SECRETS DEPEND ON ORIGINS An understanding of just how and why the Nicholas Range is peculiar or special – an oddity – depends on knowing just how the range came to exist, and what forces and processes act upon it. Let us turn the clock back a few million years, or so…
Beneath the sea A subsiding continent allowed a build up of sediment beneath a relatively shallow sea which was occasionally fairly deep. These sediments compacted into sandstones, mudstones, and turbidites (mixed up sediments which have resulted from sea floor collapses). This occurred about 350-450 million years ago (Ordovician and Silurian Periods). The pile of rocks became several kilometres thick before the crust was destabilised and uplift began. Some of the rocks were crumpled (folded), or were pushed or slid laterally (thrusted) and many were thermally altered due to changes in heat flow and, in some cases, due to the pressure and depth of burial. These rocks are known today as the Mathinna Beds and some of them can be seen near St Marys as well as to the north of the range.
Really finish the job The changes in heat flow due to the formation of the deep basin filled with sediment led ultimately to total crustal distortion and melting. Result: a suite of volcanics and granites. The molten material welled upward (being less dense than the now altered sediments) and both heated them, and shoved some of aside and upward. Who would be a rock? The first of these igneous upwellings is known as the St Marys Porphyrite – and you can see it as you drive down St Marys Pass. This process began about 390 million years ago and continued for about 50 million years during the Devonian Period.
Ever upward But the big changes had only just begun and the whole package continued to rise into a mountain range. By 300 million years it had been extensively eroded and denuded and was now covered in ice. This was the Permo-Carboniferous ice age. Glaciers ravaged the landscape and by the time it had all melted the region had been almost levelled. (This great planar surface remains visible today as the rocks which once buried it are removed and it is the reason that so much of the landscape to the west has similar maximum elevations.) The region was once again flooded by a shallow, near polar sea (with the odd ice berg) during the early Permian Period (say 280 million years ago).
Unconformity: the mark of much erosion and lost time. Permian rocks, flat lying Mathinna Beds: steeply dipping, folded Erosion surface unconformity
A different world New rocks were laid down on the eroded land which had been flooded: sandstone, mudstone, limestone, siltstone and occasional conglomerates. We drive through these in the upper section of Elephant Pass and the road up to the saddle on South Sister. This sequence of rocks is quite thin, in the range of a few hundred metres at most. Then, about 250 million years ago, the entire Earth was transformed. No one is really certain what happened but in Tasmania things which had been marine became elevated and have stayed that way. Nearly every living species died. This was the time of the greatest ever extinction of life on the planet. Nothing would ever be the same again.
The typical “look” of the Permian rocks The typical style of the Triassic coal measures
A changed climate For a few million years the rock record is missing. When it is restored (well into the Triassic Period) we see a cold desert landscape with sand being swept across plains with dry streams which flow only after great storms. These fed swamps which, as time passed and the climate slowly warmed, spread across the landscape. Great forests lived and died. The coals of the region date from this time (Triassic-Jurassic, about 200 million years ago). The climate changes were accompanied by tectonic changes: uplift and increasing volcanism. The St Marys-Nicholas Range area has preserved the only Tasmanian examples of the lava flows of the period.
Dolerite cap seen from the level of the Triassic lavas
Then, catastrophe All this action was mere precursor to serious business. The continent was in process of breaking and rifting apart and the harbinger of this was the injection of huge volumes of melted material from the lower crust. This happened about 180 million years ago (Jurassic Period). Another complete transformation: uplift, disruption and, afterwards, actual commencement of the separation of the now southern continents. The injected material, dolerite, was once a fair depth below the land surface but uplift has continued and all the rocks, once above it, have been eroded away. This rock now sits firmly and dominantly on the top of our landscape.
The distinctive dolerite of the range cap peeps over the foothills.
