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PAPE:HS AND PHOCt<:.b'DINGS OF '.f'HE ROYAL SOCIETY OF 'TAS::M:A?~IA, VoLUME 93 THE GEOLOGY OF THE PONTVILLE-DROMEDARY AREA, TASMANIA
By
IAN McDouGALL* Australian National University, Canberra
CV~lith 1 Plate, 2 F-ip;ures and 2 Maps)
ABSTRACT and the writing of the paper. Thanks are also More than 1,500 feet of flatly-dipping Permian due to D. R. Woolley for identifying many of the mudstones, sandstones and limestones have been Permian fossils. divided into nine units, and are overlain with prob able disconformity by 1000 + feet of Triassic PIIYSIOGRAPHY sandstone and shale, the former rock type being The major controlling factor of the physiography dominant. Jurassic (?) dolerite intrudes the of the area has been the Tertiary faulting, and the Permian and Triassic sediments in the form of distribution of the rock types has played a secon dykes, gently transgressive sheets and sills. The dary, but still important, role, distribution of the various rock types, along with The greater part of the area consists of rounded, the strong Tertiary tensional faulting, has con but commonly steep-sided, hills rising up to 800 feet trolled the structure of the area and hence the above the relatively broad open valleys. In the development of the present-day topography. A main the hills are of dolerite and the valleys are flow of olivine basalt, about 200 feet thick, partly eroded in softer Triassic sediments. filled the valleys of the Jordan River and its tributaries, probably in late Tertiary times. The faults in general strike in a north-westerly direction and downthrow to the north-east, and INTRODUCTION it is within the resulting downthrown blocks that The area mapped consists of two ten-kiloyard the main drainage takes place in a direction squares, 5074 and 5174, the Dromedary and Pont approximately parallel to the fault trend. The ville squares respectively, which are part of the block on the downthrown side of the Dromedary Brighton one mile (Army) map sheet. The area Fault appears to have been tilted in towards the is situated between twelve and twenty miles north fault so that a distinct trough was formed in of Hobart on the main Hobart-Launceston High front of the fault scarp along which the River way. Mapping was carried out in the field by Jordan fiows. Numerous south or south-·west walking the outcrop or contact and pinpointing fiowing consequent streams have developed on it on aerial photographs. A base map was the slopes of the tilted block together with north constructed by means of a slotted template laydown east fiowing streams from the upthrown rock. on a grid to the scale of four inches to the mile, The course of the Bagdad Rivulet may be similarly using trigonometrical stations to provide the hori controlled. zontal control. The topographical detail and the The upthrown side of the north-west trending geology were then transferred directly from the Dromedary Fault may be considered as a. physio aerial photographs by means of a rectiplanograph. graphic unit. It rises from an altitude of 700 feet All grid references within the text are given in to 1000 feet at the fault to a height of 3245 feet the normal manner as used by the Army. Speci at Mt. Dromedary in the extreme south-west mens referred to in this paper are in the collection corner of the area. For a half to one mile of the Geology Department, the University of south-west of the fault the altitude of the country Tasmania, and the specimen numbers quoted are remains similar to that of the downthrown block the catalogue numbers in that collection. but then rises by a series of cliffs and steep slopes to Mr. Dromedary, so that there has been a scarp PREVIOUS U'l'ERA'l'URE retreat of about one mile from the fault. Mt. ,Johnston (1888) makes reference to the Drome Dromedary is capped by about 1000 feet of dolerite dary Permian rocks but no detailed work was which tends to resist erosion, thus maintaining the recorded by him. However, Nye (1922) mapped relief; however, dolerite and Triassic sediments the area in a broad regional survey, and Lewis have been removed east of Mt. Dromedary, revealing 0946) has also covered part of the Pontville the Permian rocks which make up the bulk of square in a similar manner. the Dromedary bloc!<::. ACKNOWLEDGEMENTS Scree slopes, composed mainly of dolerite, are very common between 2800 feet and 2000 feet on The author wishes to express his thanks to the the slopes of Mt. Dromedary, but only rarely extend staff of the Geology Department of the University to lower levels. However, at 012428 a tongue of of Tasmania for the help and encouragement so scree material about 200 yards acros3 is found at the freely given during the field mapping, compilation stratigraphic level of the Risdon Sandstone at *Honours Thesis, U. of Tas. Grant as Re..search Student. an altitude of 1400 feet. What i:s probably a
59 60 GEOLOGY OF TIHJ PONTVILLE-DHO.MEDAHY AHEA, TASMANIA remnant of much older scree material occurs at basalt which acts as a local base level for the 027406 in a road cutting near the Upper Dromedary river above it. The stream has eut a steep village. It consists of angular blocks of fossiliferous sided gorge up to 150 feet deep in the basalt and it Permian sediments, Triassic sandstones and fresh remains in this rock type practically all the way dolerite with a matrix of finer material of the to its confluence with the River Derwent. Just same composition. The locality lies between the south of Brighton the river leaves both the basalt retreated fault scarp and the fault plane, but the and dolerite and flows in Triassic sandstones and position of the material is not consistent with the shales, but it soon turns west to nm again into present-day scarp. However, the material could basalt. At 120408 the Jordan swings sharply have been a scree related to an earlier position then south and then north-east. pnwtically of the fault scarp. back on itself. After a further to the into dolerite the Jordan A distinct, but usually not well developed, bench its estuary about 1000 yards south of Lhe boundary has formed at the top of the Ferntree Mudstone of the Pontville square. as a result of its relative resistance to erosion as compared with the overlying Triassic sandstones. Terraces are commonly developed in the valleys At the stratigraphic level of the Rayner Sandstone of the major streams of the area, particularly a similar bench tends to be present. where these streams have Triassic sediments as bed rock. The streams are now almost invariably North and north-east of the Dromedary block incised into their terraces a.lthough restricted the Jordan River flows in a generally south-east areas of flood plain, which are related to the direction, roughly parallel to the Dromedary Fault, present river levels, do oecur. Several setB of terraces which has controlled its course. The Jordan can be recognised. Generally a well-preserved River in pre-basaltic times continued in a south terrace occurs from 5 to 14 feet above the present easterly direction as far as