Over the late-Pleistocene global sea levels had fallen to 125 metres below current levels. Then 20,000 years ago, as an Ice Age warmed up, they rose dramatically and drowned the winding river gorges to create the calm, dreamy, meandering waterways of our GeoRegion.

Our landscape was uplifted in the Miocene and Pliocene and even as late as Quaternary. Rivers that meandered across the old land surface became entrenched as winding gorges, the technical term for which is incised meanders, and this includes most of the larger streams in the Ku-ring-gai GeoRegion. Smaller tributary streams were stranded in shallow valleys on the tableland surfaces, their larger parents cutting down at a faster rate. They now flow to them via precipitous drops, either as tumbling rapids, a progression of small waterfalls or a single drop of several tens of metres, guided by the particular local character of the sandstone strata. These sharp breaks in slope marking the upstream progression of valley deepening are known as knickpoints and knickzones.

Sea levels rose and fell many times during the Quaternary glacial era, peaking in the previous interglacial period 125,000 years ago when the sea was 4 to 6 metres higher than today’s. Sea level again peaked around 1.75 metres above present around 6000 years ago but the critical factor is how low it went in geologically recent times – actually around 125 metres below present roughly 20,000 years ago.