The rocks that underlie Ourimbah Creek are mostly buried under sediments that accumulated during the sea level rise between14,000 and 8,000 years ago. This was a result of the melting of the ice that had accumulated around the poles, together with ocean warming as the previous ice age ended.
These sediments, steadily accumulating over several thousand years, have mostly been derived from the erosion of the rocks of the surrounding hills. These are a sandstone and shale formation called the Narrabeen Group. They contain more of the element phosphorus than the Hawkesbury Sandstones immediately to the south of Ourimbah. Rainforest has a greater requirement for phosphorus than the eucalypt forest and so becomes more common where the Narrabeen Group outcrop.
Another geological influence on the rainforest can be seen at the southern end of our site, where a freeway underpass has exposed the Narrabeen shales and sandstone. Running vertically through the rocks is a vein of subsoil that is deep red in colour. The colour is characteristic of soil derived from basalt, suggesting that underneath there may be a basalt dyke. Basalt soils are particularly rich in phosphorus and other minerals required by plants. High fertility combined with easily-accessed groundwater may explain why the site supports such exuberant vegetation.