The Central Highlands, also referred to as Los Altos, are mountains oriented from northwest to southeast with altitudes ranging from above sea level. The western highlands are displaced faults, while the eastern highlands are mainly folds of sedimentary formationsmainly limestone, shale, and sandstone. These mountains, along the Sierra Madre of Chiapas become the Cuchumatanes where they extend over the border into Guatemala. Its topography is mountainous with many narrow valleys and karst formations called uvalas or poljés, depending on the size. Most of the rock is limestone allowing for a number of formations such as caves and sinkholes. There are also some isolated pockets of volcanic rock with the tallest peaks being the Tzontehuitz and Huitepec volcanos. There are no significant surface water systems as they are almost all underground. The original vegetation was forest of oak and pine but these have been heavily damaged. The highlands climate in the Koeppen modified classification system for Mexico is humid temperate C(m) and subhumid temperate C (w 2 ) (w). This climate exhibits a summer rainy season and a dry winter, with possibilities of frost from December to March. The Central Highlands have been the population center of Chiapas since the Conquest. European epidemics were hindered by the tierra fría climate, allowing the indigenous peoples in the highlands to retain their large numbers.