The land that the Oodnadatta Track passes through varies between red sand dunes and gibber plains.
The 'gibber plains' are areas of land covered by small stones known as 'gibbers'. The gibbers are small and smooth, and vary in colour and size. The predominant colour is red-brown, but in some areas this varies from black through to white.
It's an interesting sensation to walk across the gibbers. In some areas the ground underneath is soft, crumbly or sandy, so, as you step on the stones, they sink into the ground, providing a cushioning effect. Of course, in other areas the ground is rock hard and the stones are very uncomfortable to walk on.
From the tourist guide, 'These are polished fragments of the original duri crust (a layer of silica formed by the deposition of silica, iron oxides or calcium carbonate) that capped the plain some 65 million years ago. Fine abrasive material has swept past, wearing them down and rounding them off during rain and wind scour. Today they remain as surface lag, protecting the underlying soils from water and wind erosion'. No, I don't understand it either!