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CHAPTER – IV GEOLOGY AND GEOMORPHOLOGY 4.1 Geology The very first memoir of the Geological Survey of India (GSI) was published in 1858 carries an article on the geology of Nilgiri hills (Blanford, 1858). He mentioned the geological observations made by Benza, (1836) and Ouchterlony, (1848). Later in 1862, Blanford studied the surface exposures of sedimentary rocks and correlated them to the cretaceous formations of England and France. Benza has given an account on the geology of the terrain between Madras and Nilgiris. Prinsep, (1832) has published an article on the iron ores of Salem, which had attracted the geologist of the 19th century. Shortly, Newbold, (1842) made a geological note on the occurrence of Chromite in Salem district. The mapping of geological formations was started in 1940 and extensive research works were made available about the geology, structures and tectonic evolution of the area. These detailed studies have opened the path for understanding the relationship between stratigraphy, sub-surface facies variation, and formation of various litho units in the study area. The geological formations, especially the occurrence of charnockitic rocks in Tamiraparani basin was investigated by Narayanaswami and Lakshmi, (1967). They concentrated mainly on the occurrence of charnockitic rocks in the western part of Tirunelveli district and adjacent Western Ghats. They considered that the charnockites in Tirunelveli district represents a geosynclinals association of sediments with basic intrusive and volcanic, and inferred that the charnockites were subjected to polyphase deformation, as a result of multiple folding. Other significant work on the geology of Tamiraparani basin was conducted by Balasubramanian et al., (1985); Balasubramanian and Sastri, (1987). They pointed out that the entire basin is occupied by three distinct WĂŐĞͮϲϳ geological formations namely archean, tertiary, and recent to sub-recent sands (Table 4.1). Around 90% of the aerial distribution of rocks type in the Tamiraparani basin consists of charnockites, calc gneisses, granitic gneiss, crystalline limestones, calc granulites, and quartzites of archean age. The tertiary formations and recent sediments occur in the form of a narrow zone parallel to the coastline with the thickness increasing towards the shore. Table 4.1 Geological formations in the Tamiraparani river basin Age Lithology Recent to sub- Soils, coastal sands, red Teri kankar, Tufa and laterites, calcareous sandstone and shell recent sands limestones Hard and compact calcareous sandstones and Tertiary shell limestone Unconformity Younger granitic and pegmatitic intrusions Composite gneiss, granitic mica gneiss Garnetiferous mica gneiss, Ambhibolites, epidiorites and pyroxene granutites Archaean Charnockites Crystalline limestones and calc granulites Quartzites Distribution Eastern region adjacent to the coastline Underlying the beach sands extended up to Srivaikundam Northern and central parts Northern part Southern and south western parts Western and northern parts of Tenkasi Central parts around Tirunelveli Ambasamudram (Source: Balasubramanian et al., 1985) A few regionally extensive and well-defined tectonic contacts, mostly traced in the Precambrian terrain can assist in grouping the rocks spatially. The terrain between the Palghat-Cauvery fault (PCA) and the Achankovil-Tamiraparani fault (AK) comprises of pink and grey migmatites, rocks of the charnockite and khondalite groups with discrete bodies of granites. Migmatised charnockite and khondalite groups of rocks are noticed to the south of the WNW-ESE trending Achankovil-Tamiraparani fault (AK). The rocks in the tectonic zone of Achankovil terrain are marked by cordierite. Close association between the rocks of the charnockite group with khondalite group is recognized in the study area (Narayanaswami, 1975). The rocks of the khondalite group are of metaWĂŐĞͮϲϴ sedimentary origin and their potentiality as a storehouse of crystalline limestone and graphite is recognized. In the southern part of the state (Kanyakumari, Madurai and Tirunelveli district), three generations of folds have been recognized. Here, NW-SE trending folds are cross-folded along N-S and NE-SW. Faults and fractures are noticed in the study area, in which Cheranmahadevi fault trends in N-S direction and the Achankovil fault trends towards NW-SE direction (Figure 4.1). The Kanyakumari – Tenkasi Belt (ktsb) is running roughly NW-SE direction to the south of the Tiruchirapalli-Madurai-Palayamkottai block (TMP). The Achankovil lineament (AK) marks the northern boundary of the belt and the ultramafic bodies near Arumanallur and two pyroxene granulite bands in the terrain may possibly represent the ophiolitic assemblages in the suture, marked by the Achankovil lineament. The ancient suture is possibly ruptured and later the emplacement of alkaline rocks such as the Putteti syenite and Chengannam alkali granite took place. Occurrence of khondalite group of rocks including garnet-sillimanite-graphite schist and gneiss, quartzite and calc-granulite are found in