25

CHAPTER - II

COASTAL GEOMORPHOLOGY

2.1. General

As the study area is a part of the Cauvery Delta, the coastal landforms

are not only of marine origin but also of fluvial and fluvio marine.

Though major part of the area is covered by deltaic alluvial plains, the shore

zone part contains marine dominated landforms. As strong littoral currents

are prevalent in the near off shore region, their effects are also observed in

the near shore landforms. As large thickness of sediments from Gondwana to

Recent constitute the coastal plain, their relation with the hinterland rocks

and the isostatic equilibrium between them was the matter of concern for

geomorphologists in early 1900. Hence in order to show a comprehensive

picture of the coastal geomorphologic setup with the neighboring landforms,

a brief description of the regional geomorphology covering the adjoining

regions is given below.

2.2. Regional geomorphology

The terrestrial morphodynamics of Tamilnadu has been a field of

major interest among geomorphologists, despite the controversy whether

the generation of planation surfaces were formed by peneplanation or

pediplanation (Vaidyanathan, 1975; Babu 1975; Subramanian et. al., 1980;

Radhakrishna, 1993). The concept of peneplanation is favoured by many, as

it is evidenced by the occurrence of erosional remnants with vast plains in

between them in most part of the country. The peneplanation processes are

found to be intermingled with the block-faulting movement, resulting in the

formation of hill massifs such as the Nilgiris, Kodaikanal-Palani hills and

Pachchamalai-Kollimalai hills. As a result, the hinterland forms a peneplaned

country consisting of features mainly those that have escaped from

denudation and those of block faulting segments with intervening large

stretch of areas of plains and plateaus. But there are regions (around

Tindivanam and Chengalpet) where typical pediplain landform, formed by

coalescence of pediments and bajadas are found to occur.

26

The resultant hinterland topography, representing features ranging in

height form 100m to 2700m above MSL, is found to have been distributed in

elevations of 4 or 5 levels of planation surfaces. These features are made up

of Precambrian rocks of granites, granite gneisses, granulites, charnockites

and other associated rocks and are traversed by number of deep fractures

that have emplaced metallogenetic intrusive rocks in some places (Grady,

1971; Srinivasan, 1976; Vemban et. al., 1977). The structural trend follows

predominantly the Eastern Ghat strike lines having NE-SW trend, but they

turn to E-W towards the western part of Tamilnadu. These trend lines are, in

general, found to be parallel to the coastal configuration.

The Mesozoic and Tertiary sediments in the coastal sedimentary basins

are ostensibly the product of weathering and erosion of these crystalline

rocks. Oil and Natural Gas Corporation’s bore well data in the Cauvery basin

indicate the occurrence of more than 6000m thick sediments ranging in age

from Late Jurassic to Holocene. The resulting isostatic imbalance created by

the deposition of the huge thickness of sediments in the Cauvery basin had

been responsible for the cratonic uplift in hinterland regions in equal rate to

the sinking of coastal basins (Sastri et. al., 1981). The middle Jurassic

planation surface traced at an altitude of 2400m is considered as a result of

this continental crustal elevation. The continuation of such isostatic

movements in large scale even during Tertiary and Quaternary are well

indicated by the occurrence of huge thickness of Cuddalore sandstones of

Mio-Pliocene period and deltaic and other deposits of Quaternary age along

the east coast of India. This sequence of tectono-geomorphic events

question the long considered view of the tectonic stability of the Indian

peninsula and it is now becoming increasingly regarded that the Archean

cratonic shield also has had its Quaternary earth movements. Earthquakes

in Latur area, Maharashtra and many instances of tremors reported around

Chennai, Salem, Cuddalore and Villupuram have attested the view of tectonic

instability in the cratonic shield.

2.3 Coastal landforms

The coastal lowlands of the study area (Kattumavadi to Chidambaram)

with elevation up to 20m from MSL (except Cuddalore sandstone uplands

27

which rise upto 80m) exhibit wide-ranging features of fluvial, fluvio-marine

to marine origin. Biogenic and planation forms are also observed. The shore

zone part of the area consists of discontinuous late Pleistocene and Holocene

barrier beach ridges. In between the beach ridges swales, linear depression

with clay and silt enriched, are found to occur. The barrier beach ridge

complex is bordered in the west by wide ranging coastal landforms.

As the coast is of open nature, wave dominated landforms are abundant.

Tidal landforms are noticed around Muthupet lagoons.

In order to understand the morphogenetic set up of the landforms and

their evolution, coastal geomorphology map of the study area was prepared.

Aerial photographs and Satellite images were interpreted to prepare a base

map for the field work. Special attention was devoted to the identification

and classification of landforms. All the geomorphic features interpreted from

aerial photos were transferred to the base map. Careful incorporation of all

the landforms interpreted from aerial photos and imagery was made. Coastal

geomorphology maps were prepared for each segment of the study area (Fig

2.2. to 2.9) and a comprehensive map (Fig.2.10) was prepared compiling all

the maps. The index map for various segments for which coastal

geomorphological maps were prepared is given in fig. 2.1. The geomorphic

features observed in the study area are beaches, beach ridges, swales, delta

plain, mudflats, paleo lagoonal plains, rivers, abandoned river channels,

point bar, lakes and tanks, mangroves and lagoons (Fig.2.10).

