The Simpson Desert is a large area of dry, red sandy plain and dunes in Northern Territory, South Australia and Queensland in central Australia. It is the fourth-largest Australian desert, with an area of 176,500 km2 (68,100 sq mi).

The desert is underlain by the Great Artesian Basin, one of the largest inland drainage areas in the world. 

The Simpson Desert is an erg that contains the world's longest parallel sand dunes. These north-south oriented dunes are static, held in position by vegetation. They vary in height from 3 metres (9.8 ft) in the west to around 30 metres (98 ft) on the eastern side. The largest dune, Nappanerica or Big Red, is 40 metres (130 ft) in height.

The sand ridges have a trend of SSE-NNW and continue parallel for kilometres.

This pattern is seen throughout the deserts of Australia. Some of the ridges continue unbroken for up to 200 km. The height and the spacing between the ridges are directly related. 

Where there are 5-6 ridges in a kilometre, the height is around 15 metres but when there is one or two ridges per kilometre the height jumps to 35–38 metres. In cross section, the lee side is the eastern slope with an incline of 34-38 degrees, while the stoss side is the western slope with an incline of only 10-20 degrees. 

According to en.wikipedia