The islands bordering Moreton Bay to its east are vegetated dunes formed during a past period of intense wind activity. Aeolian sand, as indicated by drilling, occurs to considerable depth below sea level. There is also evidence of sub-aerial weathering in bedrock samples and this implies subsequent drowning of the land surface. The occurrence of colouration (red, brown, grey) in much of the dune mass is thought to be due to decomposition of some of the disseminated heavy minerals - the older more leached dune sand being quite white in appearance. Of particular interest is the occurrence within the dunes of white argillaceous and black humate cemented sands, which have formed at the groundwater surface on inter-ception of downward percolating products of leaching.