The Making of the Unique Environment of Robertson
The following information was compiled for the Robertson Community Information Centre with the assistance of members of the Robertson Environment Protection Society.
Robertson’s fascinating geological diversity has contributed directly to the landscape and vegetation of the area. Today’s famous rich red-brown soils were derived from a basalt lava flow poured out over previously eroded shale and sandstone rocks deposited over 200 million years ago. This, along with the generally high rainfall, has supported the growth of luxurious vegetation, including the Yarrawa Brush.
Geology: it’s all about the rocks
The story of the environment the people of Robertson enjoy today begins back in the mists of geological time. To gain a little understanding of this helps one appreciate why the region looks and feels the way it does. The Robertson district lies within the major geological structure known as the Sydney Basin – a thick sequence of sedimentary rocks composed of shale, mudrock, coal and sandstone – these rocks are capped by a basalt lava flow.
Taking a quick trip back through geological time…
The early sediments were deposited during the Permian period, which began around 298 million years ‘before present’ (BP). They were initially marine sediments deposited onto folded older ‘basement’ rocks belonging to the Lachlan Fold Belt, of Palaeozoic age (comprising the Ordovician, Silurian and Devonian periods). Overlying the Permian sediments are sedimentary rocks of Triassic age comprising the Narrabeen Group, the Hawkesbury Sandstone (about 250 million years old) and the Wianamatta Group – the latter two are commonly exposed around Robertson. The Robertson Basalt was poured out as a volcanic lava flow during the Tertiary Period (between 65 million and 5 million years BP) covering the eroded land surface underlain mainly by shales, mudrocks and sandstones of the Wianamatta Group. Over the years, erosion of this basalt rock has revealed the underlying Triassic sedimentary rocks in the present landscape. A major seismic event was the Robertson earthquake. This had a Richter magnitude of 5.5 and caused significant structural damage to a number of buildings in the area. It occurred at about 7.40am on Monday, 22 May 1961 and lasted for some 30 seconds. The epicentre was believed to be ‘off the coast’ and tremors were experienced as far away as Newcastle, the Snowy Mountains, Dubbo and Narrandera.
Weather
The Robertson area is well known for being cooler than its surrounding areas, for its high rainfall and sometimes foggy conditions.
The average yearly Rainfall is approximately 1670 mm with the highest rainfall generally in autumn and the least in spring, but this can vary. This is one of the highest rainfall areas in the district and is known for large scale rainfall events every few years. A good example of this was back in June 1991 when a total of 1115 mm was recorded over 6 consecutive days! (7/6/91 – 12/6/91 BoM). The altitude and proximity to the coast helps to create Upslope Fog which is ‘stable air being adiabatically cooled to its saturation point as it is gradually orographically uplifted’. The uplifting incoming moist fog from the coast brings great relief after a hot day. Another type of fog in the area is Radiation Fog, defined as ‘stagnant moist air near the ground becoming progressively cooler during a cloudless night because of an excess of outgoing radiation’. These moist fogs and low clouds add extra precipitation to the area, not usually recorded in the rain gauge. Many plants intercept the fog condensing and directing the moisture. This is known as Fog Drip. Frosts are part of this environment as well as snow, occasionally, if you’re lucky.
Winds
Being situated near the coast, the sea breeze can find its way to Robertson creating a cooling effect, where it can be hotter further west. Westerly winds predominate especially in winter and can be strong.
The interaction of weather systems (coastal and inland) can create spectacular ‘skyscapes’ overhead.
Flora
The Robertson Village area was once part of continuous rainforest, thought to have been an estimated 2,450 hectares of the Robertson plateau. This unique rainforest is known as the Yarrawa Brush and has elements of cool temperate, warm temperate and subtropical species. Described by early surveyors and settlers as ‘virtually impenetrable’, it grew particularly well on the basalt soils, as well as on soils derived from shale and sandstone in surrounding areas. It was very difficult for the early settlers to clear this vegetation but, once achieved, the rich earth proved perfect for agriculture, cattle-raising and market- gardening. Officially known as Robertson Rainforest, it is now a protected plant community classified as endangered. Only isolated patches of this rainforest type remain today and can be seen scattered across the local hills and valleys. A good example of the Yarrawa Brush can be seen at the nearby Robertson Nature Reserve (off South Street).