association with charnockites in the south of the suture zone, within the Tiruchirapalli-Madurai-Palayamkottai block (TMP). According to Narayanaswami and Lakshmi, (1967), different types of charnockites associated with meta-sedimentary rocks and gneisses occur in the Tirunelveli and Kanyakumari districts. The types of charnockites include acid microperthitic charnockite, intermediate gneissic charnockite, basic noritic charnockite (pyroxene granulite) and ultrabasic charnockite. The acid microperthitic charnockite occurs as conformable lenses and bands of different dimensions within garnetiferous gneiss and migmatites. The intermediate gneissic charnockite is the most prevalent and it is banded and foliated. Alternating bands of light colored microperthitic charnockite and layers of pyroxene granulite are seen in sheet-like form. WĂŐĞͮϲϵ Figure 4.1Map showing major faults, tectonics and lineaments in the study area (Source: GSI, 1998a & Landsat – SRTM DEM images) Hypersthenites, hypersthene bearing diopsidites and augitites are the rocks of the ultrabasic division. They occur as sporadic lenses and bands within garnetiferous gneiss. Gopalakrishnan et al., (1976) expressed the view that the pink quartzo-feldspathic granulite and the basic pyroxene granulite of the charnockitic group in Tamil Nadu represent metamorphosed acid and basic flows and the magnetite-quartzite bands in charnockite could be of volcanic exhalative origin. However, Sugavanam et al., (1978) opined that the charnockites in the northern part of the state owe their origin to deep seated granulite facies metamorphism of pre-existing volcanic and sedimentary rocks and gneisses. Near Melur in Madurai district, incipient formation of charnockite confined to certain major joint/fracture systems in garnetiferous quartzo-feldspathic gneiss has been reported by GSI, (1998b) and similar feature in garnetiferous granulites and gneisses is also observed near Papanasam in Tirunelveli district (Whitman, 1988). 4.1.1 Precambrian rocks The southern part of peninsular India is made up of granulite facies, charnockite and their retrogressed products. During the Proterozoic, several major shear zones dissected the granulite terrain and the granulite facies assemblages suppressed the retrogression (Drury et al., 1984). The magnetic susceptibility of the granulite terrain in the northern part of the study area (Madurai-Shenkottai-Tenkasi) and near Rajapalayam is reported as 170 × 106 emu/cc and 1400 × 106 emu/cc respectively (Ramachandran, 1985). The present study area is predominantly underlain by the crystalline rocks of archean age such as gneisses, granites, charnockites and acidic intrusive rocks. These rock formations are the typical crystalline rocks of the Deccan plateau as well as the Western Ghats (Pitchamuthu, 1979). Moreover, the western part of the study area and the adjoining Western Ghats are underlain by charnockites (Narayanaswami and Lakshmi, 1967). However, these features are found to be speckled as bands and lenses in NW of Tirunelveli town and they tread towards NW-SE direction along WĂŐĞͮϳϬ foliation with the gnessic rocks in Ambasamudram and Tenkasi areas. Granitiod non-garnetiferous mica, hornblende gneisses and mixed gneisses associated with migmatites are noticed in the north eastern part of Tirunelveli and the south and southwest parts are characterized by khondalitic migmatitic assemblages of garnetiferous biotite gneisses, and leptinitic assemblages. The khondalite group in the study area is epitomized by garnetiferous biotite gneiss and isolated occurrence of garnetiferous graphite gneiss and granulite with or without sillimanite at few places. In Sankarankoil region, which falls over the north eastern part of the study area, where gneisses are observed to be moderate grained to coarse-grained quartzite, well foliated and banded micaceous granite gneisses. In Sanganeri- Tirunelveli region, calcium granulite patches have been noticed, which are medium grained in nature with grayish color and contains graphite flakes. Narrow bands (1 to 5 meter) of calc-granulites are noticed in association with charnockites and garnetiferous biotite gneiss bounded by garneiferous sillimanite graphite gneiss. Moreover, in Ambasamudram area, calc-silicate rocks associated with humite chondarite spinel-magnesian calcite assemblages’ trending in the general direction of foliation have been noticed. The width of the calc-silicate band ranges from 25-350 meter, which is separated by quartzites and is flanked by intermixed banded biotite and granitic gneiss. 4.1.2 Basic intrusive In general, basic intrusive rocks occur as dyke. About 20 km to the northeast of Kanyakumari, dykes of carbonatite in charnockites and granitic gneisses have been recorded near Kudangulam. Also, fine grained basalt and medium grained dolerite dyke occur in SSE of Kudankulam, Tirunelveli district. The dykes have a width ranges from few centimeters to 25 meter and the length varies from 100 to 300 meters, which cut across the foliation of the country rock. They are hard, dark grey to black in color and WĂŐĞͮϳϭ exhibit spheroidal weathering. Basaltic dykes are seen in well cutting, positioned in the southern part of the Tirunelveli district. They are and fine-grained to porphyritic, containing phenocrysts of pyroxenes and feldspar. 