Ourimbah's foundations were laid some 270 million years ago when a huge area of land south of the Hunter region began sinking. Great quantities of eroded material washed into the depression, firstly from the New England area to the north then later from the southwest. The base of what is now known as the Sydney Basin gradually filled, forming a wide, shallow, sandy-bottomed sea.

The first material washed down from New England became the Greta Coal Measures which, at Ourimbah, are something like a kilometre below the ground. Sand was then deposited all over the Basin, at Ourimbah some 200 metres thick and called the Maitland Group. The sea retreated around 230 million years ago and the gigantic Newcastle Coal Measures, about 300 metres thick, 500 metres below Ourimbah, were laid down as the whole area took on the identity of a huge swamp. This was the time of the first dinosaurs.

Renewed uplift in the New England region brought still more eroded material down into the Basin by means of two large, southward-flowing rivers which ran side by side, close to the coastline. The rivers emptied into the shallow Basin via broad deltas. As more and more sediment was washed down to the deltas solid ground was built up and so the mouths of the rivers advanced in the direction of flow. By this means, a sandy coastline was formed as the Basin was closed off to the ocean. All this sediment became the Narrabeen Sandstone, the top layer of which is the Gosford Formation, observable all around Ourimbah today.

Then, about 200 million years ago, uplift of the country to the southwest of Sydney tilted the entire Basin and a new phase of erosion began. It was at this stage that the Hawkesbury Sandstone was laid down. Great flows of sediment from the south, almost exclusively quartz, overlaid the more shaley Narrabeen Sandstone [Rose G, ‘Triassic Rocks of the Sydney District’ (1965) xv Australian Natural History (No 1) 22-28].

Fossils found in the Gosford Formation of the Narrabeen Sandstone include amphibians, fish and non-flowering plants. Similar findings have been made in the Hawkesbury Sandstone with the addition of insects [Brannigan D (ed), An Outline of the Geology and Geomorphology of the Sydney Basin (Sydney: University of Sydney, 1976) at 6].

Hawkesbury Sandstone covers a major part of the Sydney Basin, including the high land just west of Ourimbah — Somersby, Mangrove Mountain and Kulnura. To the north are outcrops on the high ridges of the Ourimbah State Forest. This Hawkesbury stone weathers in a particular manner. It can form beautiful caves with domes and a honeycomb effect, or large scooped-out ledges protected by overhangs.

Along the coastline of the Sydney Basin, up to about 15 kilometres inland, there are many highly-weathered examples of volcanic basalt which has filled the northwest oriented joints in the sandstone. These basalt dykes, indicative of igneous activity, have been dated to the Cainozoic Era which began about 65 million years ago and saw the extinction of the dinosaurs and the rise of the mammals [NSW Department of Mines, Sydney (Sydney: 3rd ed, 1966) 1:250,000 Geological Series Sheets S1 56-5 (map)].

By this time the Australian continent, which had been slowly drifting northwards, was entering the warmer latitudes approximate to its present position and new vegetation was evolving. The Australian native eucalypts are thought to have first appeared in the Oligocene Epoch, between 24 and 37 million years ago. They replaced rainforest vegetation in the drying centre of the continent. The eucalypts followed the rainforests in abandoning the red centre as it eroded to desert conditions. Upon their arrival in the Ourimbah area, the rainforests and eucalypts would have found the sandstone to have been eroded to the extent that the landscape would have much resembled that which we see today.

The rainforest vegetation would have enjoyed the quite fertile soil on the exposed Narrabeen deposits, while eucalypts established themselves on the sandy ridges. As the continent drifted further into the warmer latitudes, this delineation became marked but forces were to push the rainforest right into the shady, secluded gullies. The eucalypts were able to regenerate after fire, the rainforest could not. The area became warmer and drier and bushfires from lightning strikes confined the rainforest to ever smaller pockets.

There is evidence of extremely early occupation by humans of Australia and of the Sydney Basin [The Sydney Morning Herald, 8 April 1987]. Forty thousand years ago, even earlier perhaps, Australian indigenous people may have frequented the ridges and valleys of Ourimbah. Because so much of the world's oceans were frozen during the last Ice Age (approximately 200,000 to 5,000 years ago) it is likely that the earliest inhabitants resided at the then coast, some miles to the east of the present coastline and used Ourimbah as a hunting area, only coming to live there when the sea level rose by about a hundred metres at the end of the Ice Age. [Byrne D, The Mountains Call Me Back Occasional Paper No 5 (Sydney: NSW Ministry of Aboriginal Affairs, 1984).]

At that time pockets of rainforest occupied the steep, shaded gullies while thick stands of eucalypts prospered on the alluvial flats and the slopes from which the rainforest had been driven.

In the rainforest environment, the Red Cedar (Toona Australis) grew with an early burst as the crown reached for light. This produced a long trunk which, under other conditions, would have sprouted lower branches. The cedar is prey to a grub which eats the growing tips of the branches. Almost all cedar trees are subject to this sometimes fatal nuisance and the continual nipping of the growing tips forces the tree to produce more and more branches, giving this single, native deciduous its tangled crown and preventing the tree, when growing in the open, from developing a long trunk. In the gullies of Ourimbah, cedar trees would have been attaining heights of up to 150 feet with a bole diameter of 8 feet or more. It is quite certain that this outstanding and beautiful tree was well represented in Ourimbah but, as with so many other locations on the eastern seabord, it is unlikely to have existed in sufficient quantity to have been the motivation for early land grants in the area or for any more than smash and grab raids by marauding timber cutters specially engaged by wealthy opportunists to collar as much of this magnificent timber as possible.

