Sand in the Issaouane Erg, below the level of the sand dunes, has accumulated to a depth of 20 to 30 meters.[6] It has barchan dunes as well as star dunes[2] of 300 to 430 meters high.[7] The presence of both barchan dunes (which form due to unidirectional winds) and star dunes (which form when winds from various directions deposit sand) "suggests that wind regimes have changed over time".[8] NASA's Earth Observatory notes all three types of erg dunes are present in the photos of the Issaouane Erg taken from the International Space Station: stationary mega dunes, which can take hundreds of thousands of years to form; mesoscale dunes (including the stardunes), which form on the mega dunes; and yet smaller ones which migrate over the bigger ones.[9] In the NASA images, the light-blue spots in the valleys between the dunes (which are red from iron oxide) are sabkhah, or salt flats, left after the evaporation of accumulated water.[10]