4.1.3 Acidic intrusive The northern part of the study area (Tenkasi and north Shenkottai regions) is characterized by the presence of conformable bands of granitoid of different types such as pinkish granitoid, leucogranite and grayish granitoids. Pegmatite veins are observed to be intruding in all the rock types with varying thickness from few centimeters to 1.5 meter. Few quartz veins are present trending to NNW, NNW-SSE and N-S directions. Leucogranite of different dimensions are seen in northern part of Tirunelveli district frequently trending parallel to the regional trend of foliation. The width of granite rock ranging from 5 to 15 m and length varies from 1000 to 3000 m. 4.1.4 Crystalline limestone The crystalline limestones in Tamil Nadu have low magnesium content and are found in Ramayyanpatti, Talayuthu deposits of Pandalkudi, Pallavanattam and Alangulam bands. Moreover, garnetiferous gneisses are found to be interbedded with quartzites in Nanguneri and Ambasamudram taluks. The Ramayyanpatti-Talayuthu deposits forms the most imperative one in the study area forming the west of a major cross-folded isoclinal antiform. The limestones are white to grey in color and coarse-grained grading to calciphyre and calc-gneisses. The western segments of Ramayyanpatti are composed of high-grade material and inter-bedded with quartzite, which is useful for chemical industries. In the Talayuthu block, the limestones are frequently interrelated with quartzite that makes it highly siliceous and impure. Near Kuttaipatimalai, the limestone grades into a calc-gneiss with local high-grade patches. Bands of calc-gneisses and calciphyre coupled with quartzite WĂŐĞͮϳϮ and garnetiferous gneisses are recorded near Ambasamudram. Cocolite bearing streaky carbonaceous limestones are also distributed in the Panachadi, cocolite and tremolite in Adaichchani area, phlogophite near Singampatti and Pattamadi with diopside and impure types. These are all aligned NE-SE, forming the southern limit of a major NW-SE cross fold. The Pandalkudi area contains narrow bands of coarse calcitic limestone associated with banded carbonaceous graphitic bands. Along the same strike in the western direction towards Sivakasi, crystalline limestones are developed as narrow bands near Anaikuttam and extended into Alangulam deposits by repeated sinistral shifts. The Kakkivadanpatti area shows three periods of deformation commencing with the folding along ENE-WSW, but plunging at 20º followed by a NE-SW folding developed on the limbs of the first set of folds. Under its influence, the limestones along the SW of Alangulam are found in greater thickness. They must have been subjected to refolding along NNE-SSW axis but plunging to south. This has brought out the outcrop pattern of the wide bands of quartzite to the east of Rajapalayam. 4.1.5 Tertiary rock In Apparkulam (Tirunelveli district), a crystalline limestone of dolomitic grade extends in NW direction for a length of 1.2 km with a width of 20 meters. It enlarges to a thickness of over 300 m, cropping out on the western banks of Pachaiyar river. In Srivaikuntam taluk, dolomitic limestones are extended towards SingathakurichiEllainaickenpatti villages. The band is exposed intermittently over a length of 3 km with an average width of 10-20 meter. Isolated patches of calcareous sandstones and fossiliferous limestones are noticed in the Tirunelveli region (Subramanian & Selvan, 2001). Calcareous sandstone system exposed near Kudangulam is one of the prominent among them. The sandstone system is poorly sorted, fine to coarse-grained in nature. The WĂŐĞͮϳϯ outcropping of limestone near Kudangulam has an elevation of above 50 meter. This exposure at south and southwest of Kudankulam covers an area of about 3 sq km. The limestone is light yellowish to pink in color, hard and compact, containing broken shells. Based on microfossil index, the limestone has placed in the age between Upper Miocene and Pliocene (Bruckener, 1988). The presence of two types of cementing medium indicates that, first type must have been formed under submarine condition and the second under sub-aerial condition (Bruckener, 1989). 4.1.6 Quaternary deposits Quaternary deposits are traced along the coast and in major river valleys. River terraces are noticed along the major river valleys and alluvium is extensive in deltaic plains. In the Tirunelveli district 99% of the area is covered by brown soil and it is known for its widespread kankar and tuffaceous limestone patches. In the southern part of Tirunelveli district, kankar nodules abound along some seasonal stream courses. Isolated cappings of laterite over gneisses, granulites and charnockites are seen in many parts of Tirunelveli district. The thickness of laterite ranges from few centimeters to 5 meter. Teri sands of reddish color have spread over a large extent of the area in Tirunelveli district. The origin of Teri sands is still a dispute among researchers with one group attributing to the detrital Aeolian (Dowie, 1940; Ahmad, 1972) and another group attributing it to the weathering of country rocks (Zeuner & Allchin, 1956; Gardner, 1981). 