North West Also bordering on Botswana and the Kalahari desert is the province of North West, with 9.5% of South Africa's land area. Its summer rainfall is low on this border and increases towards the Witwatersrand/Gauteng area in the east, so that this part of the province is mostly typical Highveld grassland. Important topographical features include the Vaal River, which forms the south-eastern boundary, the 125km ridge of the Magaliesberg, and the extinct volcano that is home to the Pilanesberg National Park. Caves in the Sterkfontein area comprise a World Heritage Site known as the Cradle of Humankind because of the wealth and significance of the hominid remains found there. Its capital, Mafikeng, is near the Botswana border and forms a single urban area with its neighbouring town, Mmabatho. Potchefstroom and Klerksdorp are the biggest cities in the province; other main towns are Brits and Rustenburg. Mafikeng/Mmabatho, which sits on a north-south line that more or less bisects the province, is the most westerly of these towns; the dry western half of the province, sandwiched between Botswana and the Northern Cape, is sparsely populated. Mining is responsible for more than a third of the province's GDP: 94% of South Africa's platinum comes from the Rustenburg and Brits districts which produce more platinum than any other single area in the world. It produces 25% of the country's gold. Granite, marble, fluorspar and diamonds are also mined. The province provides about a third of the country's maize. Other important agricultural products are sunflower oil, fruit, tobacco, cotton and beef.