Brighton where it river levels, while a second slightly dissected terrace turned south to meet the R.iver Derwent. However, is commonly found at the 20 to 40 feet level. to basalt filling its valley at least as far This terrace in the Jordan River Valley appears as 087434, the river was displaced northwards to be degradational, whereas those occurring in so that from near the junction of the Lower the Strathallern and Bagdad Rivulet valleys Dromedary and Broadmarsh roads it now flows are covered with alluvium. The latter is well for one and a half miles in a north-easterly above the level of the basalt and is probably of direction and then east to where it age. A possible high level dissected now turns south. For a large part of course in rising from 40 to 120 feet eastwards, is this portion of the area the Jordan flows in east of the Bagdad Rivulet and also appears Triassic rocks where it has commonly developed bt pre-basaltic in age. Since in all cases the quite extensive terraces. terraces are developed behind much more resistant rocks which have acted as local base levels, the Soon after the Jordan River enterfl the north rejuvenation indicated by the incised streams west corner of the Dromedary square it traverses cannot be due to eustatic changes in sealevel. dolerite in which it has cut a rather steep-sided, This rejuvenation may be caused by climatic v -shaped and gorge-like valley, 500 feet in depth. factors, e.g., an increase in rainfall resulting in However, south-west of Strathallie Hill there is a bread open valley in Triassic sediments which a greater run-off and hence more active stream extends from Broadmarsh south-east to meet the action. This would have been aided in historical Jordan valley again near the Broadmarsh-Upper times by the clearing of the land for agricultural purposes which has allowed a ff~ster and greater Dromedary turnoff. The reason the Jordan pre run-off. ferred to make its own course in the dolerite, rather than in the softer Triassic sandstones is probably that the patten~ is superimpose?, since it is thought that dolente of Strathallle STRATIGRAPHY Hill extended across the broad valley south-west A summary of the stratigraphy of the area is set to up with that at 008460, Just east out below:- of the river, in its gorge-like valley, takes a semi-circular course in the dolerite for Quaternary System: Alluvium and river gravels. one and a half miles and then turns south to Tertiary System: Mangalore clay deposit,; now in Triassic sandstones On the aerial basalt. the river is seen follow a concentric Jurassic EJystem: Dolerite, pattern joints which formed the cooling Triassic System: C1-ranul.e conglomerates, sand·, of the dolerite. The eourse of the stones and ,shales; thickness 1000 + feet, to be controlled by these weaknesses dolerite, while the dolerite wbich eovered what Permian System: Sandstones, mudstones, lime·· .is now the broad valley to the south-west was stones; thickness 1530 + feet . eroded off by small creeks. Once the dolerite was erosion proceeded at a faster rate owing l".ERMIAN SYSrn;l\t to the cohesive nature of the Triassic rocks, producing the relatively broad valley. The Permian System, as developed in this area, consists of 1530 feet of sandstones, mudstones, After leaving the gorge north of Strathallie calcareous siltstones and limestones which can HHl the River Jordan remains in Triassic sediments be divided into nine stratigraphic units. These to ~ little south Pontville where it cuts into have been defined and described in detail by BankG IAN McDOUGALL 61 and Hale ( 1957), immediately to the south of the than double the thickness recorded in this area. area, at Mt. Nassau. The base of the Faulkner Group is a bed about Thickness four feet thick of fine-grained poorly-sorted grey (feet) sandstone containing plates of golden mica and Perntree Mudstone 620 abundant black carbonaceous remains. It is com Risdon Sandstone 20 posed mainly of quartz with some feldspar. This "Woodbridge Glacial Forma bed is probably equivalent to the lower part of the tion" 260 Rathbones Sandstone and Siltstone, the Geiss Con Cascades Group glomerate apparently being absent. The rest of the Faulkner Group here ccnsists of grey fissile Grange Mudstone 0-100 siltstone and well-sorted massive to fiaggy quartz Berriedale Limestone 250 sandstones commonly with much muscovite on the Nassau Siltstone . 60 bedding planes. The only member of this group Rayner Sandstone .. 35 which crops out over any distance is the eight Faulkner Group 40 feet thick bed of sandstone, towards the base, Bundella Mudstone 2[)0 which can be correlated with the upper of the Their age varies from Lower Artinskian to Rathbones Sandstone and Siltstone. beds Kungurian·
fossils and but few erratics; however. above this mon, especially in horizons 20 to 30 feet below the horizon the proportion of fo:ssiis steadily increases, Risdon Sandstone. In a well-exposed section, the most abundant fo;:;sil being Strophalosia jukesi, immediately below the Risdon Sandstone at 067405, in addition to spiriferids, ramose Stenopora, fenes an extremely rich fauna was found consisting of tellids, and some pelecypods and gastropods. abundant Terrakea c.f. solida and Strophalosia Erratics occur throughout the formation but are ovalis. as well as many other species. Bands in not abundant. the upper three feet six inches are so fossiliferous as to be called limestones. Erratics are not common Berriedale Limestone through the formation, but loeally may be abun The Berriedale Limestone is a massive, dark·· dant. grey, dense limestone with beds two to four feet Risdon Sandstone in thickness and two to nine inch beds of laminated calcareous mudstones between them. It is up Conformably the "Woodbridge Glacial to about 250 feet thick in this area in contrast Formation" and Perntree Mudstone is to a thickness of some 150 feet given by Banks the Risdon Sandstone, which is about 20 feet and Hale for the Mt. Nassau section. As at thick. Invariably this formation is well exposed, Mt. N;tssau the limestone is very fossiliferous, but at only one locality (067405) is the outcrop the most common types being fenestellids, spiri good enough to obtain a complete section. ferids, Stenopora, both lamellar and ramose vari The basal member, 6 inches thick. is a conu;lomer· eties and Strophalosia. Erratics are present but do ate with a matrix of sand-size particles. The pebbles not occur abundantly. are composed almost entirely of quartzite and vein quartz and are up to one foot in diameter. Several Grange Mudstone spiriferids were seen in this bed. Above this occurs This formation is not always present but may five feet of yellow, medium-grained, moderately be up to 100 feet thick, it is a facies variant well sorted, massive quartz sandstone with some of the Berriedale Limestone. Below the "Wood feldspar. Towards its base there are some spiri bridge Glacial Formation" at 027419 no Grange ferids present and erratics are by no means rare. Mudstone is present; limestone occurs right up A three inch band of iron-stained conglomerate, to