4.2 Petrography 4.2.1 Leptynite It is generally branded as garnetiferous quartzo feldspathic granulite with coarse-grained granoblastic texture. The textural relations and the absence of significant compositional zoning in mineral indicate equilibrium mineral assemblages. WĂŐĞͮϳϰ Through the mineral assemblage’s proportion may vary, the common mineral assemblage is alkali feldspar - plagioglase (An 30-40) + Quartz + Garnet + Biotite. The typical mineral assemblages and their specific characters are given below. Minerals Alkali feldspar Plagioclase Pyroxene Garnet Hornblende Biotite 4.2.2 Characteristics Typically perthite Antiperthite Mainly hypersthene of pleochroic in greenish, pinkish and yellowish colors. Rich in pink garnets (Almandine and pyrope) and also colorless garnet Olive green to brown Brown to dark brown Charnockite The name charnockite refers to an orthopyroxine bearing granite commonly found in granulite facies terrains. High temperature and pressure are considered essential for their formation. Granoblastic polygonal to interlobate microtexture characterizes charnockite from other rocks. This designates through recrystallisation at conditions on par with the granulite facies metamorphism. The metamorphic assemblages are quartz-plagioclasepotash feldspar-orthopyroxene-clinopyroxene-garnet-biotite. In spite of massive appearance, the micro-texture bestows clear evidence of post charnockitic deformation. Pyroxene are large , showing exsolution of opaque, along their cleavage planes and grain boundaries and are often rimmed by corona of bluish green hornblende and cummingtonite . Pyroxenes and plagioclase show bent lamellae. Strained quartz grains exhibit wavy extinction. Sometimes plagioclase and quartz show marginal granulation with very little recrystallisation. Porphyroplastic garnets contain inclusion of quartz and feldspar, exhibiting sieve texture. Secondary coronitic garnets have developed after the primary ones. Quartz grains are large and flattened exhibiting cataclastic texture. Stretched quartz grains are found to dominate over small lobate, sutured quartz grains. WĂŐĞͮϳϱ Bent and discontinuous twin lamellae in plagioclase indicate intensive deformation followed by bead and rod type of perthites are also noticed. 4.2.3 Granitoid gneiss Granitoid gneisses including garnetiferous biotite gneisses cover the major part of the study area (Plate 4.1). Massive rocks demonstrating a hazy foliation are transitional into the rocks of granulite facies. The trend of foliation is NE-SW with a dip of 45º to 75º in SW and WNW direction. Presence of banding is emblematic in this rock. It is an alternation between bands rich in biotite, hornblende, and pyroxene and bands rich in quartz and feldspar. In these rocks, mica, hornblende, and elongated minerals such as sillimanite, tourmaline, kyanite and pyroxene are arranged more or less parallel to the foliation. The garnets contain inclusions of quartz, biotite, magnetite, plagioclase, and sillimanite. Quartz and feldspar generally have an elongated form, although they may be approximately equi-dimensional. The potash feldspars present are usually perthitic. Thin linear bands of garnetiferous sillimanite gneisses occur as in the south western part of the study area. The bands are well foliated and have a NW–SE trend with a dip of 30° to 50° towards the southwest direction. 4.2.4 Quartzo feldspathic granulite Quartzo-feldspathic granulites are characterised by “granulitic fabric”. The assemblages are quartz-orthoclase plagioclase-garnet. The grains have interlocking boundaries. The garnet of granulites is pink almandine type with higher MgO than garnet of the amphibolitic facies. Prisms of kyanite and sillimanite lie in the plane of schistocity. The orthoclase is perpetually perthitic. 4.2.5 Pink granite Granites of coarse grained demonstrate the typical granitoid texture. Both plagioclase and mafic minerals tend to form sub-hedral crystals. Anhedral quartz occupies WĂŐĞͮϳϲ intergranular spaces. Generally, alkali feldspars are of anhedral type. The minerals present in granites are dark brown biotite, zircon, muscovite, and minor amounts of sillimanite, magnetite, almandine, and topaz. 4.2.6 Quartzite Quartzites are metamorphic rocks, mostly of non-foliated, coarse textured, granular, white to light reddish in color with streaks of ion ores and composed of quartz. These rocks occur as distinct and detached linear bands on sharp edge ridges within the gneisses and charnockites in the study area and form low hills and domes. They are mainly derived from calc-gneisses and granulites and contain inclusion of chlorite, mica and hornblende. Inclusions of minerals such as sillimanite, garnet, kyanite and chloritoid are invariably present and occurrence of muscovite and biotite are also noticed. The trend varies from NE to SW with steep dips. 4.2.7 Granite gneiss It is characterised by the presence of distinct bands, streaks, lenses, and dykes of pegmatitic material. It is made up of quartz, plagioclase, and microcline with biotite and muscovite. Presence of zircon, kyanite, sillimanite and garnet are also noticed. 