PAPI<J:RS AND PROCEEDINGS OF THE ROYAL SOCIETY OF TASMANIA, VOLUME 92 THE CRYOPLANATION OF MOUNT WELLINGTON 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.