the basal sandstone of the " Woodbridge Glacial consisting mainly of sub .. rounded pebbles up to Pormation ". However, at 006447 about 100 feet six inches in diameter, of quartz and quartzite, of Grange Mudstone is present and a similar occurs above the sandstone. However, many of thickness occurs at 0754,00. The Grange Mudstone the pebbles are strongly angular. Some thin. fiat, is a massive bufi coloured mudstone containing angular mudstone pebbles occur, indicating that abundant fossils, including Strophalosia typica water transport was at a minimum. The medium-· and very common fenestellids and Stenopora. As grained sandy matrix is mainly The upper in the other formations of this group erratics occur 13 feet of this formation of a massive, but are not common. Banks and Hale report a yellow, medium-grained sandstone. showing thickness of about 100 feet Ior the Grange Mud no bedding (Plate 1, fig. J. l''eldspar, in addition stone in the Mt. Nassau section which is compar to graphite and muscovite mica are also usually able to the thickness in the Dromedary area but, present. The sorting is fair, with fragments of as pointed out above, in at least one locality no quartz up to 5 mm. in size not uncommon, and Grange Mudstone occurs. there is an occasional small erratic, here and there, through the bed. The total thickness of the forma "Woodbridge Glacial Formation" tion exposed at this locality is 18 .feet 6 inches. Conformably overlying the Cascades Group and Ferntree Mudstone occurs nearby, immediately overlain conformably by the Risdon Sandstone is above the sandstone, but in this section the Fern the "Woodbridge Glacial Formation", a formation tree has been removed by erosion. Therefore the of siltstones and sandstones about 260 feet thick. thickness of the Risdon Sandstone could be greater The section is in all respects quite similar to that than given here, but by no more than a foot or so. at Mt. Nassau, where it is approximately 275 feet Ferntree Mudstone thick. The formation is only fossiliferous in the The Ferntree Mudstone, conformably overlying upper 60 feet and the basal 25 feet. The basal the Risdon Sandstone, consists of a series of mottled member is a coarse-grained sandstone about four grey mudstones with subordinate fissile mudstones. feet thick which varies from a reasonably well It is about 620 feet thick which is very similar to sorted sandstone at 027419 to a poorly sorted the thickness at Mt. Nassau. On weathering, the sandstone at 076401, where it contains abundant rocks assume a yellow colour. In the Mt. Drome quartz fragment;; up to 10 mm. in size in addition dary block the formation commonly has a clifiy to slate fragments. Strophalosia typica, as well outcrop. Erratics are rare, although locally they as other fossils occur in this bed. Above the may be common, but small rock fragments are sandstone there is about [)0 feet of alternating invariably present in small proportion. Fossils are sandstone and fissile siltstone; the lower beds of rare in the Ferntree Mudstone but several spiri the latter being fossiliferous. The next 140 feet ferids were observed. Small scale slump structures consists of unfossiliferous fissile and massive silt are not uncommon. In other areas near Hobart stones and fine sandstones. a formation of Permian age, the Cygnet Coal The uppermost 60 feet of this formation are Measures, occurs immediately above the Ferntree siltstones and fine sandstones abounding in fossils, Mudstone, but if it was deposited in the Dromedary particularly Strophalosia ovalis, long-hinged spiri area it must have been removed by erosion before ferids and fenestellid3. Pelecypods are also com- the deposition of the basal Triassic grits. IAN McDOUGALL 63
TRIASSIC SYSTEM size as the surrounding sandstones. The presence Overlying the Permian System, probably discon of clay pellet conglomerates indicates exposure of formably, occurs a series of sandstones, mudstones the fioor and suggests deposition in very shallow and shales with subordinate conglomerates, which water, possibly in a lake. from plant remains found in similar beds elsewhere The mudstones, which are massive to shaly silt~ in Tasmania are considered to be Triassie in age stones, are in most cases subordinate to the (Bank3, 1952) and deposited in a terrestrial en sandstone. They are buff, purple, pink or green vironment. All the rocks of the Triassic System in colour, fine-grained and normally well sorted have been mapped as one unit--the Knocldofty and almost invariably contain a eonsiderable Formation. Two distinct rock associations can be amount of small museovite mica flakes. The massive and sand siltstones normally show very fine colour banding to divide parallel to the main bedding. These fine-grained, them off into separate units, for relationships often laminated, ·3ediments are particularly suscept to one another are not at all clear. The total ible to erosion and normally it is only in excava- thickness of tho formation is probably about tions, and creek beds that they crop out. 1000 feet, the basal 650 feet or so of which is As the siltstones are of a mainly sandstone with subordinate shales. Except minor importance in Dromedary square where the Permian sediments are exposed below the beds, but in the Pontville square they are not at Knocl~lofty Pormation, the position within the all uncommon. In the area south of Jew's Hill. sequence is often uncertain since recognisable abundant shales are present, as revealed particular~ marker horizons are usually lacking, and there has ly in road cuttings, where they are interbedded with been much disturbance of the beds by the intrusion 11ne to medium grained, rather impure, quartz of dolerite and the Tertiary faulting. In the main sandstones. On the northern flank of Jew's Hill the beds of the Knocklofty Formation are gently practically no shale was observed, however. Al dipping (5°-15') to the south-west but adjacent though there is a fault running along the south to faults and dolerite intrusions the direction and eastern side of this hill, its throw is considered to magnitude of dip varies considerably. be only small and it downthrows to the north-east, so that the lack of shale to the north of the hill Wherever the base of the Knocklofty Formation can only be explained by one, or a combination of is exposed, 10 to 20 feet of buff to pink conglom two alternatives: erate, granule conglomerate and coarse sandstone occurs. The basal 5 to 10 feet consists of poorly 1. They do not outcrop owing to the suscepti ;;orted massive conglomerate composed mainly of bility of the shales to erosion; or sub-angular to sub-rounded pebbles of vein quartz 2. A facies change occurs from south to the up to two inches in diameter. Commonly, bands of north from sandstone and shale into pre conglomerate a .few inches thick are separated by dominantly sandstone. beds about one inch thick of coarse quartz sand The first alternative is preferred, but it is stone with angular particles. Above the basal possible that facies variations do play a part. conglomerate a granule