The Great Sandy Desert is mainly a tree steppe grading to shrub steppe in south. It comprises open hummock grassland of "Triodia pungens" and "Plectrachne schinzii" with scattered trees of "Owenia reticulata" and Bloodwoods, and shrubs of "Acacia" species, "Grevillea wickhamii" and "Grevillea refracta", on Quaternary red longitudinal sand dune fields overlying Jurassic and Cretaceous sandstones of the Canning, Centralian, Arunta and Armadeus Basins. "Casuarina decaisneana" (Desert Oak) occurs in the far east of the region. Gently undulating lateritised uplands support shrub steppe such as "Acacia pachycarpa" shrublands over "Triodia pungens" hummock grass. Calcrete and evaporite surfaces are associated with occluded palaeo-drainage systems that traverse the desert; these include extensive salt lake chains with samphire low shrublands, and "Melaleuca glomerata" - "Melaleuca lasiandra" shrublands.