The Bouddi Peninsula is part of the Sydney Basin.
It extends south from McMasters Beach to Box Head
at the entrance to Broken Bay and the Hawkesbury River.
Bouddi National Park covers more than half the area.
The remaining area is settlement: McMasters Beach,
Killcare, Hardys Bay, Pretty Beach and Wagstaffe.

The peninsula consists of two sedimentary rock formations:

1. Hawkesbury Sandstone - upper massive formation
2. Terrigal Formation - underlies the Hawkesbury Sandstone.

These two formations were laid down in the Triassic period
from about 220 to 180 million years ago.

The Terrigal Formation (previously known as the Gosford Formation and
is part of the Narrabeen Group), was laid down first, and the Hawkesbury
Sandstone was laid down later on top of the Terrigal Formation rocks.
This combination together with millions of years of weathering
has created spectacular landscapes, headlands, rock platforms,
dune systems, beaches and inland waterways.

The topography of the area is mainly due to water erosion
and changes in sea level.
During the last ice age, 18,000 to 6,000 years ago, the sea level
was 140 metres lower. The coastline was 20 kilometers further east
and the Hawkesbury River and Brisbane Water were dry valleys.
With the melting of the ice caps at the end of the Ice Age, the sea level rose.
The coastline receded to where it is today and the Hawkesbury River
and Brisbane Water became drowned river valleys. Melting of the
remaining ice-caps time would cause a rise in sea-level of 70 metres.

The massive Hawkesbury Sandstone dominates the higher levels
as platform-like escarpments, where the exposed edges of the formation
form vertical cliffs and overhangs.
The rocks of the Terrigal Formation are finer than the Hawkesbury
so they weather more rapidly into soil-covered slopes with open forest.

Click on the image below to see more photos of the Bouddi Peninsula coastline