Woronora hill in Heathcote National Park at 282m above sea level is the highest natural point in metropolitan Sydney. The Blue Mountains to the west are most imposing but equal elevation occurs on the gently rising Woronora Plateau in the south that lead to the Southern Highlands and 1000 square kilometres of untouched bushland, including the Special Areas set aside for water collection. The city of 5.3 million people sits inside a larger geological basin and this basin contains enough unique biodiversity to have been defined as the Sydney basin bioregion.
One species that is found only in the rocky bushland slopes of Sydney is the critically endangered broad-headed snake, an animal exquisitely adapted to the particular niche it inhabits. This snake is found on the relatively wet west facing slopes of the basin where solar radiation stores heat to assist nocturnal hunting and hatching of eggs. The snake lives on the geckos and other creatures also thriving on the particular conditions of the west facing slopes.
Contour map of Sydney basin
Geology of Sydney Basin
The Sydney Basin is a sedimentary basin with an area of approximately 44,000 square kilometres. It is defined by the great diving range in the west and the ocean escarpment in the east. The defining geology extends to the Hunter Valley in the north and south to Batemans Bay.
Around 5,000 metres (16,000 ft) thick, the Sydney Basin consists of Permian and Triassicsedimentary rocks… and contains economically significant reserves of coal.
During Gondawana times, about 250 million years ago the sandstone in Sydney Basin was deposited by monolithic rivers flowing from the south west to the east.
50 million years later (200 mya) the Lapstone fault (running north south at the foot hills of the blue mountains) caused a lift in the sandstone in the east defining Sydney’s sandstone coastline, the flattened Cumberland plains and lifted the great diving range – including the Blue Mountains.
Three Sisters at Katoomba in the Blue Mountains
Fast forward another 100 million years (100 mya) and the first snake fossils appear in tropical Asia .
Fast forward another 40 million years (60 mya) and Gondawana breaks up, Australia becomes a continent and the Sydney basin is then shaped …
into a landscape that was defined by bedrock valleys exposed into a raised plateau. Sydney’s largest rivers, such as the Hawkesbury, Parramatta, Georges and Hacking Rivers eroded the region’s deepest valleys. In this period, the Ashfield Shale got weathered to create a flatter landform with low, undulating topography and reasonably fertile soils, which heavily contrasted the plateaus, cliffs and gorges on the sandstone areas in the Sydney Region. The Botany Bay Basin was also developed at that time, which is infilled with sand.
At the same time Sydney Basin is forming (60 mya) the first fossil records of elapid snakes emerge in tropical Asia. Around 24 mya elapids made the journey to Australia. Elapid snakes are in the cobra snake family and have permanently erect fangs, which may also deliver venom. Many species can deliver a bite potentially deadly to humans (and dogs), but most are effectively harmless given the availability of anti-venom. Recorded snakebites exceed deaths by a hundredfold. There are 1 to 4 snakebite deaths per year in Australia, most are from handling or cornering the snakes.
There are around 80 species of Australian elapid snakes, including: death adder, red-bellied black snake, tiger snake, duggite, mulga snake, brown snake, yellow-faced whip snake, copperhead, golden -crowned snake, and mustard-bellied snake.
Broad-headed snake
Photo of Broad-headed snake on a tree in Heathcote National Park 2021 Tom Kristensen
The critically endangered Hoplocephalusbungaroides, broad-headed snake is a venomous snake from the elapidae family. Broad-headed snakes are possibly Australia’s most endangered snake given their dependence on the unique geology of specific sites in the sydney basin. Once relatively common, the BHS is now locally extinct in metropolitan Sydney and only found in a few of the National parks and state forests within the Sydney basin.
Unlike most elapids the BHS are at home in the trees. When the rocky outcrops become too hot in the summer the snakes will disperse into gullies where they seek refuge in hollow tree limbs. Being nocturnal ambush predators, they pick up geckos on the rocks and a wider array of prey found in the forest and leaf litter on the forest floor. Seasonal behaviour patterns over a long life span find the BHS returning to the same outcrops and hiding holes year on year.
Besides the favourable western-tilted geology the Sydney basin has produced a network of upland hanging swamps. These impermeable depression on sandstone benches offer a place for the development of peat swamps supporting dense low vegetation with ponding water. These swamps store and release water through dry periods providing an oasis to a wide range of animals, including frogs, another staple food for BHS. These biodiversity gems are destroyed by the longwall coal mining that fractures the entire geology overlying the coal seams. Unfortunately the location of the swamps within the protected Special Areas has meant that their destruction has gone largely unnoticed.
Technical Report 2: Upland swamp development and erosion on the Woronora Plateau during the Holocene Kerrie M. Tomkins and Geoff S. Humphreys Macquarie University
Slow to mature, slow to reproduce, and confined to western slopes living under the flat sandstone rocks prized by landscapers, BHS are not able to adapt to human impacts on their environment. Being a small snake under a metre in length, and very attractively marked the BHS are poached from reserves and sold to collectors. Basking snakes are also commonly squashed on the proliferating mountain bike trails as national parks are increasingly managed as a recreational resource.
BHS give birth to live young unlike other elapid snakes that lay eggs. Presumably there is an advantage for baby BH snakes to be able to hide in rocks as soon as they are born.
If you come across a pretty yellow and black patterned snake in Sydney it’s most likely to be a carpet diamond python – the southern most and highest altitude occuring carpet python. While from an entirely different snake family to elapids, diamond pythons and broad-headed snakes look very similar to the untrained eye.
Diamond python Royal National Park 2025Diamond python Royal National Park 2025
Golden-crowned snake, a small Australian elapid snake on the road at dusk, just missed by the car.Red-bellied black snake (elapid)Mustard-bellied snake (elapid)Broad-headed snakeSydney Basin and Great dividing range (red)Distribution of broad-headed snakesGolden-crowned snake (off the road)Yellow-faced whipsnake (elapid) Broad-headed snake killed by bicyclePhoto of dead broad-headed snake on walking/cycling track Royal National Park by Abigail Bradley