4.2.8 Calc gneiss They are often massive, coarse-grained and crop up as thin linear bands pass through gneisses. However, finer to medium grained and impure micaceous varieties are also commonly found. Defoliation and banding are the archetypal characteristics of these rocks. The foliation embraces a well-marked segregation into dark and light layers. Carbonate is customarily interstitial or vein-like in its distribution. The calc-gneisses are characterized by minerals such as hornblende, tremolite, actinolite, plagioclase, biotite and calcite. Pyrite, Pyrrhotite and magnetite are accessory constituents. WĂŐĞͮϳϳ 4.2.9 Crystalline limestone The texture of limestone depends to a large extend on the stage at which the development of silicates takes place. The concomitant deformation with silicates has fashioned amphibolite schist nature to limestones. The minerals frequently encountered in these rocks are tremolite, actinolite, and epidote followed by Hornblende and mica. Since it is highly metamorphic, it resembles hornblende schists and gneisses, commonly called as amphibolites. A detailed geology map of the study area is shown in figure 4.2. 4.3 . Geomorphology Geomorphology is the scientific study of landforms and the processes involved in shaping the topography of an area through the course of evolution. According to Thornbury, (1969), the earth’s surface is affected by geomorphic processes which include physical and chemical changes that in turn involved in the evolution and development of landforms. The geomorphic characteristics of a basin are determined by several morphogenetic features evolved under the influence of fluvial hydrodynamics, sub-aerial processes, tectonic activities and other geomorphic divisions such as weathering and active denudation processes. The influencing features and factors such as geology, landuse and climate are also responsible for the development of morphological landforms in an area. Moreover, the availability of land and water resources is greatly controlled by the geomorphic setting of an area. Various landform characteristics in the study area are based on the broad geomorphic division, which includes exogenic processes such as weathering and active denudation. The geomorphic evaluation of the study area was carried out qualitatively and quantitatively. Intensive fieldwork and the application of geospatial technologies such as remote sensing and GIS has been used to explain the details about geomorphology of the study area. The detailed geomorphological map of the study area is shown in figure 4.3. WĂŐĞͮϳϴ Figure 4.2 Geology map of the study area (Source: GSI, 1998) Plate 4.1: A- Bedrock exposed with garnet biotite gneiss near Sorimuthayyanar temple. BEroded surface of garnet sillimanite graphite gneiss near Papanasam. C- Fractures and joints in garnet sillimanite graphite gneiss near Agasthiyar falls. D-Weathering of gneisic structures in garnetiferous sillimanite gneiss. E-Abraided blocks in the falls. F- Weathering of garnetiferous biotite gneiss in lower Papanasam. Figure 4.3. Geomorphological map of the study area (Source: Extracted from Landsat imagery) Various litho units present in the study area with dry and wet spell of climate triggers the mechanical and chemical weathering processes, which results in varied denudation and depositional landforms around the entire basin. The vegetation cover and varied landuse are also responsible for the weathering processes and influence the landform development. 4.3.1 Fluvial processes Fluvial processes are referred to the physical interaction of flowing water through the natural channels such as streams and rivers. These processes play an important role in the denudation of landforms and the transport of sediments from upstream to downstream. River induced erosion can be categorized in two ways such as chemical erosion and mechanical erosion. The former involves in corrosion activity along the stream system while the later comprised of abrasion, hydraulic action, and deposition. Fluvial erosion can be categorized into four divisions such as vertical erosion, lateral erosion, headward erosion, and mouthward erosion. The study area is largely controlled by fluvial processes such as the work of running water mainly during the monsoon season. Two distinct monsoon periods is noticed in the study area, July to August in the up streams and October to December in the down streams. The varied topography and high slopes in the Papanasam and Manimuthar catchments are responsible for its down cutting continuously since the Tertiary period. As a result of active tectonics, the study area has experienced numerous upliftment, and resulted in continuous down weathering. During the period of intense rainfall, a phenomena called rapid run-off is observed in the plain regions of the study area. During this period, the raindrops with maximum velocity and kinetic energy tend to move the sand grains and making it susceptible to