The field photographs illustrating the landforms are shown in Plate-1.

Beach ridges and associated landforms are found throughout the

length of the shore zone part of the coast. Beach ridges raised up to a height

of 6m are observed. Cuddalore sandstone uplands are found to occur in the

northwest and middle west part of the study area. The uplands in the north

are overlain by a gravel deposit that forms a high ground. The deltaic alluvial

plain dominates the major part of the study area and contains sediments

derived from mainly by Cauvery river from hinterland and weathered and

eroded materials of adjoining sedimentary rocks.

28

Each landform explicitly indicates the type of natural processes

involved in its formation and distribution. Depositional landforms dominate

the region except low beach cliff observed in a few places, which are product

of erosion.

The landforms noticed in the study region can be classified as given in

the following tables.

Table - 2.1. Classification of coastal landforms of the study area.

Process Erosional Depositional

Marine Beach cliff Beaches Beach ridges-Older and Younger Paleo-lagoonal plains, Mud flats

Fluvio-Marine --- Deltas, Point bars

Fluvial --- Abandoned river channels

Aeolian --- Dunes

Biogenic Mangroves, Salt marshes

Others Lagoons, Tidal Creeks

Plantation Cuddalore sandstone uplands

Table – 2.2. Areal details of landforms in the study area

Landforms Area in sq.km Abandoned river channels 5.00 Back waters/Lagoons 130.00 Beach 20.00

Beach ridge plains 495.00

Cuddalore sandstone uplands 1875.00

Delta plains 6920.00

Lakes/Tanks 115.00 Mangroves 13.00 Mud flats 385.00

Natural levee 5.00

Old beach ridges 165.00

Paleo lagoonal plains 385.00

Point bars 15.00

Rivers 240.00

Sand bars 4.00

Tidal creeks 1.00

Sea 2880.00

Total 13653.00

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2.4. Distribution of landforms

Deltaic alluvial plain and beach ridge plain cover major part of the

study area. Beach ridges are prominently well-developed between

Nagapattinam and Adirampatinam. The series of beach ridges with

intervening swales form a triangular landscape between Adirampatinam,

Nagapattinam and Muthupet. Beach ridge plain to a width of 10 km is

observed here. The width of beach ridge plain decreases towards both the

north and south and a few narrow ridges are noticed north of Nagapattinam

and south of Adirampatinam. The beach ridges occurring near

Adirampatinam, Muthupet, Thiruthuraipoondi and Velankanni align NE – SW

direction (Fig 2.4). These ridges are flattened and stabilised with vegetation

and have bleached sands. They are considered as older to the ridges

occurring near shore zone. The younger beach ridges are curvilinear in shape

between Muthupet and Vettaikaranirupu and progressively the ridges have

taken two directional developments. The younger ridges show EW alignment

between Adirampatinam and Point Calimer and NS alignment between

Vedaranyam and Nagapattinam. The occurrence of younger beach ridges in

this fashion gives triangular shape to the region. Swales are present in

between the beach ridges. The swales occurring between the older beach

ridges are designated as paleo lagoonal plains and swales observed in the

sea ward side of younger beach ridges are designated as mudflats as they

border the present Muthupet Lagoon.

Fifteen abandoned channels formed by both avulsion and cut-off

processes are found traversing along various parts of the delta.

The distribution of abandoned channels explicitly indicates that the river

Cauvery has migrated from Kattumavadi in the south to the present

Coleroon in the north. Another lagoon occurs at Sembodai.

An elliptical shaped lagoon is found to occur near Muthupet.

The lagoon is bordered on the sea side by barrier ridges and on the landward

side by mudflats. Thick Mangrove vegetation is noticed around the lagoon.

All the features mentioned above are bordered in the west by Cauvery

deltaic alluvial plain that extends inland up to 70 km, where it is bordered by

30

hinterland features. In the northwest and middle west part of the study

area, Cuddalore sand stone uplands occur. These uplands are bordered in

the west again by hinterland features.

2.5. Description of landforms

2.5.1. Beaches

The land form that exists along the entire coast of the study area is

sandy beach. Beaches ranges in width from a few meters to 2 km are

noticed. Since the coast is open and wave dominated, beaches are dynamic

and subjected to frequent changes in profiles. The dominant wave actions

with large amount of input of sediments derived through long shore littoral

currents make the beaches as the most dynamic landforms of the study

area. Though the beach accretion is found in its entire length indicated by

barrier beaches, erosion is also observed around Kattumavadi, Chathram,

Rajamadam and Tarangambadi. Beaches are generally made of medium to

fine sands with moderate to well sorted characteristics. In some places

beach sands are found overlie the delta plain indicating that the beach sands

have been laid over them during the later transgression of sea over the

delta. Wide beaches are observed around Nagapattinam, Velankanni,

Poompuhar and Vedaranyam. The beaches of Karaikal, Nagapattinam,

Nagore and Velankanni are famous for tourist interest.

No marked beach is observed between Kattumavadi and Vedaranyam.

