Mt Rogers is a trig-pointed 704 m hilly ridge, covered mostly in bushland of native vegetation, above residences in the North Belconnen suburbs of Flynn, Fraser, Spence and Melba, in Australian Capital Territory.
Formed originally about 425 million years ago (see below), the hill we now call Mt Rogers is named after John Rogers. John Noble Core Rogers (c. 1898-1971) was Commonwealth surveyor general from 1949-1963. He served on the Canberra building review committee, was foundation president of the Canberra division of the institute of surveyors and instrumental in formation of the national mapping council in 1945.
Formed originally about 425 million years ago (see below), the hill we now call Mt Rogers is named after John Rogers. John Noble Core Rogers (c. 1898-1971) was Commonwealth surveyor general from 1949-1963. He served on the Canberra building review committee, was foundation president of the Canberra division of the institute of surveyors and instrumental in formation of the national mapping council in 1945.
The photo above is a view northwards from (not of) Mt Rogers.
You can identify Mt Rogers from nearby roads (such as the Barton Highway near Hall) because it’s quite high, with a mobile phone tower to which fibre-optic cables were laid, underground, in 2003. The main other large man-made objects among the trees and open spaces on Mt Rogers are 3 circular concrete storage reservoirs that were fenced-off in 1999. Tracks provide access to these and the summit. Powerlines supported by wooden poles traverse the northern edge of the area.
You can identify Mt Rogers from nearby roads (such as the Barton Highway near Hall) because it’s quite high, with a mobile phone tower to which fibre-optic cables were laid, underground, in 2003. The main other large man-made objects among the trees and open spaces on Mt Rogers are 3 circular concrete storage reservoirs that were fenced-off in 1999. Tracks provide access to these and the summit. Powerlines supported by wooden poles traverse the northern edge of the area.
Two short, 200-300 m vegetated gullies to the NE and SW respectively possibly drain water from the ridge after significant rain. Gradients to the ridge from the gravel path circling the hill are moderately steep in places and erosion on these people-formed paths is noticeable. The general area perhaps measures 800 m by 800 m, and offers spectacular views to the Brindabella Range, across to and beyond Hall, over Belconnen, and in the distance to Black Mountain with its famous tower.
There are several significant rocky outcrops around Mt Rogers. The dacite rocktype is evidence of past volcanic activity (and see the geological overview below). Smaller boulders are common and the soil is often rocky and probably thin. Kangaroos occasionally access Mt Rogers via an easement to the Kuringa Drive vegetation-belt, narrower access points through the suburbs and from Ginninderra Creek's corridor. Sometimes an echidna may be seen. Foxes are not unusual.
Westerly or NW winds predominate from the Brindabellas. Easterly or SE winds from the Lake Ginninderra valley can feel cold in winter especially if snow has fallen in the Southern ACT region. Winds can occasionally be strong or tortuous and cause damage to trees. Calmer, damp winter nights may create misty mornings.
Perhaps 50 significant 200-300 year old Eucalypts remain to indicate the area's former woodland status. A few metre-in-diameter stumps show even older trees were felled. Older trees contain hollows vital for numerous bird species and possums. Other trees may have been cleared to improve the sheep-grazing value of the land. The now-grassed areas may have been natural temperate grassland degraded by stock. The presence of some phalaris and widespread wild oats suggests “pasture improvement” during the farming days and the loss of the rich diversity of natural temperate grassy woodland and woodland in all but a few places.
Acacia (wattle) species such as A. dealbata, A. mearnsii and A. implexa occur naturally. In the early seventies, as the infrastructure for the suburbs was completed, hundreds of native trees and shrubs were planted. Hakeas, Banksias, Callistemon and Melaleuca remain from this restoration era even though they are Australian species not endemic to the area. Acacia baileyana (Cootamundra wattle) was planted in large numbers. The originals are often decrepit by now but still provide excellent bird habitat and regenerate through their seedlings. Acacia pravissima and A. vestita were also planted and are significant in that Superb Parrots feed from their seed pods during their annual migration into the Fraser area (a highlight for ornithologists and residents alike).
Walkers, cyclists, joggers and dog-walkers usually have little impact on the resident and annual-migrant birds who use the range of trees, shrubs and under-storey plants for feeding, shelter and breeding habitat. The birds also visit gardens seeking insects and seeds. Over the past five years 80 species of birds have been seen in or above the Mt Rogers area.
