PAPI
By
cT. L. DAVIES Department oj Geography, University oj Tasmania
(With 1 Text Figure)
ABSTRACT lation to Tasmania as a whole. Professor S. W. Present morphology of the Mt. Wellington summit Carey of the University of Tasmania, Mr. J. N. area can be interpreted in terms of a system of Jennings of the Australian National University and cryoplanation, which operated during the Pleisto Mr. K. D. Nicolls of C.S.I.R.O., Division of Soils, cene, but bas now ceased. This involved the rapid kindly read the manuscript and made helpful frost shattering of exposed dolerite and the removal comments. of this and previously weathered material by mass 0 movement over slopes as low as 3 • Periglacial MOUNT WELLINGTON processes of land reduction were probably wide The dolerite which caps the 4165 feet high Mt. spread in Tasmania during the Pleistocene and Wellington displays strong vertical jointing and particularly important in unglaciated highland around the slopes of the mountain are great areas, where their end products have been mis masses of dolerite boulders which have been and, interpreted sometimes as evidence for glaciation. in small measure, are still being derived by frost riving and frost shattering of the rock in situ. INTRODUCTION Some of this material lies at its angle of rest The main object of this contribution is to draw which appears to be about 25° to 30° and is prob attention to a group of important geomorphological ably still mobile to some extent. But much of processes, which, locally at least, seem to have it lies on gentler slopes, consists of residual played an important part in the evolution of Tas boulders lying in a matrix of clay, and is thoroughly manian landforms and yet so far have not received vegetated. It seems likely that much of this latter proper attention. Following the nomenclature of material is congeliturbate in origin and that a Bryan (1946), the two most fundamental of these large part of the vegetated talus on the Wellington processes are congelifraction which involves in slopes dates from a time or times when congeli tensive frost riving and frost shattering of rock fraction above was considerably more intense and surfaces, and congeliturbation or the mass move the downward movement of talus was not restricted ment of the congelifractate over very gently in in the main by its normal angle of rest. On the Glined surfaces also as a result of intense frost western side of the Mt. Wellington summit this action. The sequence of land reduction which downward movement of talus at low angles has results from these two processes was termed been channelled into well marked streams and cryoplanation by Bryan and is characteristic of these merit special mention. periglacial regions where temperatures fluctuate Between Mt. Wellington and the Mt. Arthur ridge around freezing point and rapid alternate freezing to the west is a broad, shallow, treeless depression, and thawing of moisture results. between 3200 and 3900 feet above sea level. which Today, these processes are characteristic of today is occupied by the headwaters of the North subarctic and subantarctic lands, but during the West Bay River. It is well shown in photographs glacial ages of the Pleistocene they operated much presented by Martin (1940) in a study of the nearer to the equator and their fossil end products vegetation of Mt. Wellington. Although the de have now been widely recognized in North America, pression may have originated as a stream cut Europe and Northern Asia. In recent years New valley, its present form cannot be explained in Zealand workers have begun to interpret land this way. Martin, who acknowledged that his forms in terms of these processes (for example geological information came from A. N. Lewis, Stevens, 1957, who gives a bibliography) and attributed it to glaCial action and described Dead Jennings (1956) has reviewed the first evidence Island as a nunatak. But Lewis seems to have that some landforms in south-eastern Australia been in two minds about the existence of glacial may also be explained in this way. Referring to ice on Mt. Wellington since, although he envisaged Australia, Jennings remarked that "the state of it .in some publications (Lewis, 1924, 1945), on our present knowledge is well illustrated by the another occasion (Lewis, 1946) he referred only fact that, although about one-fifth of the area of to permanent snow accumUlations on top of the Tasmania is thought to have been glaciated in Mt. Wellington plateau to the expressed exclusion the Pleistocene, there is scarcely a passing refer of glacial ice. There is in fact no disc.ernible ence to periglacial features to be found in the evidence that this area has been glaCiated. The published literature on that state". The present broad, shallow, valley-like depressions do not head study examines the action of cryoplanation pro in cirques and Lewis appears to have thought in cesses on Mt. Wellington near Hobart and then terms of a very small plateau ice cap. He com suggests some of the probable implications in re- pared it (Lewis, 1924J with Ben Lomond in north- 151 152 C?,YOPLANATION OF MOUNT WE:LLINGTON
;fI ·l I ~ /
I !
