Geology

 

Outcrops and boulders of granitic gneiss are a common feature in the Park and can be seen on aspects such as ridges, beaches and offshore islands. The granitic gneiss was developed 530 to 1100 million years ago under the same mountain building process that caused the super continent Gondwana to break up.

The eastern third of the Park is a mosaic of large, rounded outcrops (tors, knolls and inselbergs) embedded in a matrix of sandy gravels and clays. Widespread erosion around 280 million years ago exposed these outcrops and led to the foundation of the valleys that contain Meelup and Dolugup Brooks.

Lateritic hills and plateaus and gravel slopes constitute the western and central thirds of the Park. Laterite was formed through the chemical weathering of the feldspars of granitic gneiss and the gradual transformation of iron and aluminium hydroxides into gibbsite, boehmite, hematite, and maghemite. Subsurface drainage segregated these secondary minerals into four zones: the upper zone (or duricrust), the friable and mottled zone, the pallid zone and then the core stones of un-weathered granitic gneiss.

In the Park, the laterite profile is 0.5 to 1.5 metres thick and is very prone to deteriorate into gravels and sands. The laterisation process took place 25 to 40 Million years ago in the Eocene- Oligocene Era when the South West of Western Australia had a tropical climate comparable to modern Indonesia.

The Park’s four beaches are confined between granitic gneiss headlands; their beach sand is mainly derived from the material transported by its brooks and streams with additional contributions from long shore drift. Semi-mobile sand dunes line the shore and back-beach margins. The position of boulders and cobbles is the result of the pounding waves, the dominant beach profile and tidal movements and storms. Cobbles and pebbles are occasionally cemented by calcium to form beach conglomerates.