Why do rainforests grow at Robertson?
Rainforest grows in this region because many ideal conditions exist in the area, including:
Some canopy trees here include, Sassafras (Doryphora sassafras), Possumwood (Quintinia sieberi), Coachwood (Ceratopetalum apetalum), Brown Beech (Pennantia cunninghamii) , Blackwood (Acacia melanoxylon) and Lilli Pilli (Acmenia smithii).
Two important trees of the Yarrawa Brush not seen in the nature reserve are the Pinkwood (Eucryphia moorei) and the Brown Barrel (Eucalyptus fastigata).
The Pinkwood tree is near its northern limit here. It usually grows along creek lines with some large old specimens around. They can put on a great flowering display, with large ‘plum tree like’ white flowers usually around February-March. Some planted Pinkwoods can be seen around Robertson especially when they are in flower.
The Brown Barrel is a tall gum tree, and is at home growing on Robertson’s red soil. Large old examples may still be seen, for xample, at the Robertson Showground.
Another rare forest type exists here, more on the outskirts of the red soil area. It is known as Robertson Basalt Tall Open Forest which is also luxuriant in its growth, involving further fast growing eucalypts. Sadly, this forest type is listed as Critically Endangered.
There are key natural attractions close by to Robertson – large National Parks and many beautiful waterfalls. These areas have their own distinctive features and are well worth exploring. Barren Grounds, to the south-east, has a heath environment and is known for its spring wild flowers and some rare fauna.
Fauna
One of the first things a visitor notices when visiting the Southern Highlands is the large number of native birds and the cacophony of different calls and sounds they make; familiar species such as parrots, cockatoos, magpies and kookaburras, but many more as well…
Sitting quietly in the rainforest you may notice understorey birds like the White-browed Scrubwrens and Thornbills, or an Eastern Yellow Robin.
The Whipbird – the male has a remarkable call, starting as a whistle sound and ending in an explosive ‘whip crack’ noise – the female replies with a few simple notes.
The Satin Bowerbird – another interesting resident. The male is a glossy blueish black with purple-blue eyes. He puts on a show at his bower, which is decorated with small blue objects and some yellow, to impress the females.
The Wonga Pigeon is a local, that mainly browses the forest floor and is known for its beautiful plumage and haunting repeated call.
Up in the canopy may be Crimson Rosellas, or a pair of King Parrots.
Overhead the ‘primitive’ sounding calls of the Yellow-tailed Black Cockatoo can remind one of the ancient ‘Gondwana’ connection this rainforest area has.
Most birds of the area are also attracted to many gardens around town.
Wombats are common here at night, especially on the roadsides, so please be careful when driving at this time. Echidnas are occasionally seen wandering around, even in town.
The Swamp Wallaby is our local rainforest roo, usually shy and solitary you might just here it bounding away. Antechinus are common in the rainforest and sometimes take up brief residence in houses. These are small rodent sized marsupials.
Possums abound, including the cute Ringtail Possum. Gliders range from the smallest, the Feathertail Glider, to the largest, the Greater Glider (more at home in the nearby Tall Open Forest).
Another big thing the Robertson area is known for is its big earthworm (Notoscolex grandis). These earthworms can go deep into the basalt soils and are usually only seen on the surface after a big rainfall event (see ‘Weather’). They can grow to a metre long!
The diverse range of fauna found here is enhanced by bordering large tracts of forest to the east, south and north, and Wingecarribee Swamp to the west, making it part of an excellent wildlife corridor.
You never know what you might see next.
The following information was compiled for the Robertson Community Information Centre with the assistance of members of the Robertson Environment Protection Society.