The ACT's Urban Services' Canberra Urban Parks & Places has a program of maintenance for places like Mt Rogers. As examples of their input, the area is regularly mown, the gravel path has been restored, seats have been installed, playgrounds have been modernized, and noxious weeds such as Serrated Tussock, African Lovegrass and Briar Rose have been sprayed. Since the fires of January 2003 most of the trees and shrubs occurring between the gravel path and residences have been cleared as fire-hazard material. Groups of small birds have had their habitat reduced significantly as a result.
There are several areas where grassland and woody grassland species show minimal disturbance and are significant reminders of the beauty & diversity of these nationally endangered ecosystems. Removal and constant monitoring of invasive weeds is a priority near these remnants.
Dedicated people have, over many years, shown their appreciation of Mt Rogers by making consistent, voluntary contributions to its maintenance. They have regularly removed rubbish, reported problem behaviour or activities, weeded-as-they-walked by removing Paterson's Curse and St John's Wort, cut and sprayed woody weeds, observed and recorded plants and animals, liaised with authorities and tried revegetation. Most importantly, many informal networks have been formed where greetings, observations and enthusiasms for 'our outdoors' are shared whilst people unwind and exercise.
Threats to Mt Rogers include:
- arson in the summer months when the wild oats & grasses are attractively dry
- abandoning or trashing of cars in the carpark
- rubbish dumped or left in carpark
- the entry of cars in spite of deterrent fencing and rock placement
- the entry of trailbikes motorbikes ridden on & off tracks
- 'drug-parties' &/or ocasional 'camp fires' at the summit for which timber is taken
- removal of timber for firewood
- occasional dumping of mowings, garden rubbish and prunings
- rocks overturned, moved & not replaced when in search of reptiles
- weed invasion when seeds are brought into the area from gardens or spread by mowers
Mount Rogers, the (very) early days
By Aidan Moore
By Aidan Moore
Back in the Cambrian and the Early Ordovician epochs, close to five hundred million years ago, the landscape of the ACT and Eastern Australia didn't exist. The continental landmass of Gondwana, that included Australia, was bounded by marginal sea-basins (back-arcs), with volcanic island arcs on the outward, oceanic side, resembling today’s island arcs in the Caribbean, Japan, The Philippines, New Guinea and the Solomon Islands, Tonga to New Zealand, and the Indonesian island arc in the Indian Ocean. Outside them the planet's crust underneath the great world-ocean boiled slowly, like porridge in a pot, moving an inch or two every year toward the continent, pushing against the island-arc rampart and eventually plunging underneath it. The hard rocky crust of the islands scraped the top layer of oceanic ooze off the moving, plunging ocean crust, much as a bulldozer blade scrapes the soil off the rocky subsurface. The ooze contained the siliceous skeletons of oceanic radiolarians - no calcite skeleton could survive at these abyssal depths under several kilometres of ocean water.
Eventually the pile of ooze - the Pittman Formation and the State Circle Shale - built up and was pushed on to the lower margins of the island arc - called the MacQuarie Arc - in gentle folds. This deepwater terrain was the fore-arc basin. As the Ordovician ended and the Silurian epoch began, around four hundred and thirty four million years ago, the shale was covered by sandy sediment - the Black Mountain Sandstone - brought to the coasts of the MacQuarie Arc by rivers and cascading down the ramp of the offshore slope to fill the muddy trough at the bottom. Thus was laid the foundations of eastern Australia and New Zealand. The pile of sediments of the fore-arc basin, eventually extending hundreds of kilometres to the east of the ACT and getting ever thicker, was pushed up the slope, and the coral reefs of the Canberra Formation began to grow in the shallowing water around the island arc. The area that would become the ACT, pushed along like a surfer on a wave, overlaid the plunging oceanic plate which, hundreds of kilometres below, had melted and was sending plumes of volcanic magma up toward the surface as the planet laboured to shed the heat generated by the Core.
A dog loses the heat generated in its active muscular body by panting. The Earth does the same, but her exhalations each last a million or more years. You can see the process most clearly today on the island of Tanna, or at Rotorua. A dog goes through cycles of panting and silence. The Earth’s equivalent is called an orogenic cycle and lasts about 50 million years before falling quiet for about 10 million, then beginning again as the heat builds up inside. The episode described above is named the Benambran Orogenic Cycle.