conglomerate or coarse sandstone occur3, which is also poorly sorted. The Triassic sediments in the fioor of the north west trending Tea Tree Valley are predominantly Overlying the basal grits are a series of sand sandstone. Outside the eastern boundary of the stones with subordinate mudstones and shales, Pontville square, towards the head of the valley at which, at Cobb's Hill, are probably at least 650 an altitude of 700 feet plus, pink shales and buff feet thick. All the Triassic sediments of the siltstones occur with sandstone, and to the north Dromedary square are of this type, as well as those of Maiden Early the Triassic rocks, which are of the Pontville-Winton Hill, Shene, Tea Tree almost completely surrounded by dolerite in plan, valley and Summerville areas in the Pontville are sandstones with a high proportion of shale and square. The sandstone, which i:s generally yellow in siltstone above the 300 foot contour. Lewis (1946, colour and commonly strongly iron-stained, is p. 194) reports the presence of plant fragments, medium-grained, massive and well-bedded. Usually probably Schizoneura and Johnstonia, from this the sediments are well sorted and composed mainly locality. It would appear in this area, therefore, of quartz but with significant amounts of feldspar, that sandstone and shale overlie the sandstone. muscovite mica and graphite. In some beds, parti Similarly, on the eastern slopes of Elliott's Hill, cularly at a locality 600 yards north-east of Winton sandst,one and shale, as fioat, occur. Home;:;tead, marked heavy mineral concentrations occur in the sandstones. Sedimentary structures Wherever reasonable outcrop occurs in the Trias such as cross-bedding and slumps are very well sic sediments within the dolerite in the north-east developed in the sandstone (Plate 1, fig. 2). corner of the Pontville square, abundant mudstones Beds of conglomerate up to one inch thick are and shales are present, interbedded with the sand of fairly general occurrence in the sandste:ne. stone. The maximum thickness of sediments, They vary from quartz conglomerates with sub which are considered to be high up in the Knock angular to sub-rounded fragments, to clay pellet lofty Formation, is about 500 feet in this area. conglomerates, with all gradations between. The At 157434 on the Tea Tree golf links, a thin clay pellets are usually fiat ellipsoidal bodies up sliver of "Feldspathic Sandstone" occurs adjacent to three inches in diameter which are norma.lly to the Bagdad Fault. The rock is a medium found parallel or sub-parallel to the bedding. In grained moderately well-sorted sandstone contain most cases the clay pellets and quartz pebbles are ing much feldspar in addition to quartz. Plant set in a matrix of the same composition and grain remains are present. Evidence from elsewhere in 64 GEOLOGY OF THE PONTVILLE-DROME:OARY AREA, TASMANIA
Tasmania indicates that the "Feldspathic Sand so that about 50% of the rock consists of this stone " occurs stratigraphically above the Knock mineral, which appears to have been produced lofty F'orJUation. directly by the thermal metamorphism of the Thus. above about 20 feet of basal grits, occurs calcareous mudstone. The quart.z, which consti approxima,tely 650 feet or sandstone with shale. tutes some 35ry;) of the rock, is fresh and occurs However, within this part of the Knocklofty !<'orma as sub-angular to sub-rounded grains. Its shape tion there may be facies variation;:; into a higher is very similar to that in the umnetamorphosed proportion of mudstone and shale. The evidence rock (8323). The clastie texture is still quite presented above suggests that higher in the sequence recognizable. mudstone and shale become more abundant and Baking of the Triassic sediments by doleri.te has that the total thickness of the formation exceeds produced hard, white, ;sparkling quartzites from 1000 feet in the area. The presence of the sliver sandstones, and dark-blue, extremely hard and of "Feldspathic Sandstone" at 157434 shows that fine-grained cherty hornfelses from the quartzose sediments of this higher formation were deposited mudstones. in the area but most of them have been removed Since the temperature of the dolerite magma by erosion. was probably about 1000' C., the lack of recrystal lization of the intruded sediments for any appreci able distance from the contaets is surprising. However, it is suggested that the reason the contact JURASSIC (?) SYSTEM aureoles are so narrow is because of the absence Large volumes of dolerite intruded as sills, gently of a catalyzing fiui.d being expelled from the transgredsive sheets and dykes into the Permo crystallizing magma and not to a very limited Triassic rocks, brought to a close the deposition supply of heat as suggested by Edwards (1942, p. 59). of the sediments of the Triassic System. The age The metamorphism was of the true of the dolerite is not known for certain, but it contact type, the reconstitution which has taken is considered to be Jurassic (Hills and Carey, 1949; place being due exclusively to heat ef:l:'ects, and the narow aureoles are a result of the poor conduc Banks, 1952). tivity of the intruded sediments. In general, the dolerite occurring in the area Scattered over the surface of Carnelian Hill is a medium to fine grained rock with an even at the north-western corner of the texture and a blue-grey colour when fresh. How square, numerous boulders of chalcedony ever, near intrusive contacts, the dolerite becomes including such varieties as carnelian, agate very fine-grained, dense and fiirtty. A thin section of prismatic quartz crystals (8351J cut from one such rock has an between bands of chalcedony. line texture and consists of 'I'his is found very closely associated with of orthopyroxene set in a fine-grained dolerite-Triassic sandstone contact. The author of granular and prismatic clinopyroxene and is in with Nye 0922, p. 38J in laths or anhedral crystals of' plagioclase. The that is related genetically to the intrusion. bulk of the dolerite is medium·-gra.ined and consists is considered that the silica was introduced by essentially of plagioclase laths (usually a labra solutions from the dorite), pyroxene ~~·-D_R_O_M_E_DA_R_Y__ ··----.,-, ..... f-.,,-. -ce-,--'--P-O_N_T_V_IL-.L-E-.