I I I I
- - -, _. - EDGE OF BLOCK STREAMS
TOE OF BLOCK STREAM
"PLOUGHED FIELD" SECT IONS OF BLOCK STREAMS
BLOCK FIELD SADDLE S
- - - - EDGE OF SUMMiT PLATEAU
~ SUMMIT PLATEAU STACKS
==ROAD
1--'" DIRECTION AND APPROX. I"") 10°,,\ ~3- AVERAGE ANGLE OF f ' \ SLOPE I.tJ
FIG. I.-The Sumndt area of ML 'Yellington. J. 'L. DAVIES 153
eastern Tasmania, which certainly seems to have tions were fed by rapid congelifrilction of the supported such a glacier, and suggested that the dolerite on the Mt. Arthur ridge and along the two were contemporaneous. But an examination edges of the Mt. Wellington summit plateau. Also of aerial photographs suggests that the glaciated at the various saddles or divides it appears that surface of Ben Lomond is freshly ice scoured and there were small block fields or Felsenmeere where relatively unweathered, whereas the summit the dolerite was being shattered on the surface of plateaux of Mt. Wellington are much weathered the depression and removed in either direction as and exhibit tor-like features which could not have congeliturbate. survived glaciation. Lewis's evidence of glaciation The apparent direction and approximate aver seems to have comprised" accumUlations of rocks age gradient of these block streams is thus in resembling moraines and almost certainly ice borne. dicated on the map. It will be seen that gradients At least man)" of them could not have rolled into vary between 3° and 9° and are therefore far out their present position and it is clear that they side any possible angle of slope of dolerite talus reached their present position before the present under normal conditions. vegetation grew up" (Lewis, 1924). Such a des Although such block streams are well known cription would apply admirably to the congeli from other parts of the world and their connec turbate material described earlier. Soil scientists tion with periglacial conditions is well established have started to interpret the soils of Mt. Wellington (compare, for example, descriptions and photo in terms of periglacial processes and Loveday graphs in Smith, 1949), the exact mechanisms by (955) mapped the dominant soils above about 2000 which flow takes place are not yet properly eluci feet as "yellow-brown soils on solifluction de dated. Two such mechanisms are most widely posits ". suggested. In the first it is assumed that the The map shows the general plan of the valley frequent periodic melting of snow and ice lubri like depressions and also the direction and degree cates the surface of the shattered blocks and also· of the slopes involved. Their surface is covered the finer weathered material between them. It by a more or less dense mat of alpine vegetation is probable that the blocks actually moved in a including a variety of small shrubs. From this matrix of finer material, in which case the existing surface rise dolerite boulders which sometimes, as clay matrix may be of periglacial origin or, more to the south of Dead Island, are clearly in situ, probably, older. The second mechanism is pro but more often, judging by their lack of joint vided by the equally well known process of frost orientation, are not. Between the latter is the heave where the surface of saturated material is normal clay matrix which is covered in part by alternately pushed up and lowered by successive peaty swamps. But here and there the clay matrix freezing and thawing. Such a heaving process has been washed away to reveal the true nature of would give rise to a resultant movement downhill the surface of much of this depression, for, where along very slight slopes of larger material lying the matrix has been removed, are masses of on the surface. weathered dolerite boulders forming what locally On the northern summit plateau of Mt. Welling are called" ploughed fields". The location of these ton are several stacks of dolerite in situ, standing patches is shown on the map and two of them are up to about 20 feet above the plateau surface and traversed by the road ascending Mt. Wellington. well weathered along the joint planes after the Because of the removal of the matrix, some degree fashion of tors. Some of them support perched of subsidence has taken place so that the surface is boulders. Whether they come fully within the concave and generally lower than that of the un genetic definition of tors as suggested by Linton disturbed and still