Introduction

Two distinctive types of islands occur on the Great Barrier Reef (GBR), the continental or high islands (hereafter referred to as high islands), and the low or reef islands (hereafter referred to as reef islands). Earlier workers defined high islands as those composed of elevated reef limestones or of non-limestone lithologies (Stoddart and Steers, 1977); on the Great Barrier Reef they are essentially continental outcrops that were separated from the mainland as the Holocene transgression flooded the continental shelf (Chapter 3). The high islands exhibit a range of topography and morphology similar to that observed along the mainland coast, which in large part reflects lithology and structure. Some high islands such as Stone Island (20° 03′ S, 149° 15′ E) are relatively low and gently sloping, but others such as Hinchinbrook Island (18° 20′ S, 146° 15′ E) are high, rugged, and drop steeply to the sea (Fig. 2.6). A good account of the relationships between mainland and high island geology and morphology is provided by Stoddart (1978). Reef islands are, in contrast, usually relatively low-lying accumulations of reef-derived sediments, portions of which may be lithified. On the GBR, reef islands have formed on reef platforms that have reached sea level after the mid-Holocene (see Sections 3.4.3 and 11.7), and which focus incident wave trains at a consistent locus over which deposition and accretion can take place (Fig. 4.11).