Karoo Supergroup of the country, and in the Lebombo mountains along the Mozambique border. Once the layer of hard lava was eroded away, the softer Karoo sediments over the rest of the basin eroded even faster. However, the dolerite sills resisted erosion, protecting the softer Beaufort and Ecca shales beneath them. This created numerous and widespread flat topped hills, known as Karoo Koppies ("koppie" being the Afrikaans term for hill), which are iconic of the Karoo, and, by extension, the South African landscape. The dykes, or vertical fissures which brought the lava to the surface stand out today as linear ridges extending
Ryn Desert can drop to a low of . Many small towns are scattered throughout the Ryn Desert, and population density is between 1 and 15 people per square mile. The Ryn lies in a semi-arid climate zone, and receives very little rainfall. High winds sweep across the desert, and in 2001 a dust storm in the Baltic Sea was determined as originating in the Ryn Desert. A study of long-range dust transportation to the Baltic Sea region—by analyzing dust pollution in Scandinavia, showed that aerosol concentrations there were influenced more by the Ryn Desert region than the Sahara Desert in Africa.