vegetated, contiguous areas, but (1955) is difficult to say. They undoubtedly re there is every reason to suppose that both vegetated present residuals resulting from the removal of the and unvegetated areas are essentially similar in weathered surrounding rock westward as congeli nature and' origin. Most convincing of all in this turbate over the plateau surface and eastward to respect are the boulder accumulations in the ex the scarp face, but it seems likely that this was treme south-west where the North West Bay River part of a single cycle of cryoplanation. Today, the enters a comparatively narrow valley. Here is to northern summit plateau slopes gradually west be seen the toe of the congeliturbate standing in ward with slopes varying between 3 ° and 8 0 and its a steep cliff about 20 or 30 feet high and cut into surface is covered with weathered dolerite boulders by the river. lYing without preferred orientation of jointing in The depressions are fringed almost entirely to a matrix of clay and matted alpine vegetation. the west by partly vegetated dolerite talus des Along occasional snow runnels, the matrix has been cending at a steeper angle from the Mt. Arthur removed to reveal loose, weathered boulders. ridge. To the east and south they abut on the In the area occupied by the summit stacks are summit plateaux of Mt. Wellington along a line of numerous monoliths, formed by individual joint slopes of between about 10° and 20° where the blocks in situ. These are absent from the plateau dolerite has been considerably shattered and large surface generally, but re-appear on the convex blocks appear to be in various stages of falling slope immediately above the free face separating to the edge of the depression below. the summit plateau from the block streams. This This combination of circumstances, which is distribution is what would be expected from a two difficult, if not impossible, to explain by other level cryoplanation of the summit area . Along the means, can readily be interpreted in terms of the convex slope too, are great masses of weathered arrested cryoplanation of the Mt. Wellington dolerite boulders which can only have been derived plateau. The broad depressions at the head of the from the plateau surface itself and have· been North West Bay River represent a system of fossil stranded there after having moved westward as block streams which under past periglacial condi- congeliturbate. 154 CRYOPLANATION OF MOUNT WELLINGTON
There were thus two" free faces" supplying con Deductive rea$oning would suggest that this idea gelifractate to'the block streams from the east: may be applicable in some Tasmanian instances. one represented by the westward faces of the Certainly it must be assumed that a more or less summit stacks and the other represented at a deep mantle of weathered material overlay the lower level by the steep slope between the summit Tasmanian highlands at the end of the Tertiary plateau and the block streams themselves. and that, under ensuing periglacial conditions, this Although some amount of frost shattering is still material would be available for translation into going on, block streams and congeliturbate slopes congeliturbate. At this stage frost shattering are now completely fossil. could only take place on steep scarps and cliffs and it would require the removal of the Tertiary rego lith for the full system of cryoplanation to be CRYOPLANATION AND THE EVIDENCE FOR initiated. GLACIATION Weathered block streams and other congelitur To what extent these deductions may be read bate material may be extremely difficult to dis into the Tasmanian landscape cannot yet be tinguish from glacial till and in the field this is assessed, but they must clearly be borne in mind. often impossible, especially in Tasmania, where so many highland areas are uniformly capped with dolerite. Congeliturbate deposits will consist of CRYOPLANATION AND SCARP RETREAT unsorted materials which have moved from their Along the high, dolerite capped scarps of Tas point of origin and in which a large proportion mania, frost shattering has been most active. Here of the bigger constituents have their major axes there is no protecting waste mantle and the aligned parallel to the direction of movement so strongly jointed dolerite provides an admirable that when subjected to detailed examination they target for the freeze-thaw processes. The