The solid geology and rocks of the floodplain along Ourimbah Creek are mostly buried under sediments, the Quaternary Alluvium that has accumulated since the sea level rise at the end of the last Ice Age (about 14,000 to 8,000 years ago).
These sediments, steadily accumulated over the last 8 - 10 thousand years, have mostly been derived from the erosion of a sandstone and shale formation called the Narrabeen Group. These rocks 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 is particularly rich in phosphorus and other minerals required by plants. Whether or not this is the reason, the soils on our site support an exuberant vegetation.