 By 7400 years ago the sea levels stopped rising and the cliffs and rock platforms at Kurnell were eroded by wave action to form sheer cliffs.Between 9000 and 6000 years ago the Kurnell isthmus began to form as the mud and sand of the Georges River built up.The locations now known as Sliver Beach and Bonna Point were subject to a buildup of sand from about 6500 years ago.