Folding and faulting apparently are absent in Vigan-Bantay Plain
except in the eastern and southern margin of the plain. In the eastern
margin, a north-south trending fault occurs transecting the Pre-Quarternary
rocks. The fault line runs from Sinait down to Bantay and Santa,
while the southern extension of the plain is bounded by another
structural line that trends northwest and southeast occurring from
Santa down to Narvacan and Santa Maria.
The land area of Vigan is generally flat. Slopes of 20 are found
at Brgy. V, Tamag, and Paoa. There are no forest reserves in the
city in terms of potential and raw materials. Bamboo is the only
minor forest product in Vigan.
The soil type of the city is classified into five (5).
First is the Bantay Loam, which is located at the southern portion
of Raois and is good as pastureland has a land area of 36.67 hectares.
The second is Umingan sandy which is good for upland rice and vegetables
and has a land area of 1191.57 hectares. The third is San Manuel
clay loam, which is best for bananas, cotton and vegetables has
an area of 1244.57 hectares. The fourth is Bantog clay - considered
as the most significant type for soil, which is best clay material
for making pots, jars and all earthenwares has a land area of 31.65
hectares. The fifth is the beach sand, which has a land area of
7.28 hectares, is generally found in the shoreline of all the beaches
of Vigan and is best for coconut plantation.