-----, Jt I I MILE FfG. L-Diag:r-amraatic 1nap oi the fault Bystem of tb<: DrmnedHry~Pontville area. R.S.5 66 GEOLOGY OF THE PONTVlLLE-DROM.EDARY AlUcA, TASMANIA tilted towards the fault plane producing a fault here the Bagdad Fault. At 153436, in a cutting on angle depression--a com.mon feature of tension the Tea Tree road, sandstones which have a dip faulting, and this may be caused by the fault of 20° to the north-east are exposed and this plane becoming less steep at depth. Within the has been interpreted as drag dip on a fault with upthrown Dromedary block several faults of a the east side downthrown. At the top of Thomp throw ot' about 100 feet have been recognised. In son's Hill several feet. of white, glassy quartzite general, they strike in towards the Dromedary occurs overlying the dolerite, and it is suggested Fault. At 010401 in a road on the line of one that this is a remnant of the roof rocks of the such fault, a very hard fault breccia some sixteen Jews Hill intrusion, which has been faulted down yards in width ls exposed. This consists of to its position. Nevertheless it is possible strongly angular Permian mudstone fragments that quartzite may have been a small raft of up to several inches in size. sediment floating within the dolerite" On the There are two horsts in the south-eastern portion that the baked sediment is in fact a of the Dromedary square. The one, the of the roof rocks of the Jews Hill body, Cobb's HEl horst, consists of sediments, a throw on the fault of some 800 to 1000 feet is which have been upfaulted, as indicated by drag required, downthrowing to the east. In a trench dips, into dolerite intrusive .into the Knocklofty 057434) on the line of the postulated fault a thin Formation. At Cobb's Hill basal Triassic grits sliver of " Peldspathic Sandstone '' occurs. Its occur at 077404 with some 600 feet of sandstone boundary with the dolerite is much shattered and (and some shale l above them. There is no sign of there is much lime present. All these facts taken dolerite in the section, so that the dolerite intru together suggest the existence of a fault. r,ewis sive into the Knocklofty Formation north of the 0946, p. 185 l interprets the dolerite/-Triassic horst fault is most probably in horizons at least contact at Thompson's Hill as intrusive because 650 feet above the base of the Triassic sediments. of the strong baking along the dolerite-sediment The horst fault therefore has a throw of at least contact running south from 168423; this point 650 feet and it could be as much as 900 feet. The lying on the fault. However, on p. 196 I.ewis states western boundary fault of the horst joins up with that at the Tea Tree golf links, near the Tea the Dromedary Fault resulting in a decrease in Tree road, the contact appears to be faulted. throw along this fault of the order of 900 feet. It is significant that the axis of the pre-basaltic The second horst, consisting of Permian sedi valley and also of the present-day valley is a con ments, occurs within the Cobb's Hill horst and it tinuation of the Bagdad Pault. Between the Wybra is remarkable that the shape of this block is very Hall Permian rocks and the Triassic sediments at similar to that of the surrounding horst. At Shene the ftctult explains the lack of stratigraphical 075403 the throw of the boundary fault is about rank across the valley; however, the exten3ion 900 feet and this throw possibly remains constant. of this fault further north is somewhat doubtfuL The faults of this block disappear into the Derwent It could cut across the extreme north-eastern Hiver to the south. From 078405 to 080444 a corner of the Dromedary square, dropping the fault with a north-north-east trend joins the two Knocklofty Formation down to dolerite, which systems of horst faults resulting in Ferntree Mud further south is intruded into Ferntree Mudstone, stone being upthrown against Knocklofty Forma but there is some doubt since several blocks of tion on the west and dolerite on the north and thermally metamorphosed sandstone were observed east. The throw is probably about 500 feet. near the boundary, suggesting rather that the contact is an intrusive one. At 048434 a floater of hard white quartzite was found near the Knocklofty ,Formation~dolerite The relatively high country rising westwards boundary, suggesting the possibility of an intrusive from the main Hobart-Launceston Highway from contact. However, no other definite contact effects Pontville north must be considered, therefore, as were observed and the boundary is most probably a dissected, retreated fault scarp with the Bagdad a Tertiary fault, as has been assumed above, the Rivulet formed in the fault angle; the highlands quartzite having been brought up by the faulting rising to the east possibly bei.ng the result of tilt from an intrusive contact below. of the downthrown block in towards the fault. The Bagdad Fault could be continued southwards The Jews Hill Fault. strikes in a generally west approximately along the axis of the Back Tea north-west direction along the south-western flank Tree Valley, but dolerite occurs both east and west of the hill and the drag dips on the Triassic of the proposed line so that there is no proof for sandstones and shales indicate that the south-west. its existence in this area. side is upthrown. The throw on the fault at a minimum is 150 feet, but it is probably somewhat A fault with a throw of the order of 100 feet greater than this. The dolerite overlying the sedi·· was recognised at 185423, south of Elliott's Hill, ments on the upthrown side has been removed by striking just north of west. The Triassic sand.. erosion. The fault can be traced with minor stones have a strong dip ( 40° l to the south, indicat inflexions in an east-south-easterly direction at ing that the northern side is upthrown. The least as far as the southern boundary of the displacement of the dolerite contact westwards Pontville square, but it cannot be found extending on the upthrown side of the fault supports the view towards Brighton from the western slopes of .Jew's that the floor of the intrusion dips towards the Hill. west. The doler.ite-Knocklofty Formation boundary At 027461 the reversals of dip and the trending north--west along the western side of high angles of dip recorded in the Triassic Thompson's Hill appears to be a fault-called stones probably indicate a fault passing through B l''W" 2.---A. Contoured stereographic projection of the poles to 70 joint planes in dolerite. B. Similar pojecUon for poles to 45 jcint planes in sediments. 68 GEOLOGY OF THE PONTVILLE-DROMEDARY AREA. TASMANIA the area~ but it was impossible to obtain a consis trusions have generally been removed, but in most tent pictme of the movement from the dips nor ca0es at least 500 feet of dolerite is exposed, and at could a fault be traced in the area. Mt. Dromedary there appears to be about 1000 feet. .Tointing of t.he Doler-ite The large mass of dolerite extending north of Strathallie Hill has strongly discordant contacts Jointing is very well developed in much of the with the rocks. Along the southern side dolerite and up to five different directions have of Stmthallie a sliver of much shattered and been observed in the one exposure. In strongly thermally Ferntree Mud- outcrops there is a predominant series of stone up to 200 yards wide one mile planes, which, in general, are sub-vertical and out at a altitude than closely spaced so that thin plates an inch or so sediments. is in thiclcness are frequently found. Columnar up along the jointing is rarely developed. In some places it of the dolerite. has been observed that the stril~e of one .ioint of system approximates to the trend of the dolerite River for some sediment contact, but this is by no means apparent suggests the possibility of a centre of intrusion everywhere. in this area.. The poles to seventy joint planes in the dolerite