water mark, boundary of the Llanherne airport. South of although they lay just below the high-water mark the road is the line of Llanherne dunes, recently of the Milford sea. One of them, to the east of cut through in the laying down of the new airport the main road, has been quarried for its shingle runway. To the north of the road is the Llanherne content. These structures resemble mid-channel beach, which is succeeded by the Milford shoreline tidal banks, but it seems unlikely that weak tidal crowned with a narrow strip of low dunes and currents, such as those operating today, could succeeded in turn by the raised Milford beach move ·;shingle and it is more probable that the banks and present-day salt marsh. are associated with a time just before the sand spit closed the strait, when stronger current action prevailed in the narrow opening. RAI,PH BAY NECK Finally the sea dropped from the Milford level The South Arm peninsula consists of a series of to approximately that of the present day. In former islands tied by marine deposition at several doing so, it built a series of sand beach ridges along former high sea levels. Ralph Bay Neck provides F'rederick Henry Bay in front of the sand spit and a good example of the influence of the two most a series of shell ridges on Ralph Bay in front of recent of these high sea level;:; on the development the shingle spit. These shell ridges have been of the present coast (Pigure 3). obliterated to some extent the building of the The Llanherne shoreli.ne can easily be traced road across the Neek, but can still be seen from the southern foot of Single Hill to a point immediately north of the eanal. analysis just west of the Hobart-South Arm road, although of the sand ridges indicated a in sea level of in two places it is somewhat obscured by alluvial about two or three feet while were being fans which have been built across it. On the formed (Davies, 1958). At the same time the southern side of the Neck this shoreline is less Milford salt marsh and mid-channel banks were distinct. but runs from Richardson's Hill to Mount uplifted and surrounded by new salt marsh devel Mather. At the Llanherne sea level then the oped at the present sea leveL Neck did not exist and a strait separated South Arm MARION BAY SPIT from the main part of Tasmania. Such a strait was also in existence in the early stages of the The spit at Marion Bay is about five miles long Milford sea level, although now more constricted. and almost closes o1I Blackman Bay from the open The Milford shoreline runs from the northern end ocean 4l. A narrow and changeable channel. of Roches Beach to H.alph Bay and forms generally at its tip is kept by tidal scour. indistinct low cliffs, a foot or two high, cut into Behind the spit. to the west, Llanherne shore- the partly erosional and partly depositional plat line is plainly visible as a line of high cliffs cut form left by the recession of the Llanherne sea. into ba.salt and sandsLone. In froi:1t of these As in the cas·e of the Llanherne shore, the Milford cliffs is a comparable to that at the shore is not so distinct at the southern end of northern of Ralph Bay Neek and to tbat. on the Neck and there is no extensive platform inter which the Llanherne airport stands. This termi vening between the two levels as there is in the nates at its seaward end in low cliffs a few feet north. This is clearly to be correlated with the high cut by the sea at the Milford level. significantly greater exposure to wave attack in It was at the Milford level that the modern spit the north. first appeared. Initially Jt grew southward from After some initial corrasi.on, the Miford sea an old headland of dolerite immediately south of began to build two spits across the strait, which the point where Bream Creek originally reached must have been shallow at this time. One, on the the coast. This first spit extended the full length Frederick Henry Bay side, was composed of sand of the nresent spit and salt marshes develooed and eventually grew so as to close the strait behind it at its northern end and in front of 'the completely. At this stage it must have resembled low cliffs at the foot of the Llanherne platform. on a small scale the bars tying the two halves Later, as the sea fell from the Mi.lford level, a raised of Maria and Bruni Islands today. Its establish sand beach and shell ridges broadened the old ment was helped by the formation of the second spit to the west (F'igure 5) and new foredunes were spit. This grew from a. small promontary in the added to seaward. In part the new dunes rest on north-eastern corner of Ralph Bay and was com a second spit emanating from a