Moreton Bay was formed when the sea level rose after the last ice age from 12,000 to 8000 years ago. Then sand washed northward by the ocean currents was deposited along its perimeter to form the enclosing islands of Stradbroke, Moreton, and Bribie. Within the bay, as the sea levels rose, only the mountain tops remained to form islands. Peel Island is one of these.
The soils of Peel Island have been formed from basalt, laid down by regional volcanic activity 28-20 million yBP (years before present). Underneath this basalt are sandstones from the Jurassic period, 213-144 million yBP. In turn these sandstones cover metamorphic rocks which are from 360 to 248 million years old.1
All the rock units on the island have been deeply weathered to form krasnozems – strongly structured, clay-textured, red acid soils. The clay content is characteristically more than 50% throughout but the surface soil behaves like a loam, owing to its humus content. The subsoil is moderately to strongly acid and its water holding capacity is low. The krasnozems of this area are residual soils occurring on modified old land surfaces but they have formed in parent rocks that weather to produce much clay, such as basalt. There is a lateritic variant containing much ironstone gravel.1
Laterite is not uniquely identified with any particular parent rock. It is a rock product that is a response to a set of physio-chemical conditions, which include; an iron containing parent rock, a well drained terrain, abundant moisture for hydrolysis during weathering, relatively high oxidation potential and the persistence of these conditions over many thousands of years.1
(1 = Prepared by Paul Cannard and David Weir, when students at the Moreton Institute of TAFE)