Another remnant of a lost plateau: St Patricks Head
The landscape becomes modern Much of the cover on the dolerite was removed during the Cretaceous Period and by Tertiary times (about 60 million years) new forces were at work. The final breakups were under way (Tasman Sea and New Zealand in the east; the Southern Ocean and Antarctica in the west and south; and warping and stretching of Bass Strait in the north). This activity led to faulting and rifting within Tasmania and to warping, tilting and uplift in different regions at various times. These impacts are important since they account for many of the oddities of the region. Great rivers drained the rifts formed during the Jurassic (note that it was a much bigger land mass than now) and these were re-arranged by the extra rifting of the Tertiary Period.
The Break O’Day The modern Break O’Day River is a rather pathetic stream in a huge, broad valley which joins the South Esk at Fingal. Once, however, it flowed out to sea but, as the land was tilted and raised, its flow was reversed. This is also why the South Esk now flows toward Conara. The subsidence toward Bass Strait is why it also flows north toward the Tamar. These are complete reversals effected in the last 15-20 million years. This uplift and tilting has not stopped. In the midst of all this activity the climate stepped back into play.
A cooling-off period Things had been cooling for quite a while (since the formation of the Southern Ocean and changes in current circulation) but became decidedly cold about 3 million years ago. Apart from a few warmish periods (including the one we now live in) things have been icy. During the colder times most of the major vegetation has disappeared, the land has been partly denuded and soils have either not formed, formed slowly, or been lost entirely. Some pockets of ancient weathering, thick soils, and botanical assemblages survived. During these changeable times there was extensive erosion and surging rivers. Erosive debris draped the slopes of the valleys.
The dolerite-derived debris (talus and scree) which drapes most slopes
Little was stable Extreme climate action, debris piled on slopes, and variable water flows and content meant that few slopes were stable. Landslides were endemic and material collapsed or slid to the valley floors. Parts of the more solid geology were involved in some of the larger failures. Drier times have held the slopes but any changes involving slope changes (like human excavations or cuts) and anomalous water changes will permit more failures. The landscape is barely stable and several risk areas are noted on the Mineral Resources Tasmania data base along the range.
Today After all this drama we now have the present landscape and distribution of rocks and soils. An eroded capping of dolerite; slopes draped with (mainly) dolerite debris; wide valley floors with alluvials and gravels; variable soils. Everything appears in a variety of sun aspects, on various slopes, and in a range of hydrological conditions. It is a recipe for variety and a modern geological map indicates these factors. And – a few elements of this history are rarities (the Triassic basalts for example) in Tasmanian context.
Geological map of eastern Nicholas Range north of St Marys
Sections across the range: note the unconformity, lava zone, coals and surface drape deposits.
A final geological thought The range appears to stand in isolation – and it is now isolated – but it is really a part of Fingal Tier, now separated by the massive valley of the Break O’Day, a river which does not deserve its valley. The rest of the great plateau which once extended north to Bass Strait is gone but, off in the distance other remnants are apparent: Mts Victoria, Albert. So much that was, so much that no longer exists. Nothing is forever on this planet; it is all about change – a lesson humans have yet to learn.
Some climatic realities The “long” history of the region has involved some significant changes in climate. There have been warm, wet periods and some very cold and dry periods – and most variants in between. The last three million years have been particularly cold – with a few moderately warm (inter-glacials) periods. The intense cold, glacial period of 20 000 years ago began to lift about 12 000 years ago and we have experienced a warm intermission over the past 8 000 years. The great changes in climate, and ice cover, have also determined sea levels. The Nicholas Range area has been affected by all these changes even though never permanently covered with ice.
Associated changes Changes in climate have affected the nature of vegetation, and the extent of its coverage. There will have been times when the land was barren. Land stability is largely determined by the nature and extent of precipitation, especially when vegetation has been removed. Within this variability there have been some constants – due to the scale of Tasmania, and the layout of surrounding seas. Westerly systems have predominated, along with occasional Southern Ocean depressions, and Bass Strait passage depressions: much as now.
