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This chapter describes the geology of reef islands of the Great Barrier Reef (GBR) of Australia. Australia's GBR extends over 15° of latitude. Clearly visible from space, the GBR is probably the largest structure on Earth built by plants and animals. This reef is made up of over 2,900 individual reefs and about 750 fringing reefs attached to the mainland or the numerous islands of the Queensland coastline. The GBR forms a true near-continuous barrier only in its most northern third. Reefs vary in size from small isolated pinnacles to massive structures over 25 km in length and 125 km 2 in area. Changes in the reef-top geometry are also important factors in long-term changes to reef islands. In the long term, reef islands are merely a temporary store of sediments in the total reef system, a store that may increase or decrease in size according to internal storage characteristics, internal reef factors, or completely external factors over which the reef itself has no control.

Belperio (1979) has suggested that lack of evidence for higher relative Holocene sea levels in the coastal plain of North Queensland negates previously presented research. Lack of evidence in particular areas is here suggested as insufficient argument and the features and radiometric dates which lead to the conclusion that higher relative sea levels have existed are reviewed in the light of Belperio's comments. In particular Belperio's claim for uniform Holocene stability in eastern Australia is challenged in the light of recently reported 6 m of uplift in the last 6000 years in the Broad Sound area.

A mapping programme of all reefs of the Great Barrier Reef Marine Park area and compilation of a computer-based gazetteer containing locational and morphological data have permitted the first comprehensive spatial and morphometric analysis of the Great Barrier Reef. As previously suspected the analysis shows that a degree of organisation exists in the complexity of the Reef and clear regional and latitudinal patterns are recognised Reasons for this, particularly in relation to regional structure and the influence of the pre-Holocene foundations of the modem reef, are discussed The usefulness of a morphometric approach is suggested and an example related to Acanthaster planci outbreaks presented

Continuous seismic profiling on the continental shelf, central Great Barrier Reef Province, Australia, has delineated a major sub-bottom reflector interpreted as the pre-Holocene surface. Channels eroded in this surface are infilled and overfilled by post-glacial sediments. Filling is ascribed to estuarine backfilling during sea-level rise followed by deposition of delta front deposits. In many cases these fills create positive relief over buried channels. Hence modern shelf morphology may not be an accurate guide to the positions of Pleistocene channels across the shelf.

SUMMARY Although legislation has been introduced to allow stricter control of activities within the Great Barrier Reef region, paradoxically there are increasing pressures due to greater use of the Reef, and population expansion and economic development on the adjacent mainland. In addition, greater understanding of the functioning of the reef system has allowed for recognition of more subtle anthropogenic effects on the Reef. On the outer Reef some localised degradation may result from intense use, accidents or from pre‐legislation activities. Runoff from the mainland has the potential to introduce increased turbidity levels, reduced salinities and some chemical pollutants particularly on nearshore reefs and in the Cairns region. Through global scale atmospheric and oceanic circulation the Great Barrier Reef is open to more distant sources of perturbation.

Evidence for Holocene shorelines from the Queensland coast, off which the Great Barrier Reef lies, has epitomized the problems of eustatic fluctuations over the last 6000 years. While some areas of southern and central Queensland show evidence of no sea level higher than the present over this period, other areas, particularly within 150 km of Townsville on the mid-North coast, have provided radiometrically dated evidence for an emergence of up to 4.9 m. The area in which the 1973 Expedition worked has been described previously by several authors, and evidence for higher shorelines in the form of cemented platforms, raised reefs and related features suggesting higher sea levels, though without isotopic dating, has been noted. Research was aimed at confirming and accurately measuring and dating such evidence and relating it to the pattern described elsewhere. Any divergences must then be explained in terms of spatially and temporally varying oceanographic or geomorphic conditions and Earth movements of tectonic and/or isostatic origin.