the overland flow. This process is more active in the alluvial plains and triggers surface soil erosion. The study area exhibits a number of fluvial erosional features such as scraps and gully WĂŐĞͮϳϵ erosion. The slopes in the study area play a major role in landscape development. This is evident to the western part of the study area comprising Kalakad, Manimuthar, Papanasam, and Courtallam hilly tracts. The hilly terrains are moderately exposed with bare outcrop of the bedrock. These areas are densely forested while the pediments zone is vegetated and under agricultural practice. Numerous potholes of various sizes and shapes have been observed during the field work in the rocky exposure of the channel of Tamiraparani river. This is due to the grinding action of sand grains and pebbles over a long time period. River bank erosion is another important feature observed along the Tamiraparani river. The occurrence of bank erosion is found to be confined in the high energy zones with feeble vegetation cover and loose sand deposits. Rills are observed along the foothills of Kalakad and parts of Manimuthar region. Formations of rills are possible where slopes are steep (30° to 70°), heavy rainfall run-off with loosely packed soil material. As erosion intensifies, rills instantly begin to integrate into larger channels to form gullies. The river terraces are one of the important geomorphic features present in the study area. There are found along the river channels, mostly flat topped, blunt edged and covered with riparian vegetation along the river channels. The terraces are developed by Tamiraparani river and its tributaries are alluvial in nature consisting of gravel, sand and fine sediments. The terraces of the study area are mainly controlled by neotectonic activities as well as climatic factors. The depositional landforms in the study area are influenced by channel gradient and stream discharges along with regional geomorphic settings. The weak bank materials are easily eroded along the river course, which results in channel widening, and sediment loading. The alluvial floodplain in the study area is the most common depositional feature formed by the combination of channel and over bank deposition. These features are commonly seen near to stream diverge, where flow and WĂŐĞͮϴϬ transport rate is decreased, which favours deposition of sediments. The lateral and vertical accretion within the channel influences the floodplain formation in the study area. 4.3.2 Geomorphic settings of the study area The geomorphology of Tamiraparani sub-basin has been studied based on Landsat satellite images along with digital elevation model (DEM) and intensive field verification. The major geomorphic units in the study area are structural hills, denudational hills, residual hills, inselberg, valley floor, valley fill, shallow floodplain, pediment, pediplain, shallow weathered pediplain, shallow buried pediplain, upper bazada, linear ridges, point bar, channel bar, channel lag, and active channel belt. 4.3.2.1 Structural hills The Western part of the study area is comprised of rigid structural hills, which are well visualized from the satellite imageries merged with hillshade layer derived from DEM, and they cover nearly 40% of the study area. It is observed that the hills are controlled by tectonic and neotectonic actions. Numerous fault lines and lineaments are observed in this region, which is formed by active tectonics. Moreover, they have steep slopes and high relief and represents the base of the Western Ghats. Due to which, the erosional dissection is controlled by the fracture pattern of the bedrock. The elevation trend is observed from East to West direction with increasing elevation towards West. The weathering and erosion in this region has produced variously shaped ridges and valleys, which further results in numerous erosional landform features such as rills and gullies. Gully erosion is common in steep structural hills and headward erosion is noticed through the rills. The first order stream network provides space for rill erosion that affects the degree of dissection and leads to sediment transportation from the catchment. WĂŐĞͮϴϭ 4.3.2.2 Denudational hills Denudational hills are formed by the natural process of weathering and they occur as exfoliation domes with partial scree or debris covered at the foot slopes and they are noticed near Kalakad-Ambasamudram area. The shape of the denudational hills is controlled by the lithology, structural features, weathering and external forces. The denudational hills in the study area is composed of gneisses with a little soil cover and moderate vegetation and falls under Mampothai and Kolundumadai reserve forest. The tropical weathering conditions in this region lead to various geomorphic forms such as exfoliation domes, domed inselberg with steep slopes along with boulders of various sizes. The altitude of this hill type varies from 300 to 400 m above the pediplain forming an important geomorphic unit. 4.3.2.3 Residual hills The residual hills are isolated, steep sided, usually smoothened, and rounded hill rising abruptly above the surroundings over the plains in tropical regions. These hills exhibit conical to rounded forms with steep to very steep slopes and are noticed in Nanguneri-Kalakad stretch and isolated patches are seen in Thiravianagar, Kilakadayam, Kunnathur, Melapattam, Perumalkulam, and Sivanthipuram. The altitude of this Hill type varies from 100 to 200 m above the pediplain. 