rate of may simUlate the fabric of till. Because they may frost shattering would have been greatly accelerated move a long way along very gentle slopes, con during the Pleistocene and the resulting great in geliturbate boulders may give the appearance of crease in the rate of scarp retreat would have been being glacial erratics. assisted by congeliturbate processes. At the base The actual angle of slope over which congeli of the Western Tiers scarp in central Tasmania, turbate material may have moved will be deter below the zone of active scree, are fields of mined by local circumstances. In particular per weathered and vegetated talus lying at angles well haps it will vary with the wetness or dryness of below the normal angle of rest of dolerite boulders. the local type of periglacial climate (Troll, 1944; Even more significant perhaps are the " rivers" of Tricart, 1950) and consequently it will be difficult talus which extend for more than a mile in places to make a general rule. It may be argued from to the foot of the scarp (see Map square 4885 in the Mount Wellington example however that slopes McKellar, 1957). These appear to be true peri at least as shallow as 3 0 carried congeUturbate glacial block streams for they lie at angles as low in comparable Tasmanian conditions. as 50 and if this is so, it is particularly interesting that they extend down to 1000 feet above sea level. It follows from these considerations that, in those parts of highland Tasmania where glaciation has been inferred only from deposits which were assumed to be till, the evidence needs particularly REFERENCES careful appraisal. The most important case in BRYAN, K., 1946.-Cryopedology-The· Study of Frozen Ground point is probably the eastern part of the Central and Intensive Frost Action with Suggestions of Nomen Plateau where there seems to be a complete lack clature. Am. J. Sci. 244: 622. of erosional evidence and glaciation has been COSTIN, A. B., 1952.-Mass Movements of the Soil Surface with Spe'cial Reference to the Monaro Region. J. Soil Con assumed in the past (for example, Lewis, 19i13) servation Service~ N.S. W. 6 : 75. on the basis of superficial deposits which were not JENNINGS, J. N., 1956.-A Note on Periglacial Morphology in explicable in terms of normal erosion and there Australia. Biuletyn Peryglacjalny, No.4: 163. fore were thought to be glacial moraines. Many LEWIS, A. N., 1924.-Notes om a Geological Reconnaissance- of Mt. Anne and the Weld River Valley, South-Western of these "morainic" deposits could easily be con Tasmania. Pap. and Proc. Roy. Soc. TaB., 1923 : 9. geliturbate and the problem needs re-examination -----, 1933.-Note on the Origin of the Great Lake in this light. and other Lakes on the Central Plateau. Pap. and Proc. Roy. Soc. TaB., 1932 : 15. -----, 1945.-Pleistocene Glaciation in Tasmania. Pap. ONE CYCLE OR TWO and Proc. Roy. Soc. TaB., 1944 : 41. ---- ,1946.-Geology of the Hobart District. Hobart. Up to now it has been more or less assumed here LINTON, D. L., 1955.-The Problem of Tors. Geog. Jour. that congeliturbate material originated as congeli 121 : 470. LOVEDAY. J., 1965.-Reco-nnaissance Soils Map oj T'asmania. fractate but this was certainly not always so, for Sheet 82-Hobart. C.S.I.R.O., Adelaide. it is probable that among the congeliturbate would MARTIN, D., 1940.-The Vegetation of Mt. Wellington, Tas be material derived by chemical weathering in mania. Pap. and Proc. Roy. Soc. TaB., 19'39 : 97. pre-glacial times. Linton (1955) has suggested that McKELLAR, J. B. A., 1957.-Geology of Portion of the Western Tiers. Rec. Q. Victoria Mus., Launceston. n.S. No.7. tors in unglaciated Britain are the result first of SMITH, H. T. U., 1949.-Periglacial Features in the Driftless deep chemical weathering in warmer Pliocene Area of Southern Wisconsin. J. Geology 57 : 196. climates and second of movement of much of this STEVENS, G. R., 1957.-Solifiuxion Phenomena in the Lower Hutt material downslope by periglacial solifluction dur Area. N.Z. Jour. Sci. Tech. E, 38 : 279. TRICART, J., 1950.-Le modele des pays froids; 1 : Le modele ing the Pleistocene. Earlier, Costin (1952) had in periglaciaire. COUTS de geomorphologie. Paris. voked a similar two cycle theory to explain the TROLL, C., 1944.-StrukturbOden, Solifiuktion und Frostklimate tors of the Monaro Plateau in New South Wales. der Erde. GeologiBche RundBchau 34 : 482.