There are two prominent geographic features in northern Luzon. One is the beautifully picturesque mountain system, the Caraballos, the most important range of which is the Caraballos Occidentales, extending north and south throughout the western part of the territory. This range is the famous "Cordillera Central" for about three-quarters of its extent northward, beyond which it is known as "Cordillera del Norte." The other prominent feature is the extensive drainage system of the eastern part, the Rio Grande de Cagayan draining northward into the China Sea about two-thirds of the territory of northern Luzon. It is the largest drainage system and the largest river in the Archipelago. The surface of northern Luzon is made up of four distinct types. First is the coastal plain -- a consistently narrow strip of land, generally not over 3 or 4 miles wide. The soil is sandy silt with a considerable admixture of vegetable matter. In some places it is loose, and shifts readily before the winds; here and there are stretches of alluvial clay loam. The sandy areas are often covered with coconut trees, and the alluvial deposits along the rivers frequently become beds of nipa palm as far back as tide water. The plain areas are generally poorly watered except during the rainy season, having only the streams of the steep mountains passing through them. These river beds are broad, "quicky," impassable torrents in the rainy season, and are shallow or practically dry during half the year, with only a narrow, lazy thread flowing among the bowlders. This plain area on the west coast is the undisputed dwelling place of the Christian Ilokano, occupying pueblos in Union, Ilokos Sur, and Ilokos Norte Provinces. Almost nothing is known of the eastern coastal plain area. It is believed to be extremely narrow, and has at least one pueblo, of Christianized Tagalog -- the famous Palanan, the scene of Aguinaldo's capture. The second type of surface is the coastal hill area. It extends from the coastal plain irregularly back to the mountains, and is thought to be much narrower on the eastern coast than on the western -- in fact, it may be quite absent on the eastern. It is the remains of a tilted plain sloping seaward from an altitude of about 1,000 feet to one of, say, 100 feet, and its hilly nature is due to erosion. These hills are generally covered only with grasses; the sheltered moister places often produce rank growths of tall, coarse cogon grass.[5] The soil varies from dark clay loam through the sandy loams to quite extensive deposits of coarse gravel. The level stretches in the hills on the west coast are generally in the possession of the Christian peoples, though here and there are small pueblos of the large Igorot group. The Igorot in these pueblos are undergoing transformation, and quite generally wear clothing similar to that of the Ilokano.