4.3.2.4 Inselberg Isolated inselberg of 50 to 100 m above the pediplain are found in the study area and are mostly composed of gneisses. Due to spheroidal weathering, they occur as boulders embedded in the highly oxidized weathered state. They are noticed between KalakadSingikulam stretch, Karaisalpatti, Padmaneri, Melapattam, Tenkulam, Mayamankurichi, and Govindaperi (Plate 4.2). WĂŐĞͮϴϮ 4.3.2.5 Valley fill The valley fills are common along the drainage system where boulders, cobbles, pebbles and clastic materials are deposited. In contrast to terrace sediments, the valley fill sediments are probably yellowish to greyish finer silty sand to find sands. However, coarse sand is also found in these areas due to occurrence of erosional processes near to hilly terrains. In the study area, the valley fills are noticed along the major stream networks and in the northern part of the study area. 4.3.2.6 Shallow floodplain This geomorphic landform is formed by recent alluvium and is found along the active floodplains. They consist of unconsolidated gravels, sands, silts, and clays formed by seasonal flooding when the river overflows their banks. Generally, floodplains form a higher level than the active floodplains and are not frequently inundated by flood waters. The alluvial plains are used as croplands for widespread cultivation along the river banks. Floodplains are generally depositional landforms, which are found adjacent to a channel and made up of sediment transported by the flow regime (Nanson & Croke, 1992). In the study area, the shallow floodplains are seen along the Tamiraparani main channel and are encroached by shrubs and vegetation. 4.3.2.7 Pediment Pediments are gently sloping erosional surfaces of low relief and were developed on bedrock or older unconsolidated deposits. Their occurrence may be influenced by lithologic, neotectonic and climate settings and their formation may be sub-aerially exposed and covered by a discontinuous to continuous veneer of alluvial deposits. Moreover, the subsurface weathering processes have strongly influenced the pediment development in the study area and such processes are critical to the formation or modification of pediments on crystalline bedrocks. In the study area, the pediment WĂŐĞͮϴϯ formations are seen in the foothills of the Western Ghats and also noticed in isolated patches throughout the study area. However, there is a close relationship between pediment formation and other structural features such as hills, linear ridges, and inselbergs. This highlights that the formation of these landforms are due to the presence of gently sloping erosional surfaces in the study area. 4.3.2.8 Pediplain The pediplains are characterized by gently undulating to flat topography overlaid above the bedrock and are found on either side of the main channel. They are formed from older alluvium and they gradually developed from topographic highs within the alluvial plain. The pediplain exhibits a gentle slope towards the main channel as channel overflows feeds the alluvial deposits to the pediplains. In the study area, the pediplains are noticed along the main channel of Tamiraparani river and adjoining tributaries. Moreover, the variation in the characteristic features of pediplains influences their spatial distribution in terms of composition of landscapes, drainage system, overburden, and minor reliefs. The cumulative effect of these features results in the formation of shallow and moderately weathered pediplains (Plate 4.3). 4.3.2.9 Shallow weathered pediplain Shallow weathered pediplains are flat and smooth surface of buried pediplain with a thickness of 0 to 5 meter and composed of shallow overburden of weathered derivative material. They are uniformly distributed in the study area and this geomorphic landform is good for groundwater prospecting in shallow aquifers. Any modification of landuse and land cover over these landforms may have a direct impact on the groundwater resources. WĂŐĞͮϴϰ Plate 4.2: A- Structural hills near upper Karayar. B- Isolated denudational hills near KalakadNanguneri. C- Dome type residual hills and inselbergs near Kalakad. D- Eroded hard rock channel bed near Papanasam-Agasthiyar falls – result of tectonic activity. E- Bank erosion in the upstreams of Gadana. F- Shallow weathered pediplain with moderately vegetated residual hill near Kadayam. Plate 4.3: A- Exposed bed rock due to stream action in the upstreams of Tamiraparani river. BHuman intervention near Reddiyarapati, a road network is cutting through a linear ridge. CBoulders in the channel bed of upper Tamiraparani river. D- Pot-holes in the channel bed of upper Tamiraparani river near Sorimuthayyanar temple. E – Flood plains of Tamiraparani river near Kokaraikulam. F- Outcrops of Hornblende-biotite-gneiss in the pediplains. 