A narrow stretch of beach is only observed in these regions. Alluvial and

deltaic plains are observed underlying these sandy formations very near to

the shoreline indicating that the sea is under transgression making the older

deltaic plains to submerge under water. Beaches are observed in small

patches (1.5 km) North of Setubava chattram, south of Manohara (200m),

near Agniar river mouth (1km) and east of Rajamadam (300m). Beaches

are continuous in the area between Vedaranyam and Kodiyampalayam. Here

the beaches ranges in width from 20m into 1.8 km are observed.

The width of the beaches at Vedaranyam is 1.7km, at Vettaikaranirupu 3km,

at Velankanni 2.5km and at Thirumullaivasal 3km. Total area covered by the

beaches is 20 sq.km

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2.5.2. Beach ridges and Swales

Beach ridges are defined as linear sandy-shelly swash deposits with

intervening sandy plain or swales occurring parallel or sub-parallel to the

shore formed by periodic wave impounding action (Short et. al., 1989).

Similar to beaches, beach ridges are also found to occur as

discontinuous ridges along the entire coastal stretch of the study area.

These beach ridges ranging in height from 0.5 to 6 m are noticed in the

shore zone parts of the delta. The width of the beach ridges plain varies from

2km to 10 km. The widest beach ridge plain is observed between

Adirampatinam and Nagapattinam. In the area North of Nagapattinam a few

ridges in discontinuous patches are noticed. In the area South of

Adirampattinam a narrow stretch of younger beach ridges are observed.

There are two sets of beach ridges observed in the study area and they are

named as younger and older beach ridges.

The older ridges contain yellow colour bleached sand and occur along

the land ward margin of the beach ridge plain. The direction of older beach

ridges changes from NE-SW direction to EW in the area around

Thiruthuraipoondi and Adirampattinam and to NS direction in area North of

Nagapattinam and South of Adirampattinam. The older beach ridges are

stabilized and flatted by the terrestrial processes. The sands are bleached

and yellowish in colour. The sands are fine in nature and well sorted. The

comparison of the alignment of configuration of these older ridges with

respect to the present day shoreline helps to trace the direction of

progradation of delta and evolution of the coast. The younger beach ridges

contain recent loose sands and unbleached unlike older beach ridges. In the

Point Calimer region these younger beach ridges form a barrier environment

that helps to develop Muthupet lagoon. The creeks in the barrier serve as

inlets for the lagoon. The older ridges have been dated to 1, 25, 000 years

BP and younger ridges to 6000 years BP (Bruckner 1989, Loveson and

Rajamanickam 1988, Banerjee 2000). In between the older and younger

beach ridges paleo lagoonal plains are noticed. The occurrence of the

lagoonal plains suggests that lagoons existed in the region behind the

32

barriers when the older beach ridges were formed similar to the present

Muthupet lagoon occurring behind the barriers now.

a) Older beach ridges

Older beach ridges are observed in the area between Rajamadam in

the south and Chidambaram in the north as discontinuous patches.

Prominent older ridges are observed around, Muthupet, Thiruthuraipoondi

and Kollidam (Fig 2.4 & 2.9). Older beach ridges are absent between

Kattumavadi and Rajamadam (Fig. 2.2 & 2.3).

In the area north of Rajamadam older beach ridges are observed

about 1km from shore line (Fig 2.3). The width of the ridges ranges from 50

m to 350 m and length extends up to 5km. These ridges are mainly found in

the area between Agniar and Nasuyunniar. They are bordered by mudflats in

the seaward side and by deltaic alluvial plain in the landward side. The ridges

align NE-SW direction and more or less parallel to the present shore line.

The ridges contain yellow sands of medium to fine size with well sorted

nature.

The older ridges are prominently well developed between

Adirampattinam and Velankanni as discontinuous linear patches (Fig. 2.4).

The ridges align N60° E around Muthupet, N40°E around Thiruthuraipoondi

and N25°E around Velankanni.

The ridges around Muthupet extend to a length of 23km and to a

width of 1km to 1.5km (Fig. 2.4). The ridges are bordered in the seaward

side by younger beach ridges and paleo lagoon plains and in the landward

side by paleo lagoon plains and deltaic plains. The older ridges occur

overlying the paleo lagoonal plains.

The ridges around Thiruturaipoondi extend to a length of 17km and

the width varies from 1km to 5.5 km (Fig. 2.4). The ridges are bordered on

all the sides by paleo lagoon plains except a small portion which is bordered

in the landwards sidle by deltaic plains. The ridges contain yellow sands,

medium to fine grain size and moderately sorted nature.

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In the region south of Velankanni an older beach ridge to a length of

8.5km and width of 1.5km is observed (Fig. 2.4). The ridge occur over lying

the paleo lagoon plain. The ridge is bordered on all sides by paleo lagoon

plains. The ridge contains yellow sands in medium and well sorted

characteristics.

Older ridges are observed in discontinuous small patches between

Nagore and Thiruvattangudi (Fig. 2.5 & 2.6). South of Nagore, a beach ridge

to a length of 3.6km and a width of 900m is observed. The ridge aligns in the

NS direction and parallel to the shore line. In the area North of Nagore, a

beach ridge to a length of 6km and to a width of 1.7km is observed.

