WORKING GROUP 8
HERATH
455
The Hinterland
It is possible to subdivide this into two landform units. The origin of the
first of these, in the south, is attributed to the material deposited by the Maha
Oya (Oya = river). The remainder of the hinterland is characterised by broad
valleys separating well rounded hills. These prominent hills do not show any
type of bedding or lineation. This rules out the possibility of their being sed
imentary in origin. They could possibly be granitic out-crops and their smooth
ly rounded nature could then be explained by exfoliation. The broad valleys
may have some relation to the lowering of the sea level during the Pleistocene
period when the rejuvenated rivers cut deep valleys which were filled up with
sediments during the post glacial rise of the sea level. Most of the valleys
disappear as they approach the alluvial plain. On closer examination it was
noticed that the two major valleys make a sharp turn at the region of contact of
the alluvial plain and the hinterland. This indicates that their present courses
have been determined by the former coastline. Few scattered hillocks emerge
in the area marked as the Old Alluvial Plain.
The Coastal Plain
The alluvial plain is completely devoid of rounded hills and broad valleys
(except for two at its edge). It appears to be predominantly a sandy plain,
part of which is completely flat and shows no structure on the air photographs.
In some other parts distinct sand ridges occur running from north-west to south
east, which in turn are truncated by a series of more pronounced ridges and
moats which are presumably younger. This is a clue to the coastal history of
the area. It may be assumed, with justification, that at some stage of its geolog
ical history, the coastline has been located inland from its present position.
The interruption of accretion in the north, due to some phenomena, and the
consequent double truncation, has finally resulted in the situation of the inner
northern part of the coast.
The appearance of successive beach ridges with pronounced moats is clear
evidence of an advancing coastline. The depressions are commonly referred to
as moats, swales, slashes, or furrows. There are many views on the origin of
these moats. Gilbert is of the opinion that each ridge is related to some kind
of exceptional storm which has thrown up sand and debris to form a ridge,
while La Fonda and La Jolla observe that hurricanes and tsunamis tend
more to destroy a beach than to build it. Davis strongly supports the view
that beach ridges are predominantly the work of successive wave action and
that they derive their material from the sea bottom. Changes in the course of
a river also tend to make uncomformable beach ridges. In the case being
studied, no such fossil river beds are seen in the photographs.
When one examines the present coast, its remarkably smooth, straight
character is very striking. However, the seaward side of the bar appears to be
slightly dentate. It is a well known fact that a coast of emergence is generally
