The Petén lowlands are formed by a densely forested low-lying limestone plain featuring karstic topography. The area is crossed by low east-west oriented ridges of Cenozoic limestone and is characterised by a variety of forest and soil types; water sources include generally small rivers and low-lying seasonal swamps known as "bajos". A chain of fourteen lakes runs across the central drainage basin of the Petén; during the rainy season some of these lakes become interconnected. This drainage area measures approximately east-west by north-south. The largest lake is Lake Petén Itza, near the centre of the drainage basin; it measures . A broad savannah extends south of the central lakes; it features a compact red clay soil that is too poor to support heavy cultivation. This resulted in a relatively low level of pre-Columbian occupation. The savannah has an average altitude of above mean sea level with karstic ridges reaching an average altitude of . The savannah is surrounded by hills with unusually steep southern slopes and gentler northern approaches; the hills are covered with dense tropical forest. To the north of the lakes region "bajos" become more frequent, interspersed with forest. In the far north of the Petén the Mirador Basin forms another interior drainage region. To the south Petén reaches an altitude of approximately as it rises towards the Guatemalan Highlands and meets Paleozoic metamorphic rocks.