4.3.2.10 Moderately weathered pediplain Moderately weathered pediplains are composed of smooth surface of buried pediplain. They have a thickness of about 5 to 20 meter overburden of weathered derivative material and are found along Pachaiyar river stretch and the palaeo-channels of the study area. These units have good potential for groundwater resources and have less seasonal variation. 4.3.2.11 Upper bajada These are fan shaped deposits and are formed along a mountain front by the action of stream induced sediment deposition at the base of the mountains. The coarser sediment deposits at the base of the mountain and the finer sediment moves outward to forms a fan shaped feature away from the mountain face. They are commonly found in those areas where flash flood induced sediment deposits occur over a period of time. In the study area, the bajada formations are seen in the foot hills of Kalakad and adjoining areas. 4.3.2.12 Linear ridges The linear ridges observed in the study area are narrow, low in relief with moderate vegetation. The structural features in linear ridges are mostly strike controlled and are found in isolated patches along NW-SE direction with a parallel resemblance to the Western Ghats. The Idaiyankulam-Melkarai stretch has numerous ridges with parallel alignment and similar observations are also noticed in Veeravanallur, Alwarkurichi, Vickramasingapuram, Kalladikurichi, and near Alangulam. Moreover, a well established linear ridge system is observed between Anjan Kattalai and Muthur with the height ranges from 1 to 2 meter above the pediplain. 4.3.2.13 Point bar Point bars are notable geomorphic units and are often found to be associated with meandering streams. Also, the sediment deposition on point bars has a great influence on WĂŐĞͮϴϱ shaping the river channels due to lateral migration of meandering river at the time of flooding. In the study area, the point bar features are found to be associated with the major tributaries of Tamiraparani river. They are developed from the lateral accretion on the inner banks of the river channel. The area between Aladiur and Kodaganallur along the Tamiraparani river stretch is notable for such features and it can be well discriminated from the satellite imagery. In the smaller streams such as the tributaries of the Tamiraparani river, point bars are noticed as simple depositional features on the convex sides of the meander dipping gently towards the channel. Moreover, these point bars are encroached with vegetation cover and forms a stable landmass thus shaping the course of the channel. 4.3.2.14 Channel lag The transported material in a river contains a wide range of sediment particles and they differ in their size. Based on its size fraction they can be classified as silt, clay and coarse sand. Silt and clay fractions are transported in a much faster rate and are often deposited in flood plains and the coarser sediments, gravels and pebbles lag behind and accumulate as discontinuous lenticular patches in the deeper part of the channel and called as channel lag deposits. These deposits are later accumulated with fine sediments and are preserved in the base of the channel. They are invariably discontinuous and occupy the lowest part of the channel and represent the base of the channel. The study area, the channel lags are often seen in Papanasam-Servalar confluence near to Irumbupalam bridge under low stream flow condition. 4.3.2.15 Channel bar Channel bars are often associated with the braided character of a stream. The braided condition of the channel indicates the sediment carrying capacity of a river. This condition prevails when excess materials fall out of transport when it enters the plain. The WĂŐĞͮϴϲ sediment yield is one of the contributing factors for the formation of channel bars in the stream network. In the study area, channel bars are often seen along the Tamiraparani river and its tributaries. The old bars are associated with vegetation cover whereas, the new bars are bare and composed of coarse to fine sediments and migrates during high flow condition. 4.3.2.16 Active channel belt The active river course of the present-day channel is called active channel belt and they occupy the youngest geomorphic surface in the fluvial system. Before the construction of reservoirs, the main channel had significant width with full water flow condition especially in the monsoons. However, the scenario changed after the construction of reservoirs and the water flow was marginalized. As a result, the full flow condition of the river has greatly reduced and the flow is restricted to the deep narrow channels of the main river and it can be noticed in the down streams of Tamiraparani river. The old channel is now facing anthropogenic encroachment resulting the river degradation both physically and biologically (Plate 4.4). WĂŐĞͮϴϳ Plate 4.4: Aerial view of few geomorphic features in the study area. A- The active river course of the present-day channel. B- Point bars and channel bars near Manamangalam. C- Linear ridge near Anjankattalai. D- Bajada formation near the foothills of Kalakad.