headland to the posed of small shingle and shell. It reached only north of Bream Creek. The result has been that 94 SEA LlcVEL CHANGE AND SHORELINE DEVELOPMT';NT TN S.K TASMANIA the creek has been sandwiched between the two ( spits and enters the sea in the middle of Marion Bay. The mouth of Bream Creek oscillates within \ about 600 yards along the shore, but, apart from this minor wandering, it has probably been stable throughout the life of the spit at present sea leveL There is no evidence of an older mouth elsewhere and the present mouth is marked by blow-·out dunes of varying ages, indicating a fair degree of permanence. Convenient partial sections of the uplifted portion of the spit are now visible in the south-west where small storm waves, g•cnerated in Blackman Bay have eroded the inner tip of the spit into a straight line perpendicular to the direction of greatest fetch. Semi-concretionary sand with bedded shells, lying at and slightly above present mark here, is representative of the spit the Milford level. Since the sea reached approximately its present level, extensive new salt marshes have been built and a short account of these is given by Curtis and Somerville 0947 J • DISCUSSION JivfAll!ON The Llanherne level is marked by strong cUffing and by relatively wide platforms, in part wave-cut. This is especially true of the Marion Bay site, which faces the open ocean. Depositional features at this level are limited. In any case. they are no longer in contact with the present FihOP SAND.R.iOGES AND FOREDUNES and of tidal marshes behind the spits is part of the cycle of erosion consequent upon submergenc•e of SHELL. AiDGES a coastline. The slight emergence has only accen tuated these features and to some extent speeded L- thei.r construction. Fw~ 4.----Evolution o.f J\ia·don Bay Splt. ,J. L. DAVIES 95 FIG. 5.-----CToss profile· of inner edp;e of Marion Ba:y Spit. The indicate ridges, all emnposed _prirnarily of the It would be very difficult to interpret the succes break between marine processes at the Milford sion of events between the initiation of the Milford level and those at the Llanherne. No proee;c;s initi sea l•evel and the present as other than essentially at-ed at the Llanherne level carried through into the continuous. In particular the beach ridges and Milford. In keeping with this, there is a pronouneed frontal dunes show a transition in soil gap between the degree of podzolisation of Milford profile from the raw, calcareous sands of the sands and that of Llanherne sands. Loveday present-day to the incipient podzols of the Milford (1957) noted this and remark-ed upon it. The level. Ther'e Is no break other than possibly tentative inference is that the Lianherne level dates a recent one, indicated perhaps by the present from the last interglacial or an interstadial of the erosion of ridges and dunes. But this break, if last glacial ag'e. it has existed, represents only a very small interval of time and may be due to a slight contemporary submergence, or as suggested elsewhere (Davies, REFERENCES. 195'll, to an increase in storminess. In short, the BASCOM, W. N .. Control of St-team Outlets by "vVave present cycle of erosion started at the Milford sea Refraction. 62: tiOO. CURTIS, W. M., AND SOMERVT;cu::. J-., 1947.--Roomet· lVIarsh-----A level and has hardly been interrupted by a small Pre-Hrninary Botanical and Historical Survey. Pnp. P·roc, emergenc•e and possibly an even smaller and most Roy. Soc. Tas., 1B46: 151. recent submergence. If this conclusion is correct, DAVIES, J. L., 1957.-The Importance· of Cut in the then the Milford level must represent the limit Development of Sand Beaeh Hidges. Aust. J. 20: ] 05, of post-glacial subm Milford level and the small amount of podzolisation LEWIS, A. N. 1 1935.-CorJ·ela.tion of the 'f'asn1anian Ple-istocene which has taken place in the Milford sands are in Raised Beaches and IUver Terraees in Unglat3iated Areas. sympathy with this. Pap. _F'roc. Roy. Soc. Tas., 1934: 75. ------~------, 19'40.---Geology of the Hoba.,rt Distriet. 1-Iobart. In contrast to the relationship between the LovgnAY, ,J ., 1957.-·SoHs of the Sorell-Cadton-Copping Area. Milford level and the present, there is an evident Tasn1ania. C.S.LR.O .. Soil Pub!. No. 8. PAPERS AND PROCEEDINGS OF THE ROYAL SOCIETY OF TASMANIA, VoLUME 93 PLATE 1. FIG. 1.-The Milford shoreline at the north-western corner of Ralph Bay Neck: a low, vegetated, raised beach of semi concretionary shell and small shingle is being eroded at present sea level where wave action is now extremely small. FIG. 2.-The Llanherne shoreline at Marion Bay: abando ned cliffs and raised marine platform. F96.