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]