The ridge aligns N 12° W. The ridge contains yellow colour sands with

medium to fine size and moderately sorted characteristic. An older ridge is

observed around Thiruvattangudi. It extends to a length of 16.8km and the

width ranges from 1km to 2.2km. The ridge is found 2km away from the

shore line. The ridge aligns in the NS direction. The ridge contains yellow

colour bleached sands in fine size and well sorted characteristic.

Around Tarangambadi, older beach ridges are observed (Fig 2.7).

Ridges to a length of 5.7km and width of up to 4km are observed.

The ridges align NS direction and occur 3km away from the shore line.

The ridges are bordered an all sides by deltaic alluvial plain. The ridges occur

over lying deltaic alluvial plain.

A small patch of beach ridge is observed in the region west of

Thirumullaivasal (Fig. 2.8). The ridge is bordered by paleo lagoon plains in

the landward side and by younger beach ridges in the seaward side.

The ridge occurs over lying paleo lagoonal plain. The ridge aligns in the NS

direction parallel to the present shore line. The ridge occurs 3km from the

shore line. The length of the ridges is 3km and width is upto 1.2km.

Older beach ridges are observed around Kollidam in scattered patches

(Fig.2.9). The patches occur from 4km to 15km away from shore line.

The ridges occur over lying paleo lagoonal plains. They are bordered by paleo

34

lagoonal plains and younger beach ridges in the seaward sides and deltaic

plain in the landward side. The ridges align parallel to the shore line.

The ridges extend to a length of 10km and to a width of 7km.

The alignment of older beach ridges marks the direction of paleo

shoreline existing at the time of formation of the older ridges. The shoreline

of the past existed in these places would have had the configuration similar

to the configuration of older ridges i.e. the older beach ridges denote

strandline.

b) Younger beach ridges

Younger beach ridges are found all along coast of the study area

except in the region between north of Nagapattinam and Tarangambadi.

In the area between Kattumavadi and Ammanichattram, younger

beach ridges are found parallel to the shoreline as discontinuous ridges

(Fig 2.2).

The ridges occur 0.3 to 0.7 km away from the shore line and extend

all along the coast. The width of the ridges varies from 0.5km to 1.5km.

The ridge aligns almost parallel to the shore. The ridge is bordered in the

seaward side by mudflats and on the landward side by deltaic plain.

The ridges occur over lying deltaic plain. The mudflats also occur over lying

deltaic plain. Number of lakes and tanks (both natural and manmade) are

observed in the deltaic plain region. The ridges contain buff colored sand

with medium size and well sorted characteristics.

The younger ridges from Ammanichattram area continue in the north

upto Rajamadam (Fig 2.3). The ridges around Manohara and Malipattinam

occur near the shore line. The ridges to a length of 17km and to a width of

1km are observed. The ridges are bordered in the east by recent beaches

and in the west by deltaic alluvial plain. The ridges align parallel to the shore

line. The ridges contain buff coloured sands with medium grain size and

moderately sorted nature. Numerous lakes and tanks are observed in the

deltaic plain in the region south of Agniar River.

35

Prominent younger beach ridges are found to occur from

Adirampattinam to Nagapattinam (Fig.2.4). The distribution of younger

beach ridges makes this region in to a triangular prograded region.

The younger beach ridges plain extend up to 10km in land around

Vedaranyam. Along the western side of the younger beach ridge plain, the

ridges align in an arrow shaped fashion. These ridges progressively bifurcate

in two directions towards the seaward side. In the region between

Adirampattinam and Point Calimer the ridges align EW direction and between

point Calimer and Nagapattinam in NS direction. The younger beach ridges

are bordered by paleo lagoonal plain in the landward side and by mudflats in

the seaward side. In the area between Point Calimer and Nagapattinam the

younger beach ridges occur adjoining the shore line.

The ridges are continuous all along the coast from Adirampattinam to

Vedaranyam to a length of 41km and from Vedaranyam to Nagapattinam to

a length of 43km. The width of the ridges varies from 2.8 near

Adirampattinam to 15.3km near Vedaranyam in EW direction. The width of

the ridges around Vettaikaranirupu is 3km and around Nagapattinam 2.3km.

In the area between Nagore and Poompuhar (Fig. 2.5 & 2.6) younger beach

ridges are not found to occur.

In the region between Thirumullaivasal and Chidambaram

discontinuous younger beach ridges are observed (Fig. 2.8 & 2.9).

The younger beach ridges occur 0.5 km away from the shore line and extend

up to 5km. These ridges align parallel to the shore line and extend to a

length of 18km. The width of the ridges varies from 2.5 km to 6km.

The ridges are bordered by recent beaches in the seaward side and paleo

lagoonal plain in the seaward side. The ridges contain buff coloured sand in

fine size and well sorted nature.

Though the beach ridges are generally denotes the area of

progradation, it is surmised that the study area is an area of retrogradation

due to present transgression and these beach ridges were formed during the

earlier regression that followed the last interglacial and middle Holocene

36

transgression maximum. The ongoing transgression converts the swales in

between the ridges into lagoon around Muthupet. Total area covered by the

younger beach ridges is 495 sq.km.