Anomalous weather The region is noted for its irregular and anomalous weather. Clearly, the arrangement of broad valleys, offset high ground, and coastal location is important. Onshore cloud situations can apply. The geological signals agree with the rainfall data: this is a dry area with quite exceptional rainfall patterns. Rain surges can happen at any time of year and be among the most intense in Australia. Monthly rainfall data and averages tend to be rather meaningless in this region. Rainfall and seasonal patterns are not predictable – something which shows up in river data. Up to 255 mm has fallen in 7 hours, or 352 mm in 16 hours, or 508 mm in 24 hours (March 22, 1974 – Cullenswood and German Town).
Contrasting river behaviour: abnormal v normal Note the absence of a seasonal pattern with a winter surge in the Break O’Day case
Rain at German Town An indication of the variable and erratic nature of rainfall on the range is provided by data from German Town. The monthly mean ranges from 65 mm (Jan) to 97 mm (Nov) with all other months in excess of 83 mm. The monthly median ranges from 54 to 81 mm (the amount most likely to fall). The monthly minima range from zero to 19 mm and can happen in any month. The monthly maxima can also occur in any month, and have ranged from 216 to 596 mm. Other local sites, such as Gray, present similar patterns but all such character is restricted to a zone with a radius of a few kilometres of the pub in St Marys. The only differences are, that down in the valley, the rainfall totals are likely to be at least 300 mm per year less than on the adjacent ranges.
Where does the water go? Large rain surges lead to flood surges. The river shows this direct pattern. There is no significant seasonal flow or base flow. The catchment yields an average of 2.25 ML/ha with a total of 34% as baseflow feed at Killymoon. About 80% of all water falling in the catchment is used directly (evaporation and transpiration) with about 70% rate in winter and 90% rate in summer. Plantation trees (contrasted with native tree use) lift usage by about 1 ML/ha. Long term climate changes have reduced rainfall by about 8% since 1975 and led to a catchment loss of more than 16% in the same period. (due to changes in temperature and evaporation) The catchment is not stressed (in terms of changed usage) at the present time (2004 analysis).
Water within The top of the range is rocky and dry. Water either evaporates or infiltrates into the rock mass and slope debris. Stream development is not apparent until about mid slope. High level run off is not general although sheet flow may occur temporarily in the extreme rain events. Water migrates within the rock mass (dolerite at top, coal measures below) and may reach surface at many locations – dependent on cover thickness, hydraulic properties of slope materials and rocks, and structures within the rocks. Springs are common but not predictably found. The range thus stores a huge volume of water, but releases it quite slowly to obvious surface systems.
Water is stored in pores within talus but within fractures and bedding surface in the rock mass. Various pathways may control flow within the materials or determine outlets to surface. Disturbance of materials may affect such pathways and alter flow conditions.
Linkages Diagram showing water circulation in the Nicholas Range Land stability depends on water transfer and volumes and may involve surface materials or bedrock. Diagram stresses flow controlled by coal measures and basalt, and major fractures or failure surfaces. Cross-unit flow is minimal.
A bore at St Marys The hydrology of the range, its history, and the origin of the valley, are matters which combine to provide the present water supply to the town of St Marys. The town bores near the railway seem “normal” and are taken for granted. This supply is quite unusual. It depends, in equal amounts, on the three rock types drilled (Permian, basalt, Triassic) and two of these yield water here in abnormal volumes. The basalt is a rare extra and may be the supply link to the water stored in the range. The local fault-fracture system connects all the elements.
The “odd” range The geological construction of the range is unique in Tasmania. The basalt ensures this. The geomorphology of the range is unusual; isolated, elevated and coastal. The range stands in a peculiar climatic zone; which should be, and is, dry but which receives exceptional rainfalls and patterns. The combination of elements is unique in Tasmania and we should expect an interesting and varied ecology developed on it as a result.