Robertson’s fascinating geological diversity has contributed directly to the landscape and vegetation of the area. Today’s famous rich red-brown soils were derived from a basalt lava flow poured out over previously eroded shale and sandstone rocks deposited over 200 million years ago. This, along with the generally high rainfall, has supported the growth of luxurious vegetation, including the Yarrawa Brush.
Geology: it’s all about the rocks
The story of the environment the people of Robertson enjoy today begins back in the mists of geological time. To gain a little understanding of this helps one appreciate why the region looks and feels the way it does. The Robertson district lies within the major geological structure known as the Sydney Basin – a thick sequence of sedimentary rocks composed of shale, mudrock, coal and sandstone – these rocks are capped by a basalt lava flow.
Taking a quick trip back through geological time…
The early sediments were deposited during the Permian period, which began around 298 million years ‘before present’ (BP). They were initially marine sediments deposited onto folded older ‘basement’ rocks belonging to the Lachlan Fold Belt, of Palaeozoic age (comprising the Ordovician, Silurian and Devonian periods). Overlying the Permian sediments are sedimentary rocks of Triassic age comprising the Narrabeen Group, the Hawkesbury Sandstone (about 250 million years old) and the Wianamatta Group – the latter two are commonly exposed around Robertson. The Robertson Basalt was poured out as a volcanic lava flow during the Tertiary Period (between 65 million and 5 million years BP) covering the eroded land surface underlain mainly by shales, mudrocks and sandstones of the Wianamatta Group. Over the years, erosion of this basalt rock has revealed the underlying Triassic sedimentary rocks in the present landscape. A major seismic event was the Robertson earthquake. This had a Richter magnitude of 5.5 and caused significant structural damage to a number of buildings in the area. It occurred at about 7.40am on Monday, 22 May 1961 and lasted for some 30 seconds. The epicentre was believed to be ‘off the coast’ and tremors were experienced as far away as Newcastle, the Snowy Mountains, Dubbo and Narrandera.
Weather
The Robertson area is well known for being cooler than its surrounding areas, for its high rainfall and sometimes foggy conditions.
The average yearly Rainfall is approximately 1670 mm with the highest rainfall generally in autumn and the least in spring, but this can vary. This is one of the highest rainfall areas in the district and is known for large scale rainfall events every few years. A good example of this was back in June 1991 when a total of 1115 mm was recorded over 6 consecutive days! (7/6/91 – 12/6/91 BoM). The altitude and proximity to the coast helps to create Upslope Fog which is ‘stable air being adiabatically cooled to its saturation point as it is gradually orographically uplifted’. The uplifting incoming moist fog from the coast brings great relief after a hot day. Another type of fog in the area is Radiation Fog, defined as ‘stagnant moist air near the ground becoming progressively cooler during a cloudless night because of an excess of outgoing radiation’. These moist fogs and low clouds add extra precipitation to the area, not usually recorded in the rain gauge. Many plants intercept the fog condensing and directing the moisture. This is known as Fog Drip. Frosts are part of this environment as well as snow, occasionally, if you’re lucky.
Winds
Being situated near the coast, the sea breeze can find its way to Robertson creating a cooling effect, where it can be hotter further west. Westerly winds predominate especially in winter and can be strong.
The interaction of weather systems (coastal and inland) can create spectacular ‘skyscapes’ overhead.
Flora
The Robertson Village area was once part of continuous rainforest, thought to have been an estimated 2,450 hectares of the Robertson plateau. This unique rainforest is known as the Yarrawa Brush and has elements of cool temperate, warm temperate and subtropical species. Described by early surveyors and settlers as ‘virtually impenetrable’, it grew particularly well on the basalt soils, as well as on soils derived from shale and sandstone in surrounding areas. It was very difficult for the early settlers to clear this vegetation but, once achieved, the rich earth proved perfect for agriculture, cattle-raising and market- gardening. Officially known as Robertson Rainforest, it is now a protected plant community classified as endangered. Only isolated patches of this rainforest type remain today and can be seen scattered across the local hills and valleys. A good example of the Yarrawa Brush can be seen at the nearby Robertson Nature Reserve (off South Street).
