Katandra Bushland Sanctuary

Katandra Geology

Hawkesbury Sandstone

Katandra Bushland Sanctuary was established following a gift of land to the people of NSW by Harold Seymour. Katandra is dedicated to the study and preservation of native flora and fauna of the Hawkesbury Sandstone country.

Hawkesbury Sandstone is a sedimentary rock made from sediments that were laid down in the Middle Triassic period between 180 and 220 million years ago, around the time when dinosaurs first appeared on Earth. At this time almost all the Earth's land mass was concentrated into a single "supercontinent", centred more or less on the equator and spanning from pole to pole, called Pangaea. About this this time this land mass

Rock types in the Sydney basin. Hawkesbury Sandstone (yellow) lies on top of the older Narrabeen group of rock layers (blue) and under the younger Wianamatta shales (pink). (Source – Australian Plants Society)

began to break into two masses known as Gondwana and Laurasia. Gondwana itself then also broke apart forming most of the landmasses in today's Southern Hemisphere, including Antarctica, South America, Africa, Madagascar and the Australian continent, as well as the Arabial Peninsular and the Indian subcontinent which have now moved entirely into the Northern Hemisphere.

The sand that formed Hawkesbury Sandstone was washed from the Broken Hill area, and laid down in a bed that is about 200 metres thick. Currents washed through it, moving the sand about, leaching outmost of the clay and minerals leaving a sand that is purer than usual. This resulted in a harder sandstone which is more strongly bonded and more resistant to weathering. The water movement washed out channels in some places, while in others, the currents formed sand banks that show a characteristic current bedding or cross-bedding that can often be seen in Hawkesbury Sandstone.

The lack of minerals also produces the nutrient poor soils found in Sydney when the rock does erode. It is thought that nutrient poor soils assisted the evolution of the most characteristic of Australian trees, the eucalypts. As plants cannot afford to lose leaves to herbivores when nutrients are scarce, they defend their foliage with toxins. In eucalypts, these toxins give the bush its distinctive eucalyptus smell.

Most of the city of Sydney lies on Hawkesbury Sandstone. This approximately 200 metre thick layer of rock lies on top of the slightly older but much thicker Narrabeen group of sedimentary rocks which includes layers of sandstone, siltstones, claystones, conglomerates and shales. The rocks of Sydney form a saucer-like basin. Away from the city the bottom of the Hawkesbury Sandstone is above sea level. Escarpments caped with Hawkesbury Sandstone box in the Sydney area on three sides: to the west the Blue Mountains, and to the north and south, the Hornsby and Woronora plateaux.

Layers of rock forming the Sydney Basin (Source – Australian Plants Society)

Between Dee Why and Long Reef is a fault line which has seen rock layers to the north lifted to expose the Narrabeen layers. Headlands to the south of Long Reef are steep blocked cliffs of Hawkesbury Sandstone. To the north are the layered cliffs of Narrabeen sandstones and shales. The tallest cliffs, Bangalley and Barrenjoey Headlands, are capped with Hawkesbury Sandstone on top. Long Reef itself is made of much softer, chocolate-coloured Bald Hill Claystone, also one of the Narrabeen series of layers. This layer gradually slopes down to the north, disappearing below sea-level after Bilgola Headland. The escarpment along which Katandra exists is made of Hawkesbury Sandstone on its upper levels overlying the Narrabeen group layers which are exposed much lower down the slope.

Rock types along the Northern Beaches headlands (Source – Reefcare)

The escarpment along which Katandra exists, located about 2 km inland from the coast, is made of Hawkesbury Sandstone on its upper levels overlying the Narrabeen group layers which are exposed much lower down the slope, below Katandra's boundary . The steepness of the escarpment produces some beautiful caves and rock formations.

Geological phenomena existing in Katandra include

Cross-bedded units - varying in thickness and lateral extent.
Overhanging cliffs and "caves" - generally formed by different rates of weathering of softer and harder stone and gravitational collapse of jointed blocks of sandstone.
Jointed blocks of stone gravitated away from clifftops.
Sculptured weathering effects on a poorly cemented sand layer.

Slump structures / contorted bedding - thought to be a consequence of disturbance (by earth tremors or other causes) of the sediments when they were in a semi-consolidated, plastic state.
Dripstone formations resulting from downward moving water depositing iron oxides, silica and minor amounts of calcium carbonate on the roof and floor of caves.
Washout or scour and fill structures - formed by streams washing out weakly consolidated sediments and then depositing new materials.

Source - Katandra Bushland News (Autumn 2017 edition)

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