The cruise boat went from Macleay Island to Calypso Bay and back past Stradbroke Island. The sands have shown remarkable erosion and the roots of the trees and almost entirely visible as they hang down the sandstone cliffs. Some trees have slid down the sand but are still hanging on precariously along the edge.
. In this way the islands take the form of a spit (Text-fig. 1). The resemblance being emphasised by the line of the ocean beach. From Point Danger, at the New South Wales border, there is no significant interruption in the long curve to the Broadwater and Point Lookout. The sweep of the shore along the north coast of Stradbroke
Island and then to Moreton island merely continues the scalloped figure of
the coast further south.
Each island which extends out from the mainland shore, Stradbroke
Island and Moreton Island in the south, and Fraser Island in the north, is
elongated in the direction of beach-sand drifting. Each island is composed of
sand, except for small and nearly insignificant areas of hard rock. Different researchers state…..in an imaginative essay, that Stradbroke and Moreton islands had in fact originated as barrier islands, that is, they appeared first as beach ridges and spits built out from the known areas of hard rock, which were then considered to be islets
in the open sea.
The islands therefore are best regarded as part of a drowned landscape, formed by partial submergence of a land surface that included several
high areas of blown sand. They originated when the sea was far out on the
continental shelf, in an early glacial age
The main part of Strad broke Island – or, more correctly, North
Stradbroke Island, for the southern part was cut off by the sea at Jumpinpin
in 1898 (Steele 1972)- is formed of eroded dune sands and high fixed
dunes. Volcanic rocks form high bluffs at Point Lookout. Weathered sandstone gives Dunwich its solid foundation, and greenstone occurs at the shore
opposite Canaipa Point.
The rear margins of the coastal strand plains are marked almost
everywhere by cliffs which rise abruptly from swampy ground. They were
obviously formed by marine erosion of the island’s central core but erosion
did not occur all at the one time, as relationships with beach ridges show.
The cliffs mark former shores cut close to present sea level by wave action.
The fresh scarps and the limited weathering of the coastal sands prove that
the cliffed shore was abandoned recently.
The broad swamps and lagoons between the beach ridges are surface
exposures of the water table, and have developed as the sea has drawn back
from the cliffs. The water table underground slopes towards the sea, and as
the coast builds forward the ground-water level increases to maintain the
grade to the shoreline.