were plotted on a stereogram (Fig. 2al which was Eastwards from this centre a sill-like body of then contoured. A maximum of 10'/o occurs in a dolerite was intruded into the Ferntree Mudstone and the lower beds of the Knocklofty Formation, direction of 55 o, corresponding to a set of vertical joints striking at 325', and at right to this lifting the large block of Mangalore sediments a 7% maximum occurs corresponding vertical above it. The dolerite is generally concordant joints striking at 55° Sub-maxima (5o/a) occur in but in local areas it is slightly transgressive, since an east-west and north-south direction represent it rises stratigraphically towards the north through ing vertical joints striking north-south and east the Ferntree Mudstone into the basal beds of the west respectively. Sub-horizontal joints are not Knocklofty Formation. North of the Mangalore uncommon, as shown by the presence of a 2 to 5 r;;;; Creek, near the northern boundary of the Drome area towards the centre of the diagram. dary square, two blocl{s of Ferntree Mudstone oceur fioa.ting as rafts in the dolerite, and these Both the trend of the Tertiary faults and are probably remnants of the sedimentary cover strike of the most developed system of of the intrusion, separated from the main body of joints is the same. 'rhe main set of joints roof rocks by dykes of dolerite. is at right angles to the trend of the faults. The coincidence of direction may be purely fortuitous, The bloclz of Ferntree Mudstone half a mile long or the two prominent sets of joints may be related and up to thre•e hundred wide cropping out to the tensional forces which produced the faults half a mile north of the to Mangalore on or it might even be possible that the Tertiary the Broadmarsh road, is also a portion of the roof faults were controlled by the joint directions in the rocks overlying the dolerite. Towards the east the dolerite. dolerite invaded lower in the Ferntree Mudstone If the joints in the dolerite were produced at the till near the base of the latter it became dyke-like, same time as the Tertiary faulting, it might be the contact running in a straight line down through expected that the sediments also would show such the " Woodbridge Glacial Formation," north of joints. A contoured stereographic projection of the Wybra Hall, to be cut off by the inferred Bagdad poles to the joint surfaces, mainly in Permian Fault from the Triassic ;sediments further east. rocks, was prepared (Fig. 2bl and two good maxima North and north-west of the Creek occur at 100° and 10', corresponding to two sets Valley the hiils, which consist of rise of vertical joints striking at 100 and 100' respec- rather steeply to about 1000 feet so tha,t the lack tively. Although only 45 were obtained, of sediments above the dolerite here indicates the maximum of nearly significant. that it became more transgressive northwards. These two directions do to those in Dolerite connected with the Strathallie Hill the dolerite nor to the general of the Tertiary intrusion and the sill underlying the Mangalore faults. Creek sediments rose up through an east-west. frac ture into the Knocklofty Formation, where i.t THE FORM OF Tim DOLERITJ<: INTRUSIONS spread southwards as a sill. The dyke-like contact Dolerite occupies more than a third of the area runs along the northern fiank of the Winton Hill- and, except within the horsts and the Dromedary Goat Hill which consists of the eroded fault block, the hills are almost invariably capped dolerite sill. Mangalore Creek sediments are by dolerite. It is intrusive into both Permian and th·erefore i.n a large block (four miles long and Triassic sediments, but by far the greater volume averaging one mile in width), isolated from the is found in the latter beds in this area. Th.e sedimentary basement by dolerite. Lewis 0946, form of intrusion is extremely variable, from con p. 191) explained the occurrence of the Mangalore cordant or slightly transgressive sheets to strongly Permo-Triassic block as being due to faulting transgressive dyke-like bodies. Sill-like bodies of along east-west lines before the intrusion of the dolerite are most common, although in detail the dolerite, but this is incorrect-the faulting was contacts are in many cases discordant. The thick cognate with the intrusion. On the southern slopes nesses of the sills are usually impossible to deter of Winton Hill the dolerite becomes transgressive mine since the sedimentary roof rocks of the in- down through the Triassic beds. IAN McDOUGALL 69 In th·8 extreme north-eastern corner of the along ,ts southern flank. The northern boundary Dromedary square the dolerite is possibly faulted is approximately concordant. but the base of the up against Triassic sediments along the postulated dolerite rises eastwards from 250 feet on the Bagdad P'ault, but the presence of some quartzite western side of the hill to about 680 feet at 165418, sugg·ests tlmt the boundary is in fact intrusive. east of which it decreases by 200 feet in 500 yards. If the contact is an intrusive one the dolerite would The dolerite, becoming strongly transgressive, have to be upwards, since north to meet the Bagdad Fault 400 yards the field relations the Triassic heds of Thompson's Hill. At least 600 feet of would have to constitute the fioor of the intrusion. dolerite is exposed on Jews HilL The T.hompson's The dolerite extending .from Lodge Hill west Hill-Elliott's Hill intrusion is considered to be the wards to the western edge of the Dromedary square same sill faulted down some 800 feet by the Bagdad is considered to be the southern continuation of Pault with the baked Triassic sandstone on top the Goat Hill-Winton Hill sill, the Jordan lUver of Thompson's Hill possibly a remnant of the having cut down through it to reveal the underlying roof the quartzite here could Knocklofty Formation. The dolerite remains sill- well be a raft in dolerite. The base of like for the most part,, in the area between the sill is exposed on the eastern of Elliott's t,he Broadmarsh road, of the Mangalore Hill and strikes in a The and the Dromedary Pault where the contact is in northwards dolerite discordant contacts with the intruded from 181428, sediments. This dolerit·e sill is faulted off on the The three-quarter mile wide Tea Tree Valley, south and south-west by the Dromedary Fault and eroded in sandstone, separates the Elliott's Hill the Cobb's Hill horst faults. An irregularly-shaped dolerite from another intrusion, which has the block of Triassic sandstone and shale, completely same general trend and which is part of the large surrounded by dolerite, except on one side which is mass of dolerite in the north-eastern portion of the faulted off by the Dromedary Fault, occurs in the square. This large and extensive intrusion of area around 040435. The contacts are discordant dolerite remains in the Knoeklofty Formation, but and the sediments are strongly thermally meta becomes higher stratigraphically towards the north morphosed. It is uncertain whether this block east. There are many blocks of sediment which, in