2.5.3. Abandoned river channels

Abandoned river channels are stream channels that have been

abandoned either by cut off process i.e. the sudden abandonment of a part

of the channel or by avulsion i.e. abandonment of the whole of a channel

course (Reineck and Singh, 1986). Generally cut-off processes are associated

with sedimentation and sea-level oscillations and avulsion is associated,

either with tectonism when the sudden abandonment of whole of a channel

course or with the filling up the channel due to extreme increase in the rate

of sedimentation and reduction in depth.

15 Abandoned river channels have been located traversing along the

deltaic alluvial plain (Fig. 2.11). Both avulsion and cut-off have involved in

the abandoning process. All the abandoned channels found to occur in the

area south of the present river course (Coleroon) have been formed by

avulsion process except the two abandoned channels (No. 14 & 15) formed

by cut off processes. The network of abandoned channels and their pattern

clearly exhibits the northerly shifting of the river course from Kattumavadi in

the south to present Coleroon river in the north.

The abandoned channel (No.1) noticed near Kattumavadi ends in the

coastal region where a paleo micro delta occurs. The occurrence of micro

delta and the abandoned channels suggest that delta progradation had been

initiated in this region. The micro delta has number of in land lakes and

tanks. The abandoned channel merges with the river Kakkar found at the

middle of the micro delta.

Three abandoned channels are noticed (No.2, 3 & 4) around

Orathanadu. The channel No. 2 extends from base of the Cuddalore

sandstone upland to the river Maharaja samudram. The channels No. 3& 4

(Rajamadam branch canal and Kaliyan odai branch canal) extend from the

region at the base Cuddalore sandstone to the region where strand line

37

formed by older beach ridges occurs. The channel No. 4 merges with the

river Vadavar canal. All these abandoned channels contain clays to a depth

of 2.5 m with sand at the bottom.

An abandoned channel (No. 5) is observed extending from the bank of

Vettar River to Muthupet. The channel bifurcates into two (No. 5 & 6) south

of Mannargudi. The channel No. 6 extends up to Thiruturaipoondi.

The channels 5 & 6 also contain clay and silt in alternate layers up to a depth

of 3m and sand at the bottom.

An abandoned channel (No. 8) extending from the bank of Vettar River

to Thiruthuraipoondi is observed. The channel is trifurcated into channel

No.7, 8 & 9 in the region north of Kattur. All these channels extend up to the

stand line formed by older beach ridges and paleo lagoon plains.

These abandoned channels are filled with clay to a depth of 3.5m and sand

at the bottom.

Another abandoned channel (No. 10) north of Vettar River aligning

parallel to the river is observed This channel bifurcate into two (No. 10 to11).

The channel 11 extends up to the region north of Nagapattinam and merges

in to Kaduvaiyar River. The channels also contain clay plug and sand at the

bottom.

The abandoned channel No. 12 is observed parallel to Thirumalaranar

River. The channel commences from the southern bank of River and extends

up to Thirumalairajanpattinam.

The abandoned channel (No. 13) observed parallel to the present

Cauvery River extends from Kumbakonam to Tarangambadi. The channel

starts from the bank of Cauvery and align parallel to the Cauvery up to

Mayavaram and turns to SE towards Tarangambadi.

Two abandoned channels (No. 14 & 15) formed by cut off process are

observed on the southern and Northern banks of River Coleroon.

These channels contain clay with sand at the bottom. The channel No. 14

38

has been formed by neck cut off process and channel No. 15 has been

formed by chute cut off process.

All the abandoned channels contain channel fill deposits.

The observation along well sections and hand auger data indicates the

occurrence of clay plug in it with a layer of sand at the bottom.

The abandoned channels are filled with mainly clays indicating that the

channels had stagnant water for long time. In many locations at the bottom

of the clay sand layers are observed. The grain size analysis of a sample

indicates 95% of silt and clay. The process of filling up of the channel has

taken place faster at the margin than at the centre, as observed elsewhere

by Allen (1965).

2.5.4. Delta plain

In broader terms, a delta can be defined as those coastal deposits,

both sub-aqueous and sub-aerial derived from river-borne sediments.

Also included are those sedimentary deposits moulded by various marine

agents such as waves, currents and tides that are found within the deltaic

plain (Coleman, 1981). As the delta development is controlled by various

agents like river, waves, tides, currents and wind wide ranging geomorphic

features are usually associated with the delta plain.

A large deltaic alluvial plain interspersed with distributaries and

abandoned channels covers major part of the study area. The plain is made

up of a large thickness of alluvial soil that makes the region fertile, mainly

for the cultivation of paddy, sugarcane, banana and coconut plantation.

The delta has its apex near Tiruchirappalli and spread in a triangular fashion

to a total area of 13,650 sq. km. The Cuddalore sandstone uplands

occurring north of the river Coleroon and around Orathanadu region restrict

the spreading of delta and forms western boundary to the delta plain.

The delta plain forms more or less a horizontal to very gently inclined

surface. The altitude of the delta goes up to 20 meters near Kumbakonam,

Valangaiman and in adjoining regions. The alluvial plain extends up to the

shore in the region between Nagapattinam and the mouth of Coleroon.

