Geomorphic Systems

Geographical Inquiry Question

How does an understanding of geomorphic systems help us to understand the geology of this area today?

Background to the Geology of Ku-ring-gai Chase National Park

Ku-ring-gai Chase National Park's landscapes include drowned river valleys, estuaries, steep sandstone cliffs and plateaus.

The landscapes of Ku-ring-gai Chase National Park started forming between 190 and 225 million years ago when Australia was part of the Gondwanaland supercontinent and the Sydney region was a large freshwater lake.  This was slowly filled up by deposits of sand, mud, silt and pebbles washed in by large streams.  Over millions of years, these sediments were gradually overlaid by others and compressed into sandstone, mudstone and shale.

Two to 12 million years ago, the Hornsby plateau was uplifted to between 150 and 250 metres above sea level.  As rivers flowed across this plateau, they gradually cut deep V-shaped valleys into it leaving narrow sandstone ridges behind.  During the past million years, rising sea levels flooded the lower valleys. The sea cut off some of the plateau peaks, forming islands like Lion Island, and Scotland Island.

What is a Drowned River Valley?

Drowned river valleys are formed when rising sea levels flood existing river valleys.  (Also known as Rias).

The sea levels of the coastline at Ku-ring-gai Chase National Park rose after the last ice age, about 10,000 years ago, flooding previously eroded V-shaped valleys.

Hawkesbury Sandstone

Most of Ku-ring-gai Chase National Park's geology consists of Hawkesbury sandstone.

Hawkesbury sandstone is made up of medium to course grained quartz, which sometimes sparkles when the sun shines on it.

It has low fertility, is very acidic and has very low levels of nitrogen and phosphates.  It is easily eroded, especially after fire and heavy rain.  Therefore, Hawkesbury sandstone is not suitable for agriculture.  It mainly supports dry-sclerophyll bushland.