is a raft of sediment in the dolerite or is connected plan, are completely surrounded by dolerite; their with the country rock below the dolerite. A diameters range from 100 yards to half a mile. In similar type of occurrence is found at 087412. many cases these to be rafts of sediment, The small dolerite body at Brighton and the floating in the which were torn off from two masses south, on the eastern side of the River the country rocks invading magma. are considered to be extensions of the South-east of Tea Tree the dolerite contact dolerite. with the country rocks is steeply dipping, but north 'I'he dolerite capping Mt. was intruded of Tea Tree it becomes concordant, with the at least 400 feet above the base the Triassic dolerite overlying the sediments. Dolerite extends beds which, at 007404 on the south-eastern slopes eastwards to the edge of the square from this of the mountain, is at, an altitude of 1830 feet. contact probably in the form of a fiat dipping The dolerite intrusion appears to be a sheet. North of Maiden Early the dolerite remains sill so that its exposed thicknes;s is of the concordant, except in the area around 158468, of 1000 feet, since the altitude of Mt. Dromedary wher-e it becomes strongly transgressive, The is 3245 feet;. It is considered to be the same sill south-west striking spur of dolerite north-west as that exposed in the downthrown side of the of Maiden Early is probably continuous with that Dromedary P'ault. Within the Dromedary block extending north from Thompson's and Elliott's dolerite has also been intruded, as a sill, 100 Hills under t,he basalt. The contact with the to 250 feet below the top of the Bundeila Mud Knocklofty !<'ormation on the northern side of the stone. Since dolerite occurs in tho lower Permian ris·es towards the north-east and then sed.iments immediately to the to north-west, striking in this direction F'ault and also in the block of the Pontville The to the 'fault plane, intruded into Triassic south-west 120497) then there are at least bodies of thermal at different stratigraphic horizons in this rising to the part of the area. this discordance continues depth is uncertain. In the north-western corner of the Dromedary block dolerite has been intruded into Perntrce A large intrusion of dolerite occurs in the north- three of the four boundaries are Tor- east corner of the Pontville This contains but the fourth is dyke-like dolerite blocks of Triassic and these are contact. the key to the Jorm of the intrusion, If the \Vest of the Strathallie Hill intrusion and divided the north-ea.st from it by the Broadmarsh in sandstone Of the ~COUUJlt;'il is an arrowhead-shaped, intrusion of by erosion. base of the dolerite which may be · of the Strath- one north-east of Shene is at about 850 feet, allie Hill body Irom definitely con- but the Triassic-dolerite contact one mile further nected with large to t,he north and west. north-east at l6:Cl495 is at 1400 feet so that a Jews Hill consists of a sill of dolerite intruded of one in four is on the base of into 'Triassic sediments and it has a fault running dolerite for it to reach However, 70 GEOLOGY OF THE PONTVILLE-DlWMEDAHY AHEA, TASMANIA the contact 500 yards north of Sheehan's Hill is intruded sediments, in this part of the area, will only at about 970 feet so that a gradient of only onlv become clear wben further detailed mapping one in seYen is here. Since the dolerite is carried out in adjacent areas. nearly surrounds the of 'Triassic sediments A small oval-·shaped intrusion of dolerite, 300 the base of the dolerite would have to be very yards by 200 occurs 200 south-east irregular to explain the field data by this form of of the junction of Bagdad witll the intrusion. By means of faults, one along each Jordan River. It is a plug-lik·e mass which is, of the two creeks which join one mile north of hovvever 7 conformable for a short distance its M·erriworth, it is possible to the presence of eastern margin. Triassic rocks on the slopes of both valleys, but since no is further SUMMARY south the throws on 1500 feet of marine to decrertse southwards. siltstones and (which are in the same sensel must be times followed dis- several hundred feet where the two faults 1000 feet of Triassic The Mt. D.ismal block between the two shales. In the ,Jurassic faults would have to be tilted towards south intrusions of dolerite west to produce such a system of faults and there sediments in the form would have also to have been several hundred feet of downward movement the eastern block at the edge of the square with respect to the dolerite in the Sheehan's Hill area. The blocks of Triassic sediments are still nol:. "'L'~''"'~"'''·' explained by this hypothesis. A sill-like at a time and detached blocks of sediment so that they remained HEFERENCES. the field are found rafts of sediment so that if this interpretation is correct there must be some controHing factor on this .intrustion which is not operative. s. W. Carey (personal suggested that if dolerite di:fferent centres of intrusion were intertlnger. then an outcrop pattern as found in area may result. Blocks of sediment would be isolated from the rock in such a zone of interfingering. This hypothesis, at the the most reasonable is considered that the dolerit·e and the PAPNHS ~'i..ND PttocEenr:NGS OF' 'rHg RoYA.L SociETY oP TASJ\'LANIA, VoLlTME PLATE l. FIG. 1.-rfypiea.l outcrop of massive H.isdon Sandstone on the Upper Dromedary road. FIG. 2.-Triassie quartz sandstone showing well-developed current bedding and slump structures. F70. GEOLOGY OF TASMANIA 5074 ONE INCH SERIES-UNIVERSITY OF TASMANIA,GEOLOGY DEPARTMEHT DROMEDARY -:- FAULT WITH OOi.vNTHROWN SIDE INDICATED Quaternary System LEGEND RECENT SERIES KEY MAP SHOWING MAGNETIC DECLINATIONS - - FIIULT- POSITION APPROXIMATE Q~;; ALLUVIUM SECULAR VARIATION lMINS.PA System ~~ - - - FORMATION BOUNDARY-- POSITION APPROXIMATE I Dolerite Boundaries CONCORDANT SILl DISCORDANT INTRUSIVE l:lOUNDAIIIES Compilation from Aenal Photograph5 VERTICAL JOIHTS Station Control by 75___ STRIKE AND DIP OF JO\NTS Deportment of Hobart and the I Service IJ MAPPED AND COMPILED BY !.M,:OOUGALL1vs5 _l_ 5,000 lo.OOOFEET GEOLOGY OF THE DROMEDARY AREA SHEET 5074 PH'r'SIOGRAPHY The area has in general o relief of obout 800', with rounded, but commonly steep-sided, hills rising tip from relatively brood volleys The physiogrophy hos been controlled mainly by the Tertiary faulting striking in o north-westerly drrectron. The Dromedary Fault is the motor fracture and has a throw of 2,000 feet or more decrPosing in mngni tt1de to thP north-west. On the up thrown (south-west) side of thi', fault the relief rs in excess of 2,000'. In the downthrown block, which was tilted towords the fault plane, the moin drainogc of the area tokes place porollel to the fault witi1 smoii consequer11 streams flowinC) in at opproximotely right angles from both blocks. The distribution of the resrstont dolerite and the relatively soft sediments hos olso been on rmportont foetor in controllrng the physiogropl-ry. The Jordon River north of Strothollie Hill, has