The beach ridge plain is narrow in this region. The archaeological remains

39

that occur in the off shore regions of Poompuhar suggest that sea is

transgressing along this part of the coast. It is surmised that the

transgression had submerged the beach ridges formed earlier and make

beach ridge plain narrow here. It also denotes that the delta had extended

for beyond the present limit into the sea during the regression of the sea

following Middle Holocene transgression.

The network of abandoned channels and other related features like

flood plain and natural levees indicate that the development of modern delta

commenced along the south and southeasterly portions i.e. the areas around

Kattumavadi (Fig 2.2). Shifting the domain of sedimentation towards north

successively, the river Cauvery changed its deltaic regime from time to time

and at present only the northern margin of the delta bordered by river

Coleroon is found to be active. The details of delta characteristics are

provided in table 2.3.

Table – 2.3. Characteristics of Cauvery Delta

The sea ward boundary of the delta plain from Kattumavadi to

Nagapattinam is in an arc shape and from Nagapattinam to Kodiyampalayam

is in a straight line. The southern part of the delta plain around Kattumavadi,

Rajamadam and Pattukottai contain number of inland lakes.

Basin Climate Humid sub-tropical

Rainfall (mm) 750

Basin area (sq.km) 85000

Channel length (km) 1280

Channel type Straight in the upstream and meandering in the

coastal plain

Climate Humid sub-tropical

Delta area ( sq.km) 6920

Tectonic setting Subsiding basin

Shoreline length(km) 225

Delta channel Pattern I order

40

In the middle and northern part of the delta such lakes are only a few.

A large inland lake (Veeranam Eri) is observed south of Chidambaram

bordered in the west by Cuddalore stand stone.

The delta plain contains mainly fluvial sediments. In the subsurface

samples collected in the bore wells at Valangaiman and Nannilam, the

sediments exhibit fluvial characteristics only. The marine sediments are

completely absent in the bore wells up to a depth of 50 m. Except near shore

regions where older and younger beach ridges with associated landforms are

noticed, the entire deltaic plain shows no trace of marine influence.

The delta plain is made up of sediments of fluvial process derived by the

river Cauvery and its distributaries. The sediment characteristics of these

bore wells samples are described in Chapter 3.

The sub-surface geology of the delta plain of the study area has been

well studied since 1960’s by Oil and Natural Gas Corporation Ltd, India,

during their petroleum exploration. The sediments ranging in age from

Middle Jurassic to Recent are present here. But large scale development of

the delta is believed to have commenced only during Middle Cretaceous as

indicated by the occurrence of coral reef limestone of Lower Cretaceous age

in the western margin of the delta. The thickness of sediments in the study

area is found to be over 6000m and hence, the deltaic alluvial plain is only a

surficial part of the larger basinal sediments. The areas occupied by the

outcrops of Mesozoic sediments became areas of non-deposition positively

during late Paleocene and only the areas occupied by Cuddalore sandstone

continued to be under deposition till Pliocene (Datta and Bedi, 1968). The

alluvial tract further to the east up to the coast became positive only during

Quaternary and the sedimentation is still continuing in the offshore region.

With the help of geophysical studies, mainly gravity and seismic, the

basement configuration and structure of the study area have been brought to

light (Sastri and Raiverman, 1968; Raiverman et. al., 1966; Kailasam,

1968). The basement rocks of study area have undergone number of

periods of subsidence along NE-SW aligned faulted segments. While most of

the areas in the Cauvery basin underwent subsidence during one or more

41

cycles of subsidence, certain areas remained tectonically positive indicated

by the basement ridges of the present day. As a result, the basement is

formed by a series of ridges and basins (Fig. 2.12). The occurrence of such

ridges in the basement and their reactivation during Quaternary had its

influence in the shifting of the regime of sedimentation. The reactivation of

the basement faults and related tectonic effects are observed during

Quaternary and even in modern days. The tectonic activity along these

basement faults made the river Cauvery to shift its channel in a preferential

direction. Total area covered by the alluvial plain is 6920 sq.km.

2.5.5. Lagoons, Salt marshes and Mangrove swamps

Lagoon is a shallow stretch of sea water, such as channel, pond or

salt-water lake, near the sea and partly or completely separated from it by a

low narrow, elongate strip of land, such as a reef, barrier island, sandbank or

spit. It often extends roughly parallel to the coast and has a narrow inlet.

A major lagoon noticed in the region is Muthupet lagoon. The lagoon

occurs behind the recent barrier bars formed between Point Calimer and

Adirampattinam. The lagoon has many tidal inlets in the barriers.

Thick mangrove plantation grows around the lagoon. The region between

the lagoon and younger beach ridges is covered by mudflats. The mudflats

bordering the landward side of lagoon indicates that the area of lagoon has

shrinked. The shrinking of the lagoons, since the Middle Holocene

transgression maximum has also been aided by infilling of sediments.

The narrow stretch of salt marshes around the lagoon marks the expansion

of lagoon during high tide time (not shown in figure as they occupy a small

area which can not be represented in the map). The lagoon expands and

shrinks diurnally during high tide and low tide respectively. The sketch of the

lagoon and associated features is shown in Fig. 2.13a. The areal extend of

the lagoon is given in table 2.4.