Why do rainforests grow at Robertson?
Rainforest grows in this region because many ideal conditions exist in the area, including:
- The volcanic soils contain more nutrients than the surrounding shales and sandstone
- The soil is structurally ideal for the growth of trees
- Rainfall is higher and more reliable than in most parts of New South Wales
Fogs are frequent - Bushfires rarely occur
Some canopy trees here include, Sassafras (Doryphora sassafras), Possumwood (Quintinia sieberi), Coachwood (Ceratopetalum apetalum), Brown Beech (Pennantia cunninghamii) , Blackwood (Acacia melanoxylon) and Lilli Pilli (Acmenia smithii).
Two important trees of the Yarrawa Brush not seen in the nature reserve are the Pinkwood (Eucryphia moorei) and the Brown Barrel (Eucalyptus fastigata).
The Pinkwood tree is near its northern limit here. It usually grows along creek lines with some large old specimens around. They can put on a great flowering display, with large ‘plum tree like’ white flowers usually around February-March. Some planted Pinkwoods can be seen around Robertson especially when they are in flower.
The Brown Barrel is a tall gum tree, and is at home growing on Robertson’s red soil. Large old examples may still be seen, for xample, at the Robertson Showground.
Another rare forest type exists here, more on the outskirts of the red soil area. It is known as Robertson Basalt Tall Open Forest which is also luxuriant in its growth, involving further fast growing eucalypts. Sadly, this forest type is listed as Critically Endangered.
There are key natural attractions close by to Robertson – large National Parks and many beautiful waterfalls. These areas have their own distinctive features and are well worth exploring. Barren Grounds, to the south-east, has a heath environment and is known for its spring wild flowers and some rare fauna.
Fauna
One of the first things a visitor notices when visiting the Southern Highlands is the large number of native birds and the cacophony of different calls and sounds they make; familiar species such as parrots, cockatoos, magpies and kookaburras, but many more as well…
Sitting quietly in the rainforest you may notice understorey birds like the White-browed Scrubwrens and Thornbills, or an Eastern Yellow Robin.
The Whipbird – the male has a remarkable call, starting as a whistle sound and ending in an explosive ‘whip crack’ noise – the female replies with a few simple notes.
The Satin Bowerbird – another interesting resident. The male is a glossy blueish black with purple-blue eyes. He puts on a show at his bower, which is decorated with small blue objects and some yellow, to impress the females.
The Wonga Pigeon is a local, that mainly browses the forest floor and is known for its beautiful plumage and haunting repeated call.
Up in the canopy may be Crimson Rosellas, or a pair of King Parrots.
Overhead the ‘primitive’ sounding calls of the Yellow-tailed Black Cockatoo can remind one of the ancient ‘Gondwana’ connection this rainforest area has.
Most birds of the area are also attracted to many gardens around town.
Wombats are common here at night, especially on the roadsides, so please be careful when driving at this time. Echidnas are occasionally seen wandering around, even in town.
The Swamp Wallaby is our local rainforest roo, usually shy and solitary you might just here it bounding away. Antechinus are common in the rainforest and sometimes take up brief residence in houses. These are small rodent sized marsupials.
Possums abound, including the cute Ringtail Possum. Gliders range from the smallest, the Feathertail Glider, to the largest, the Greater Glider (more at home in the nearby Tall Open Forest).
Another big thing the Robertson area is known for is its big earthworm (Notoscolex grandis). These earthworms can go deep into the basalt soils and are usually only seen on the surface after a big rainfall event (see ‘Weather’). They can grow to a metre long!
The diverse range of fauna found here is enhanced by bordering large tracts of forest to the east, south and north, and Wingecarribee Swamp to the west, making it part of an excellent wildlife corridor.
You never know what you might see next.
Environment Panel at the Robertson Community Information Centre at the entrance to Hampden Park