cut o steep-sided volley ond appears to follow major joints within the dyke-like intrusion of dolerite. STRUCTURAL GEOLOGY The mojor structural features ore the Tertiary foults. Apart from the Iorge Dromedary Fault two horsts occur within the southern port of the areo with the larger Cobb's Hill horst of Triassic sediments having o throw of the order of 900', and the inner, smaller horst, consisting of Permian sediments, having a throw of similar magnitude STRATIGRAPHIC TABLE Age Group Formation Lithology Thickness (in feet) Quoternory River grovels ond olluvium Tertiary Brighton Olivine bosolt flow 100 Basolt Basanite plugs Early Tertiary PI Strong Foulting Jurassic PI Tholeiitic dolerite sills and dykes Triassic Knock lofty Predominontly quartz sondstone, 650 Sondstone and Shale some shale. Basal conglomerates and granule conglomerates. Perm ion Ferntree Mudstone Mudstone 620 Risdon Sandstone Sondstone, conglomeratic bonds 20 "Woodbridge Glacial Siltstones ond fine sandstones 260 Formation" Bosal coarse sondstone Gronge Mudstone Mudstone 100-0 Cascades JBerriedale Limestone Limestone, minor calcareous mud 150-250 stone lNassou Siltstone Siltstone 60 Royner Sandstone Sandstone and Siltstone 35 Foulkner Sondstone and Siltstone 40 Bundello Mudstone Sondstones, si I tstones, mudstones 250+ IGNEOUS ROCKS In the Jurassic (!) Period there was widespread injection by tholeiitic dolerite magma into the Permo -T riossic sediments os--lorge, commonly irregulor, sill-like and dyke-like bodies. The sills in most cases hove been unroofed by erosion. Over 1,000 feet of dolerite is e!Xposed in a sill copping Mt. Dromedory in the south-west -corner of the oreo. There ore two occurrences of basanite in the form of plugs ond these ore considered to be Tertiory in oge A flow of olivine basalt of Tertiary oge fills portion of the pre-basaltic Jordon volley_ POINTS OF SPECIAL INTEREST Bosonite plug (5053E 7468NI. Horst foult (5060E 7415N) Dromedary Fault (5047E 7413NI. Fault breccia (5010E 7402NI. Steep intrusive contact of dolerite with sediments rSOSOE 7466NI Boso I T riossic grits ( 5070E 7472N). Risdon Sandstone (5067E 7405NI. REFERENCES Bonks, M. R, and Hole, G. E. A, 1957~-A type section of the Permian System in the Hobart Area. Pap. Proc. Roy. Soc. Tasm., vol. 91, pp 41-64. Lewis, A. N, 1946------Geology of the Hobart District. Mercury Press, Hobart. McDougoll, I, 1959---The Brighton Basalts, Tosmonio. Pap. Proc. Roy. Soc. Tasm., vol. 93. McDougall, I, 1959-The Geology of the Pontville-Dromedary Area, Tasmania. Pap. Proc. Roy. Soc. Tasm., vol. 93. Nye, P B , 1922--The underground woter resources of the Jericho- Richmond- Bridgewoterr Area Tas. Dept. Mines Underground Water Supply Paper, No. 2. GEOLOGY OF TASMANIA PONTVILLE 5174 LEGEND -~- FAULT WITH DOWNTHROWN SIDE INDICATED Quaternary System RECENT S£FUES Compilation from Aerial Photographs KEY MAP SHOWING MAGNETIC DECLIN4i/ONS 1957 - -- ~ fAULT CONCEALED trigonometric Station Control by SECUlAR VARIATION 7'P£R ANNUM I_Q!~J ALLUVIUM -7- FAULT INFERRED courtesy of the Deportment of Tertiary Sysf·em -- FORMATION BOUNDARY Lands and Surveys, Hobart, and the f1ifS~fJ CLAYs AND SANDS Australian Army Survey Service System Origin of co-ordinates 40o,ooo yds Dolerite Boundor1es Wdst and 1,aoo,ooo yds. South of True Origin of Zone 7 of the International Grid VERTICAL JOINTS STRIKE AND DIP OF JOINTS MAPPED AND COMPILED BY /.~DOUGALL 1956 S,OOO 10,000 FEET GEOlOGY Of THE PONTVIlLE AREA SHEET 5174 PHYSIOGRAPHY Rounded but steep-sided hills, usually composed of dolerite, rise up to some 800' above the brood open volleys which ore generally eroded in the relatively soft Triassic sediments. Faulting has also been a controlling foetor, particularly the Bagdad Fault, which the Bagdad Rivulet follows for some distance. Commonly dolerite and basalt oct as local bose levels for the streams because or their resistance to erosion. Above these bars the drainage system has sediments as bed rock and wide volleys hove been eroded in which the gradient of the streams is low. Within the bosolt and dolerite the rivers hove developed gorges. The surface of the basalt, except where the streams breach it, is markedly planar. STRUCTURAL GEOLOGY The Bagdad Fault is the main structural feature of the area and has on estimated throw of about 800', downthrowing to the north-east. The south side of Jew's Hill is olso faulted, with a minimum throw of 150'. STRATIGRAPHIC TABLE Age Formation Lithology Thickness (in feet! Quaternary Alluvium and river grovels T ertio ry Mongol ore Cloy and sand 5+ Brighton Basalt Olivine basalt flow 150 + Plug of olivine basalt Early Tertiary Pl Strong Faulting Jurassic Pl Tholeiitic dolerite sills and dykes Triassic Knocklofty Sandstone and Shale Sandstones and shales 1000 + Permian Ferntree Mudstone Mudstone 200 + Risdon Sandstone Sandstone 20 "Woodbridge Glacial Formation" Siltstones and fine sandstones 250 IGNEOUS ROCKS Jurassic Pl dolerite was intruded as rather irregular sheet-like and dyke-like bodies. Almost invariably the sedimentary roof rocks of the intrusions have been removed by erosion along with some of the dolerite, so that the maximum thickness remaining in a sill-like body is about 800 feet. The complex outcrop pattern of do\eri te and sediment in the north-eastern port of the area is possibly due to the interfingering of two sheets of dolerite originating from different centres of irruption. The Brighton Basalt of Tert1ary age has flowed down the pre-existing Jordan volley, flooding back up the valleys of the Bagdad ond Strothallern Rivulets. Only one flow is present which in the south exceeds 150 feet in thickness. The basalt is a massive to vesicular and amygda!oidal, olivine-bearing type normatively saturated in silica. Depen«ling on the position within the flow the texture varies from porphyritic with a glassy groundmoss to typically ophitic. Polygonal joint columns ore usually developed and in some cases groups of such columns present most extra ordinary forms, varying from fan-shaped groups to "synclines" up to 120 yards across. A plug of olivine basalt, probably also of Tertiary age, occurs north-west of Maiden Early. POINTS OF SPECIAL INTEREST "Syncline" of basalt columns 15115E 7400N). Fan shaped group of basalt columns (5135E 7138N). Section through basalt flow showing the strong textural variation (5142E 7438Nl. Intrusive contact of dolerite with sandstone (5139E 7438Nl. T riossic sandstones showing current bedding (5187E 7439N l and slump structures (5133E 7446Nl. REFERENCES Edwards, A. B, 195~ The petrology of the Cainozoic basaltic rocks of Tasmania Pap. Proc. Roy. Soc. Vic., voL 62 (New Series), pp 97-120. Lewis, A. N., 1946--Geology of the Hobart District. Mercury Press, Hobart. McDougall, I , 1959--Tile Geology. of the Pontville-Dromedary Area, Tasmania. Pap. Proc. Roy. Soc. Tosm., VoL 93. McDougall, I, 1959-The Brighton Basalts, Tasmania. Pap. Proc. Roy. Soc. Tasm., Vol. 93. Nye, P. 8, 1922-The underground water resources of the Jericho-Richmond-Bridgewater Area. Tas. Dept. Mines Underground Water Supply Paper, No. 2.