Table 2.4. Areal extend of Muthupet lagoon

Place Total area

sq.km Area under water sq.km

Area under mudflat/salt marshes

sq.km

Muthupet 72 18 54

42

The lagoon extends from Rajamadam to Point Calimer in the EW to a

length of 8 km and from Point Calimer to Vedaranyam in the NS to a width of

2.km.

The occurrence of Muthupet lagoon in the southern part of the study

area is also an indication for the confluence of river Cauvery here in the past,

as the places of occurrence of lagoons generally indicate the area of paleo

river mouths in other parts of Tamilnadu coast (Anbarasu 1994).

The distribution of abandoned channels, the occurrence of Muthupet lagoon,

the occurrence of a micro delta around Adirampattinam and the location of

present day channel of river Coleroon suggest that the river Cauvery had

migrated towards northward right from the place around Adirampattinam.

Though the river that debouches in to the sea near Poompuhar is designated

as the river Cauvery, on the basis of the nature, width, size and

geomorphologic setup of the river the river Coleroon is the original river

(Cauvery) that continues from Tiruchirapalli that forms the apex of the delta.

Many distributory channels of the present day river had served as main river

in the past and they are presently abandoned channels through which a

channel flows as a distributory.

A lagoon is noticed in the area south of Vetaikaranirupu in the sea

ward side of older beach ridges. It occupies a small area in the paleo

lagoonal plain and surrounded by mudflats in the East. The lagoon receives

water from the sea through a channel that runs to a length of 20km. Since

the lagoon is far away from the sea the diurnal expansion and shrinking is

not observed. But the lagoon also had expansion in the past as indicated by

mudflat observed in the east and north of the lagoon. The sketch of the

lagoon and associated features is shown in Fig 2.13b. Small Lagoons are also

noticed North of Kaveripattinam, Uppanar river mouth, Mattampattinam

village and NW of Kodiyampalayam. The total area covered by lagoons in

the study area is 130 sq.km.

Salt marshes are defined as marshy lands that are periodically flooded

by sea water. In other words, salt marshes are vegetated mud flats. While

the mud flat regions are completely submerged under sea water during high

43

tide, the salt marsh area gets water only through intricate creek network

during high tide (John Pethick, 1984).

Salt marshes are found to occur in the study area only around

Muthupet lagoon. The plants noticed in this area are Saliconia and

Arthrocnemum with sea grasses of Halodule, Cymodocea, Syringodium,

Enhalus, Thalassia and Halophilla ovalis.

The occurrence of salt marshes and mudflats around Muthupet lagoon

suggest that the lagoon has expanded during Quaternary transgressive

phases and shrunk during the regressive phase of the sea. The sedimentary

sequences of the region also exhibit progradational and retrograde

sequences indicating the rise and fall of the sea during Quaternary.

Total area covered by the Mangrove swamps and Salt marshes is 13 sq.km.

Tidal creeks are inlets, through which water flows to and from a

lagoon or other backwaters during high and low tides respectively.

Tidal creeks are found in number of places in the study area, the important

among them is located at Thiruthuraipoondi. Similar to lagoons, the width of

the tidal creeks also gets enlarged and reduced during high and low tides,

respectively.

Mangroves are trees and bushes inhabiting tidal lands in the warm

countries. The mangrove vegetation growing irregularly around Muthupet

lagoon consists of Rhizopora spp, Avicennia marina, Bruguiera cylindria,

Aegiceras Corniculata, Ceriops decandra etc., (Meher and Homji, 1973).

About 15 sq.km area is under mangrove vegetation.

2.5.6. Cuddalore sandstone upland

Cuddalore sandstone is a Mio-Pliocene formation found to occur in

Tamilnadu in detached patches from Marakkanam (Northern Tamilnadu) to

Sivaganga (southern Tamilnadu), in the western margin of Cauvery

sedimentary basin, and also in the area north of Madras which extends into

the adjoining Andhra Pradesh state. These Cuddalore sandstones develop

uplands invariably in all their locations with/without laterite capping. These

44

uplands are formed by differential erosion due the occurrence of laterite

capping which is more resistant to erosion than the adjoining formations.

The laterites are resistant to erosion because they become hard like a brick,

when exposed to rain and sun alternately. For this reason Buchanan (1807)

coined the term “laterite”.

In the study area, Cuddalore sandstone uplands are found to occur in

two regions, one around the area west of Orathanadu and other in the area

north of Coleroon River around Udaiyarpalayam. The height ranges from 10

m to 80 m. The removal of laterite from these uplands during anthropic

activity makes this upland subject to fast erosion and as a result, deep

gullies and bad land topography have been developed in some places.

The top portion of this upland is, generally, horizontal to gently slope

towards east. Many geologists call it as table land. The Cuddalore sandstone

is buff to red coloured due to the laterite cover. It contains mixture of

sediments of silts, sand and boulders. It is the host rocks for lignite deposits

occurring in Neyveli and Jayamkondam region (Not in the study area).

Total area covered by the Cuddalore sand stone uplands is 1875 sq.km.

2.5.7. Paleo - Lagoonal plains

Paleo lagoonal plains are observed bordering the older beach ridges in

the region between Muthupet and Nagapattinam (Fig 2.4).