The flat at Ourimbah was colonized by the blue gums for which it was known for some time. These large trees enjoyed the good soil and regular rainfall which washed down from the slopes surrounding the flat. Those slopes were rife with giant blackbutts around 200 feet high and some 6 feet through, while hardy ironbarks preferred the less fertile ridges, above which they towered as much as 150 feet.

Also well represented were roundleaved gum, spotted gum, white mahogany and many more. There is unlikely in the future ever to be the like of the magnificent and awesome forest that was Ourimbah. In 1917 when the Forestry Commission was set up there were virtually no mature trees in the Ourimbah State Forest [Forestry Commission of NSW (Wyong Forestry Office) Detailed Survey of Part of Ourimbah State Forest, Section 1 of about 5,000 acres (unpublished: 7 December 1917)]. The wonderful domain which once was supported a myriad of wildlife and provided a wide variety of game for the many generations of Aborigines who lived at and around Ourimbah.

So little was the Aborigines' culture understood by the first several generations of Europeans in New South Wales that the lifestyle of the indigents was soon completely upset, the great majority of those who survived becoming wholly dependent on reservations and handouts [Haigh C, ‘Ignorance, Alcohol, God-and Good Intentions’ in Haigh and Goldstein (eds) The Aborigines of NSW (NSW Government National Parks and Wildlife Service, undated), a reprint from 11 Parks and Wildlife (5)].

Very often men were ruthlessly slaughtered when they reacted to the provocation of the stealing of their land or violence to their women [Forestry Commission of NSW Red Cedar (pamphlet 1979); Vinnicombe P, Predilection and Prediction; a study of Aboriginal Sites in the Gosford-Wyong Region (a report to the NSW Government National Parks and Wildlife Service, 1980) at IV:21].

It is likely that many were gunned down as a matter of course on Cape's land at Wyong [Bennett FC (ed), The Story of the Aboriginal People of the Central Coast of New South Wales (Brisbane Water Historical Society and The Entrance and District Historical Society, 1981) at 11; Vinnicombe P, Predilection and Prediction; a study of Aboriginal Sites in the Gosford-Wyong Region (a report to the NSW Government National Parks and Wildlife Service, 1980) at IV:18; Swancott CW, The Brisbane Water Story Part IV-The Rest of the Story (Woy Woy: Brisbane Water Historical Society, 1955)].

A newspaper article of 1875 alleged that 43 years earlier a body of soldiers had been despatched to the Brisbane Water District to quieten some hostile Aborigines:

In the middle of the night, camp after camp was surprised and the occupants, men, women and children, shot down like native dogs. The poor friendly blacks fared no better than the others; and the whole affair was a horrible satire upon our civilization. [‘Arcadia at Our Gates’ Town and Country Journal 6 March 1875 at 379–80.]

The major justification for these shocking inhumanities was that because the Aborigines engaged in no cultivation they had no entitlement to the land. But the imported notion of salvation through work was no philosophy for a people who, for generations immemorial, had lived in a certain harmony with the finely balanced ecosystems which the Europeans now busily attacked with gun and axe and replaced with fence and shed. Without wheat or maize, without beasts of burden, with no written language and without the abstract idea of individual ownership of arbitrarily parcelled pieces of land, the Aborigines managed agriculture on a grand scale by the calculated and deliberate use of fire.

Soon after his arrival in New South Wales, Governor Phillip explored Broken Bay and found numerous Aborigines at both Brisbane Water and Pittwater. It was noticed that some Aborigines from Sydney were among those at Brisbane Water, suggesting friendly relations and interaction. In 1789 smallpox broke out and devastated the Aboriginal population.

Although there are no reports of the effect on the Aborigines of the Gosford-Wyong region, skeletons were reported all around Broken Bay and it is feasible that the disease greatly reduced numbers to the north of that waterway. This feasibility is further borne out by magistrate Willoughby Bean's 1827 count of only sixty-five Aboriginal men, women and children in the Brisbane Water District - fifteen each in the "Myal or Broken Bay tribe", the "Taggera Beach tribe" (The Entrance) and the "Wyong tribe" and ten each in the "Narara" and the "Erina" tribes [AONSW 4/5525].

Recent research indicates that the Aboriginal inhabitants of Gosford-Wyong were part of a tribe which covered both sides of Broken Bay and the Hawkesbury River [Vinnicombe P, Predilection and Prediction; a study of Aboriginal Sites in the Gosford-Wyong Region (a report to the NSW Government National Parks and Wildlife Service, 1980) at IVA, IV:2]. That research has found that tribes were divided into clans and sub-divided into "hordes" which in turn were made up of families [Moore D, ‘The Aboriginal Tribes of NSW’ in Haigh and Goldstein (eds) The Aborigines of NSW (NSW Government National Parks and Wildlife Service, undated)]. It is suggested that it was possibly these hordes, the food-gathering units, which Willoughby Bean referred to as tribes.

It has also been suggested that Bean may not have counted all the groups of indigenous people that were in his District. A 1968 publication by two local Historical Societies postulates twelve groups of about thirty people each in the Gosford-Wyong region, one of those groups being centred on the Ourimbah Creek Valley [Bennett FC (ed), The Story of the Aboriginal People of the Central Coast of New South Wales (Brisbane Water Historical Society and The Entrance and District Historical Society, 1981)]. Perhaps these Ourimbah Aborigines had already been broken up and dispersed by Bean's time. Convicts on the run from Newcastle back to Sydney were known to have taken some toll on Aboriginal populations in their path. It is also likely that cedar cutters investigating the extent of the cedar which occurred along Ourimbah Creek and its tributaries placed stress on the Aborigines. The cedar getters, universally described as rough, brawling men, are known to have upset the careful marital arrangements of the Aboriginal groups by taking women and dealing summarily and conclusively with protests [Forestry Commission of NSW, Red Cedar (pamphlet 1979)].