 These "flooded" or "drowned" valleys, knowns as rias, were eroded into the sandstone much deeper than the current sea level during the glacial phases of the Pleistocene ice age.When the ice caps melted about 10,000 years ago, the sea levels rose and flooded the valleys of the park.The dominant rock type is Hawkesbury Sandstone which determines the topography and the vegetation in the sandy soils.