They are originally swales developed between older beach ridges and had

been flooded by sea water for long time to form clay and silt deposits.

The land ward Margin of paleo lagoonal plains is noticed along the area west

of Muthupet and Thiruthuraipoondi. The paleo lagoonal plains are bordered

by deltaic plain in the land ward side and by younger beach ridges in the sea

ward side. The paleo lagoonal plain forms a vast stretch of horizontal

country extending to a length of 53 km from Muthupet to Nagapattinam.

The width of paleo lagoonal plains is 4 km near Muthupet 7.5 km from

Thiruthuraipoondi and 4.5 km near Velankanni. The paleo lagoonal plain

contains mainly clay and silts of marine origin. The shells of marine animals

like Cardium spp, Cardita spp, Murex spp etc are found to occur.

The sediments of older beach ridges lie over the paleo Lagoonal plains.

The paleo lagoonal plains are nothing but mudflat developed when the older

45

beach ridges were formed. To distinguish them from the mudflats formed

during the formation of younger beach ridges, they are named as paleo

lagoonal plains.

2.5.8. Mud flats

Mud flat is a flat area containing a fluid to plastic mixture of finely

divided particles of solid materials mainly silts, clay and water.

They are always associated with the sheltered environment like lagoons,

estuaries and other embayment. The difference between the forms of

sheltered environment and open coast are not of morphological changes, but

of changes in sediments.

Mud flats are observed adjoining the shore line in the area between

Kattumavadi and Manohara (Fig. 2.2 & 2.3). The beach is absent in the

region. The mudflats are bordered in the land ward side by younger beach

ridges. Palk bay in the region provides a sheltered environment with low

energy waves and high tides. This condition favours the formation of

mudflats along the shore zone part.

Mudflats are observed in the regions along Rajamadam,

Adirampattinam, Kodiakarai and Point Calimer. The mudflat borders the land

ward side of Muthupet Lagoon. The expansion of the Lagoon during earlier

transgression, high tide and storm season has favoured the development of

mudflats here. Bordering the lagoon mangroves have grown and salt

marshes have developed. The mudflat regions contain marine clay and silt

with lot of marine animals shells.

While the swales formed during the time of older beach ridges have

been made into paleo lagoonal plains, the swales formed during the time of

younger beach ridges have been made into mudflats. The swales that are

found behind the recent barrier ridges are made into lagoon.

The barrier beaches occurring in the seaward side of the backwaters

create a sheltered environment, which has favoured the development of mud

flat.

46

A small patch of mudflat is present in between Nagore and Karaikal in

the middle part of the study area. Mudflats are also observed near Uppanar

river and Coleroon river. Total area covered by the Mudflats is 385 sq.km.

2.5.9. Tanks / Water bodies

A number of tanks of fresh water in varying dimension are found in

the deltaic plain from Kattumavadi to Rajamadam (Fig. 2.2 and 2.3).

Many tanks are in crescent shape. These tanks receive water during

monsoon season and keep water for three to four months after the end of

monsoon. Irrigating the water intensive cultivation is carried out in the delta

plain.

All these tanks have man made bund in the sea ward side and many

outlets of water occur for irrigation purpose. The general slope of the area

towards east makes the storage of water in crescent shape.

The satellite images of the area explicitly show an occurrence of micro

delta around Ammanichatram. Many numbers of tanks are observed in the

area in and around the micro delta. The deltaic plain in the other part of the

study area does not have such tanks except one or two. The micro delta

occurring in the region is a bird foot type. It is common for the bird foot type

of delta to have number of such tanks and lakes. These tanks receive water

through number of rain water gullies which are invariably connect with the

neighboring main stream.

A major tank observed in the northern part of the study region

(Fig. 2.10) is Veeranam Eri. The tank is bordered in the west by Cuddalore

sandstone and in the east by man made bund. The lake receives water both

from Coleroon and Vellar (which flows north of study area) river. It is one of

the major tanks used for irrigation purpose in Tamilnadu. It is also in the

crescent shaped indicating that the slope of the region is towards east. Total

area covered by the water bodies is 115 sq.km

47

2.5.10. Point Bar

Point bar is a wedge of sand occurring within the river course and

induce braiding of river channels. Point bars are usually occur in the loop like

portion of the river course are in the place where the energy of the water is

very low and deposition is very high.

Point bars are noticed along the Coleroon river course at many places

from Kollidam to mouth of the river. Total area covered by the Point bars is

4 sq.km.

2.5.11. Rivers

Two major rivers present in the study area are Coleroon and Cauvery.

Number of minor rivers are present the study area starting from south to

north namely Kattar river, Agniar river, Koraiyar river, Vellar river,

Koduvaiyar river, Tirumalairajanar river, Arasalar river, Ambanar river and

Uppanar river etc. Cauvery is a nomenclature used for the river right from

the place of origin (Kodagu) to the place of confluence at Poompuhar. But

from Kallanai geomorphologically the river Coleroon is the original

continuation of the river Cauvery. The designated river Cauvery from

Kallanai to Poompuhar is only a tributary. All the rivers mentioned above are

tributaries of the river Cauvery. Along the many abandoned channels of the

river Cauvery also a lean flow of water is noticed.