Eventually, around 430 million years ago, the subduction zone at the ocean/island-arc plate boundary became clogged with sediments and volcanics and jammed by irregularities and entire island chains forced up against the plate boundary. The island barrier did retreat before this horizontal pressure, but slowly and reluctantly, compressing the back-arc basin behind, which was now filled up with sediments carried down by rivers from both the continental landmass itself and the mountains pushed up along the island arc. The Canberra area had risen above sea level. The Pacific Ocean plate, stalled, blocked and unable to move, retreated oceanward to the eastern edge of the mountainous mass of sediments and volcanics and began to descend there, far to the east of the ACT. The crust above the now dislocated plunging plate under the ACT, relieved of the compressive pressure, relaxed and stretched. The stretched crust fractured and the Canberra area sank down into a rift, a valley bounded by faults. The abandoned oceanic plate below it continued its plunge toward the Core, melting and sending plumes of molten volcanic magma and bosses and mushroom-shaped batholiths of molten granite upwards, seeking out every crack and cranny in the crustal rocks, infiltrating them with tendrils of superheated hydrothermal fluid, hydraulically fracturing them and finally forcing them aside to make way for the ascending volcanic lava and the granite. The sinking Canberra Rift with its marginal extensional faults provided a ready access for the ascending volcanic plumes.
The event from 428 to about 370 million years BP is called the Tabberabberan Orogenic Cycle.
At the beginning of this cycle, for about four million years from 428 to 424 Ma BP, the Canberra area was subjected to recurrent eruptions of the Ainslie Volcanics, the Walker Volcanics, the Mount Painter Volcanics, and it is this Middle Silurian suite of rocks, collectively called the Hawkins Volcanic Suite (Smh on geological maps, or just Sv) that constitutes Mount Rogers and much of the ACT. The Capital Territory was part of the ‘ring of fire’ around the Pacific rim, which is still going strong, now more than three thousand kilometres to the east and north-east of us, extending in our vicinity from New Guinea, through the Solomon Islands to Vanuatu and from Tonga to New Zealand.
While the volcanics broke out on to the surface, above sea level, sometimes in great explosive eruptions, the granitic magma rising from the plutonic depths lodged and solidified some five or more kilometers below it. Like a giant mushroom it pushed up the country rocks above it, and this stretched and broken cap-stone was vulnerable to erosion. There was little or no vegetation on the land in this early period of biological history, so the process of erosion and removal by rain and rivers proceeded faster than it would today. Kilometres of surface rock was quickly removed, and the now cold granite was eventually exposed on the surface of the land. One of the biggest granite masses became the Murrumbidgee Batholith whose frost-broken top we admire in the Namadgi National Park, with names like Legoland, The Spinnaker and Gorilla Rock. The resulting complex of plutonic, volcanic and sedimentary rocks is the eastern part of the Lachlan Fold Belt, one of a number of structural units that make up Eastern Australia.
If you study a piece of the rock found today on and around Mount Rogers - any little bit will do, and it will show better if you wet it - you will see it is a greenish-grey to purplish colour, with small light-coloured fragments of irregular shape scattered through it. It resembles the deposits found around explosive volcanoes of the type that destroyed Pompeii and Mount Saint Helens and that periodically causes havoc on Caribbean and other island chains. This dacite, rhyolite, andesite and ‘burning cloud’ ignimbrite is characteristic of volcanics that have forced their way through the upper crust and sediment. It is quite unlike the black basaltic lava that flows comparatively quietly out of volcanoes on oceanic islands like Hawaii or Iceland. That is dark, basic lava straight from the Earth's mantle and it flows like melted glass; this is lighter-coloured, acidic, viscous and explosive. It comes to the surface full of gases that have not been able to escape through the thick, viscous lava, and that now do so with violent results.
This marked acidic tendency must select and influence the plants that grow on the mountain. The first land plants began at this time, in the Silurian Period, including a famous one that towered above its understorey to a height of at least an inch, 25 to 30 millimeters, a veritable Silurian cedar. It is known around the world under the name of Cooksonia – nothing to do with the famous sea-captain, its discoverer was Elizabeth Cookson, an Australian paleobotanist. It is briefly described (with life-sized sketch) in the Geological Guide to Canberra (Finlayson et al, the principal source for this article, see Refs below) and the revolution these plants triggered in the Earth’s ecology is canvassed on page 28. The next time you see a lichen-covered rock on the mountain, have a closer look. The variety and vitality of lichens may surprise you. Speculative literature sometimes canvasses the possibility of ‘terra-forming’ Mars, making it more like Earth. On the lichenous rocks of Mount Rogers you are looking at the descendants of the pioneers that terraformed the Earth.
