that parallel the north-central coast. The series of valleys between these two parallel ranges constitute the core area of the country; as the site of burgeoning metropolitan Caracas, this comparatively small area hosts the country's densest population, the most intensive agriculture, and the best transportation network. Another broad gap separates this area from the easternmost group of mountains, a series of dissected hills and uplands that rise steeply from the Caribbean and extend eastward almost to Trinidad. The great expanse of lowlands known as the Orinoco plains extends westward from the Caribbean coast to the Colombian border between the northern mountains and the Rio Orinoco. This region is commonly known as the llanos, although it also contains large stretches of swampland in the Orinoco Delta and near the Colombian border. The area slopes gradually away from the highland areas that surround it; elevations in the llanos never exceed . North of the Rio Apure, rivers flowing out of the northern mountains cut shallow valleys, leaving eroded ridges that give the land a gently rolling appearance. South of the Apure, the terrain is flatter and elevations lower. One of the oldest land forms in South America, the Guiana highlands rise almost immediately south and east of the Rio Orinoco. Erosion has created unusual formations in this region. Comprising over half of the country, the highlands consist primarily of plateau areas scored by swiftly running tributaries of the Orinoco. The most conspicuous topographical feature of the region is the Gran Sabana, a large, deeply eroded high plateau that rises from surrounding areas in abrupt cliffs up to . Above the rolling surface of the Gran Sabana, massive, flat-topped bluffs emerge; many of these bluffs (referred to as tepuis by the Venezuelans) reach considerable altitudes. The most famous tepui contains Angel Falls, the world's highest waterfall. Climate Although the country lies wholly within the tropics, its climate varies from tropical humid to alpine, depending on the elevation, topography, and the direction and intensity of prevailing winds. Seasonal variations are marked less by temperature than by rainfall. Most of the country has a distinct rainy season; the rainy period (May through November) is commonly referred to as winter and the remainder of the year as summer. Average yearly rainfall amounts in the lowlands and plains range from an arid in the Paraguaná Peninsula – the driest part of South America outside the Arid Diagonal