The whole park lies on the Karai-Karai formation, of the Tertiary age, which is composed of sandstone, siltstones, kaolinites and grits. Underneath this lies the Gombe formation, of Cretaceous age, composed of sandstones, siltstones, and ironstones. The valleys of the Gaji, Yashi and Yuli Rivers are filled with Alluvium of more recent age. Sandy loans and clayey soils of riverine alluvium occur in the valley of the Gaji Yashi and Yuli Rivers. East of the Gaji valley is a 5–7 km wide band of very poor sandy soils that support a shrub savanna formation Wildlife Yankari Game Reserve is an important refuge for over 50 mammal species including African bush elephant, olive baboon, patas monkey, Tantalus monkey, roan antelope, western hartebeest, West African lion, African buffalo, waterbuck, bushbuck and hippopotamus. The lion population is on the verge of extinction. Only 2 lions remained in the park in 2011. Leopard long presumed to be extinct in the park, but in April 2017, one adult male was captured on WCS camera-trap. There are also over 350 species of bird found in the park. Of these, 130 are residents, 50 are Palearctic migrants and the rest are intra-African migrants that move locally within Nigeria. These birds include the saddle-billed stork, guinea fowl, grey hornbill, and the cattle egret. In recent years there have been no sightings of Critically Endangered White-backed Vultures in Yankari and species probably extirpated from the reserve. Yankari is recognized as having one of the largest populations of elephants in West Africa, estimated at more than 300 in 2005. The growth of the elephant population has become a problem for surrounding villages at times as the animals enter local farms during the rainy season. The elephants have also stripped the park of many of its baobab trees. Since 2005, the protected area is considered a Lion Conservation Unit together with Kainji National Park. Features Due to underground geothermal activity, Yankari Game Reserve also features four warm water springs. The camp is named after the most well known of these, the Wikki Spring, from the local Duguri language with “Wikki” meaning “where are you?”. The Wikki Warm Spring is the largest spring and is about 13.0 metres wide and 1.9 metres deep. It daily flows 21,000,000 litres of clear, spring water into the Gaji river. The spring has a constant temperature of 31.1 °C throughout the year during both day and