24 million years ago. It was a direct response to the continued extension of the Indian collision zone. The mountain range is still uplifting today along with the Himalaya. Karst formation One seventh of the area of China is covered in carbonate rocks (limestone and marble) which are easily chemically eroded by water, forming a karst landscape. This morphology is not well developed across the west and north of China where there is less rainfall than the main karst region in the southeast. China's karst regions are almost all formed in rocks of Devonian to Triassic age. Near Guilin the lowering of the base level, through uplift and river erosion, has formed prominent karst hills. It is estimated that the river erodes down 50–100 mm/kyr, from this the age of the karst landscape is estimated at 10–20 million years (Miocene). Desertification and loess The central China loess sequence records many periods of climatic variation. During dry (often cooler) periods wind erosion increases and loess is deposited, while during wetter and warmer periods paleosols form. The desertification of China's interior is inferred to have started 23 million years ago (Early Miocene) due to the formation of loess deposits from this time until 6.2 million years ago. The glacial and inter-glacial Pleistocene climatic cycles are also presented in the loess deposites. Extensive loess sequences were deposited during a cool period 2.5 to 2.3 million years ago and the most prominent paleosol formed between 615 and 470 hundred thousand years ago. The start of the Holocene (10 to 8 thousand years ago) is recorded as a warm and wet time in central China, most likely from the melting of snow and ice off the Tibetan Plateau. A decrease in the area of loess deposits shows that the Holocene Climatic Optimum occurred across central China 8 to 5 thousand years ago. Hainan and the South China Sea The island province of Hainan is located off the south coast of China's mainland. It was separated from the mainland by tectonic rifting and coastal erosion. The parts of the South China Sea that China claims, formed by the evolution of continental rifting into oceanic spreading during the mid Cenozoic. There are numerous sea mounts and island carbonate reefs that have developed on horsts formed during the extension. Cratons and orogenies China has three Precambrian cratons, the North China, South China and Tarim Block. These have a