The Wollumbin / Mount Warning shield volcano and its erosion caldera is a unique volcanic landform which has functioned as a refugium, over the 20 million years since it’s formation, for a core area of Gondwana rainforest flora.
The Caldera is located in a major climatic transition zone, between temperate and tropical floral and faunal species, giving high species diversity under a wide range of habitats, its volcanic origins and very high localised rainfall make this a very complex and interesting landform.
The Wollumbin Caldera is part of the Gondwana Rainforests of Australia World Heritage Area, and includes several parks and reserves protected under this UNESCO listing. These are:
- Wollumbin National Park
- Border Ranges National Park
- Nightcap National Park
- Mount Jerusalem National Park
- Mebbin National Park
- Mooball National Park
- Goonengerry National Park
- Limpinwood Nature Reserve
- Numinbah Nature Reserve
- Springbrook National Park
These areas are protected under World Heritage listing, for their rainforest ecosystems and geological significance.
20 million years ago the continent, bearing its Gondwanic rainforest flora, was moving north, away from Antarctica. The continent had been passing over one of the earths hotspots, and this had given rise to a chain of volcanic activity down the east coast of the continent, including the volcanic eruption and subsequent laval covering of the landscape which was the formation of the Mount Warning / Wollumbin Shield Volcano. The height and spread of the laval mound of the volcano is estimated at 2km high and a spread which for example extended 30km out onto the continental shelf beyond the current sea level.
Over millennia this landform acted as a focus for heavy rains and the consequent massive erosion carved out the unique erosion caldera observable today.
Exposed dramatically in the centre of this erosion Caldera is the dominant massif of Mount Warning, 1156 meters high, the ancient magma chamber, composed of erosion resistant plutonic rock which hardened underground.
The caldera, with it’s steep scarps rising 1150 m in altitude, forms an almost regular horseshoe about 15 km radius around Mount warning and acts as a giant scoop for moisture laden air along the open eastern seaboard. The products of the erosion of the caldera bowl are deposited on the rich flood plain of the Tweed River estuary.
Mount Warning itself acts as a weather triggering mechanism to the general atmospheric instability above the caldera, especially in spring and autumn. In the rainy season (late January to March) the whole Valley remains developed in a cloud for weeks at a time effectively and closing the humid damp conditions in a greenhouse effect. Along with the protective nature of the caldera rim, this undoubtedly has been the way the landfall has functioned as a refugium for the many rare and endemic Gondwanic rainforest species it has nurtured on its rich volcanic soils over the millennia.
The changes in land use over the last 150 years have resulted in the removal of 80% of the native vegetation, however the favourable climate and diverse soils have resulted in the survival of many remnant areas of the original vegetation types.
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Before the massive clearing, the western caldera floor consisted of dry eucalypt ridges grading into tall wet sclerophyll slopes. These merged into gully and gallion subtropical rainforest along the creeks and rivers.
Where the flood plain fanned out to the north and the east of Mount Warning, the lowland subtropical rainforest and paperbark, freshwater swamp forests were the main habitat zones, with many dense palm stands. Gigantic figs and numerous hoop pine were also features of this forest type. Rises in the terrain on this floodplain supported closed canopy subtropical rainforest as did the eastern caldera rim remnants, such as the Cudgen Hills, but with an admixture of wet sclerophyll elements.
Between the Cudgen Hills and the coastal dunes the vegetation was a mosaic of literal rainforest, with prolific hoop pines, freshwater lakes and brackish wetlands, interspersed with palm forests, melaleuca swamps and mangrove lined creeks draining to the ocean.
The central complex of Mount Warning and its ring dyke system basically supports similar habitats to the caldera rim, the only difference being in the parent rock in the central complex being plutonic. The lower slopes of the complex supports two types of subtropical rainforest, the Booyong Association and the palm forest type. Higher up on soils of lower fertility the more simple warm temperate rainforest occurs on the mid slopes. Wet sclerophyll forest also occur, with the elements grading into the rainforest and a limited area of dry sclerophyll occur on the northerly and westerly aspect. The summit itself is an area of Heath. This pattern is repeated on the Caldera rim scarps with the addition of cool temperate rainforest with limited areas of Antarctic beach around the rim heights.
Many species in the Caldera have limited range or are endemic to the landform.
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The original Aboriginal people of the caldera were estimated by Oxley on the first expedition by Europeans into the Tweed, in 1823, as being in “large numbers occupying substantial settlement structures.”
Over 185 plant species and gerera were available for indigenous people’s use in the caldera, more than 115 of which were, and are, to be found in the caldera rainforest. This resource base can only be described as being very rich. With over 115 species and 15 genera of food plants available over the range of habitats, the diet of these people would have been varied and nourishing. Staples available were Cunjevoi, Palm Cabbages, Bungwall, Beach Beans, Yams, Spinach Nettles, Water Lilies, Pandanus, and ai wide variety of fruits and nuts, mostly rainforest species, ensured balanced nutrition in the diet.
Medicinal plants, contraceptives, aphrodisiac and narcotics account for 53 species and 9 genera. Again a large percentage come from the rainforest. It could be expected that these would have been well utilised, especially the medicinals, given the climate in the caldera which has high humidity and rainfall, giving wet conditions for 8 of the 12 months most years.
32 species and eight genera could’ve been used in a wide range of artifacts, with fiber separately accounting for 27 species and 3 genera, and adhesives, cements and glues another 5 species.
Apendix 1 is a listing of caldera flora species and their known usages by the indigenous peoples.
Artifacts from this region exist in collections throughout the world, both museum and private. Notable is the rainforest shield, usually made from the light wood of the Batswing Coral Tree, Erythrina vespertilio, one of which can be seen in the Midjungbal Museum and Resource Centre’s collection and others are held by the Richmond River Historical Society (McBryde 1978:138).
The palm leaf water vessel, made from the leaf base of the bungalow palm, Archontopheonix cunninghamiana, is another distinctive local artifact with examples held by both the above. Many of these left Australia in the late 1800s and now reside in foreign collections. (Adrian Piper – pers. comm., McBryde 1978:138).
Fishing nets, hunting nets and dilly bags were also reported in the wide range of artifacts from this region, many of which were reported to have been of fine manufacture and examples of which are held by the above institutions.
Sullivan (1978:110-1) and McByrde (1978: 133-210) have collated the evidence for the range and scope of artifact assemblage known from this region, an assemblage which reflects the richness of the environment and diversity of its resources before circa 1850.
Appendix 1: A listing of caldera flora species and their known usages by the indigenous peoples (link).
Please note, the above description of the geological caldera and its flora is extracted from a paper by Jan Hunter, ‘The Erosion Caldera of the Mount Warning Shield Volcano’, a paper which focuses on the flora by which aboriginal people of the caldera sustained, reference library, CEC.
