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Wellington Park Geodiversity
Protecting geodiversity - the range of geological, landforms and soil features, assemblages, systems and processes
"The geology and geodiversity of the Park provides the physical foundation for the landscape, ecosystems and character of the Wellington Range – the sheer dolerite columns of the Organ Pipes, hidden caverns of Lost World, familiar features of Collins Cap and Collins Bonnet, the band of sandstone beneath the Wellington Range and mudstone waterfalls in the foothills." (Wellington Park Trust, 2020)
Geodiversity underpins many of the ecosystem services including supporting, provisioning, regulating and cultural services. The benefits can be direct - such as the provision of water and minerals - or indirect, such as the contributions to food and fuel derived from the productivity of soils. Geodiversity also contributes to the resilience and adaptive capacity of biodiversity by providing a range of habitats which determines the spatial distribution of species. To ensure a functioning ecosystem that continues to provide these services, geodiversity in the Wellington Park needs to be managed effectively. Successful management of geodiversity is dependent on the capacity to understand, assess, classify and monitor geoconservation sites (Williams, 2019).
Evolution of the Wellington Park
"The foothills feature sedimentary sandstones and mudstones laid down during the Permian Period (230-280 million years ago). These are now visible in the foothills of the Mountain around Fern Tree, South Hobart and Lenah Valley. Creamy white to grey in colour these mudstones and sandstones are visible in horizontal or gently dipping layers, sometimes up to a metre thick." (Wellington Park Trust, 2020) At the merton fossil site brachiopods, bryozoans and molluscs can be seen which have been deposited in a previously marine environment.
"Sandstones rich in quartz formed during the Triassic (180-230 million years ago) and covered the mudstones. Today these can be seen above approximately 600m in the cliffs and promontories around the Springs (including Rocky Whelans Cave), Sphinx Rock, Crocodile Rock and Snake Plains. On the western slopes of the Glen Dhu Rivulet valley the Yellow Cliffs are 50m high and extend for 1km, making it the highest and longest sandstone cliffs in the State." (Wellington Park Trust, 2020). Drones have been used to survey the Yellow Cliffs to understand the extent and provide a way of monitoring change.
"A thick sheet of dolerite intruded during the Jurassic Period 170 million years ago. Shrinkage cracks developed while the molten magma cooled, forming large vertical columns with polygonal crosssections. Block faulting is in part responsible for the main landform trends of eastern and central Tasmania. Mount Wellington and the Derwent Valley were formed some 150 million years ago by block faulting. This has influenced the erosional development of the size and form of the Wellington Range. Individual faults have vertical displacements up to 600m." (Wellington Park Trust, 2020). Collins Bonnet is an example of a large dolerite dyke that has intruded a dolerite sheet, now exposed as prominent ridge and extensive lineament, extending to Snug Tiers.
"Erosion has since exposed the 350m thick horizontal sheet of dolerite, featuring the Organ Pipes on the eastern face of the Mountain. This area has been eroded back approximately two kilometres since the escarpment was produced 10-15 million years ago. Some of the faults are considered significantly older than this." (Wellington Park Trust, 2020)
The 3D Model of the Organ Pipes shows the physical expression of Mount Wellington and provides a strong sense of place for Hobart. Click the play symbol to interact with the model.
"When exposed to the elements dolerite is the colour of rust and when unweathered it is blue-grey (bottom). It is a hard, compact rock which is significantly more resistant to surface erosion than the clay-rich and sandy Parmeener Supergroup sedimentary rocks into which the dolerite magma intruded. Large dolerite sheets are globally unusual, though reasonably common in Tasmania and are also found in the Central Plateau, the sea cliffs on the southern tip of the Tasman Peninsula and Bruny Island."(Wellington Park Trust, 2020) Despite the dominance of dolerite on Mount Wellington, there is a small Tertiary volcanic vent close to the summit of Mount Wellington containing volcanic basalt (top).
"It is estimated that more than six glacial stages have occurred in Tasmania over the last few million years. The Wellington Range has not been subject to extended periods of ice accumulation however a small amount gathered during the last Ice Age in the Pleistocene (10,000-250,000 years ago). Glaciers were not produced however dolerite forms were considerably eroded by water freezing and expanding within the columnar cracks. This resulted in the dolerite talus evident on the higher slopes. Where the soil and detritus has been washed from the crevices, block fields or Ploughed Fields have been produced."(Wellington Park Trust, 2020). The well known Disappearing Tarn is part of the blockfield called the Potato Fields and can be visualised with this 3D model .
"The cliff-base talus at the Lost World, on the eastern slopes of Mount Arthur is also significant. Beneath the 30m dolerite columns is a maze of underground passages (‘boulder caves) extending up to 360m long and 42m deep. These are believed to be the most extensive noncarbonate terrestrial caves in Tasmania." (Wellington Park Trust, 2020). Formed by the toppling of columns which is the action of freeze-thaw cycle or periglacial weathering and erosion.
"While the bedrock of Wellington Park is not unique or unusual in Tasmania, it provides the character for which the visual landscape is valued. It also provides the foundation for local soils, an important component of the living ecosystems that operate in the Park. Dolerite, sandstone and mudstone soils occur throughout the area in connection with the local bedrock." (Wellington Park Trust, 2020). The blockstream pictured here is a typical geomorphological feature of the park and is part of a debris cascade system restricted by the high elevation on the flanks of Mount Arthur and is dependent on the slope of the terrain.
"Alpine peat soils, best developed around Dead Island, are highly significant being the most southeastern alpine peats in the State. As these soils can be dated, they may provide new information on Holocene climates, rates of soil development in alpine areas, and post-glacial climatic change. As the substrate for a string bog biological community, they are also significant. Landslips and rock slides are evident throughout the Park. Soil movement affects water catchment health and drinking water quality. Erosion is managed where necessary to ensure impacts are minimised." (Wellington Park Trust, 2020)
The importance of Mount Wellington is recognised by the Tasmanian government (Dombrovskis, et. al, 1996), however significant geosites are under threat from development activities and exacerbated by a lack of understanding about how these sites evolved and why they are special. The Wellington Park is an incredible natural laboratory and provides Hobart with a strong sense of place and it is time for both the geodiversity and biodiversity of the Park to treasured and nurtured for the future.
Illa Brook Sandstone Gorge geosite
Behind the Scenery – Tasmania’s landforms and geology, 1990. Dept of Education and the Arts.
Dombrovskis, P., Flanagan, R, & Kirkpatrick, J. B. 1996. On the mountain. Hobart : West Wind Press.
Leaman, D.E, and Corbett, K. D., 2004. Mount Wellington Walk Map and Notes, Geology
Wellington Park Trust, 2020. Geodiversity Overview of Wellington Park.
Wellington Park Trust, 1996. Draft Wellington Park, Values, Use and Management Inventory.
Williams, M., 2019. Geoheritage in the Hybrid Age: Geographic Information Technology (GIT) tools used to support Geoconservation and Geotourism Initiatives (Unpublished master's thesis), University of Tasmania, Hobart, Australia.