Meta-volcanics and Meta sediment topography; on the south is the Municipality of Santa where the north–south trending fault terminates; to the west is the South China Sea; while the northern boundary opens to the northern extension of the Ilocos coastal plain. The central part of the plain is characterized by a dissected ridge of Plio-Pleistocene age that trends in a northeast–southwest direction. The ridge divides the Vigan-Bantay Plain into northern and southern plains. The northern part of the plain is drained by minor rivers namely: Bantaoay, Santo Tomas, and Nauman rivers, while the Abra River is the main drainage network of the southern portion of the plain. The Abra River meanders into the hinterland but exhibits a braided pattern across the plain. The drainage network of the eastern margin of Vigan-Bantay plain consist of deeply cut valleys whose pattern is strongly controlled by main faulting trends, particularly those in the northerly and easterly directions. Vigan is being drained by the Vigan River and Bantay River, respectively, on the north and on the south by Mestizo River whose water discharge comes from the Bantaoay head waters. The relief of the Vigan-Bantay plain is actively level to undulating with a slope ranging from 0% to 8%. Vigan in particular is generally in level with several hills in Barangays Tamag, Bulala, and Salindeg, sometimes reaching about 50 meters in elevation and has a slope ranging from 3% to 8%. The eastern margin of the plain characterized by a steep to very steep slope, about 40% to 60%, the relief often reaches an elevation of more than 600 meters especially the northerly and easterly portion. The dominant topographic feature in the eastern margin of the town is the Vigan Gap Hill where the continuously shifting Vigan River cuts its way. The Gap Hill is located approximately 10 kilometers east of Vigan. Weather and climate The city of Vigan falls under (tropical savanna), which is characterized by two pronounced seasons – the dry season which starts from the month of November and ends in April; and the wet season which spans the months of May to October. The average annual rainfall is and the heaviest registered rainfall is . The average temperature is 26 °C. The average warmest temperature is 30.9 °C and the average coldest temperature is 21.1 °C. An average of 7-10 typhoons visit Vigan annually. The average relative humidity is 81%. Demographics