We do not know what happened on Mount Rogers in the ages that followed the Silurian, because the area remained above sea level until today, and the solid evidence of history was removed by erosion. Some 20 million years later and 40 km to the north-west of Mount Rogers a shallow sea laid down exceptionally fine-grained limestone that entombed and replaced the bodies of fish with the finest details preserved. These miraculously preserved specimens from the Early Devonian (Emsian) Taemas Limestone of Wee Jasper and on Lake Burrinjuck are world-famous. They reveal the detail of development of early eyes, ears, electromagnetic senses and the intimate details of their brain-case and nervous systems. They have been described by Gavin Young of the Research School of Earth Sciences at the Australian National University. These four-hundred million year-old fish, the placoderms, are the ancestors of four-legged land creatures. The armour in which their bodies, and even their eyes, were encased, shows what a competitive and dangerous environment they lived in – giving them a sustained and strong incentive to leave the sea and walk upon the land. One of their predatory competitors, now in the ANU’s fossil collection and not normally accessible to the public, is formidable enough to endanger any man-size creature that came within range. Because Australia has no national home for these precious fossils, the very best of them are now lodged and jealously guarded in the world’s leading natural history museums. Australian scientists must go to places like London and ask permission to study them there – they are just too valuable to be lent to Australia. Darwin’s famous question ‘where are the early, transitional models of the modern eye’ is or will be answered in part by these priceless indicators of our continent’s (and our planet’s) biological history.
In the east the plate margin again and again stepped back to one side or the other of the compressed mass blocking the subduction zone at the ocean/island arc boundary. The diagram on p17 of Finlayson and co-authors (reference below) shows this happening during one short period of this complex history. After one of these plate reorganisations the Tasman Sea began opening up. New Zealand drifted away to the east. The eastern Australian coastal rim moved slowly upward, forming the elevated scarp and plateau that we cross to get to the coast. Rivers responded by cutting downward to form gorges in numerous locations, and they deposited the waste - gravels and alluvium - on their flood-plains.
I have skated over the long and intricate history of the intervening ages, when Australia, attached to Gondwana, drifted into the Antarctic Circle and the glaciers advanced. They retreated again when our continent left Antarctica behind and began its solo journey north. Giant rivers the size of the Ganges carried billions of tons of clean washed river sand down into the Sydney Basin to form the Hawkesbury Sandstone and create its spectacular landscapes. Dinosaurs roamed a lush well-wooded land watered by abundant rain.
Mount Rogers was undoubtedly part of the landscape, gradually wearing down to the charming little hill it now is. Its soils, for too long exposed to weathering, oxygenation, and leaching by rain, are not particularly fertile. Our flora have responded to these deprived conditions with a variety of ingenious strategies, extracting the maximum nutrition from a sparse and miserly larder. They deserve our admiration - and our solicitous love and care. They are unique, and if our climate changes alarmingly fast and warms as predicted, we may come to depend on this clever flora to keep our land from becoming a desert like most of the long-cultivated and much abused Middle East.
References and further reading
A Geological Guide to Canberra Region and Namadgi National Park,
Finlayson D.M. (author and compiler) and co-authors.
Published by the Geological Society of Australia (ACT Division) Canberra 2008.
(140 pages with CD of geological excursions and places of interest. Bibliography and useful websites) Can be bought at the shop in the Botanical Gardens and the National Library. ISBN 978 0 646 48734-2
Finlayson D.M. (author and compiler) and co-authors.
Published by the Geological Society of Australia (ACT Division) Canberra 2008.
(140 pages with CD of geological excursions and places of interest. Bibliography and useful websites) Can be bought at the shop in the Botanical Gardens and the National Library. ISBN 978 0 646 48734-2
Geology of Canberra, Queanbeyan and environs, notes to accompany the 1980 1:50,000 geological map,
G.A.M Henderson (compiler)
PUblished by Australian Government Publishing Service Canberra 1981.
Available from the Sales Division at Geoscience Australia in Symonston.
An Internet search for ‘Finlayson DM Seismic Imaging’ will yield a plethora of research papers by this and other authors on the complex plate-tectonic history and structure of Eastern Australia and the South-West Pacific margin. A good ocean-floor map of the same area shows its intriguing structure, centering today on Fiji.
Aidan Moore
Melba, ACT
February 2012
Melba, ACT
February 2012
