GEOLOGICAL SIGNIFICANCE OF MARINE MOLLUSKAN BEDS : EVIDENCE FROM SOUTHERN COASTAL ZONE OF SRI LANKA

: Mollusks are invertebrate animals that live in brackishwater or marine habitats. The diversity and complexity of these habitats are due to winds, waves, tides, bottom features, daytime illumination, geologic origin of shoreline and ecologic conditions of the oceans. Assembly ofbivalve and univalve mollusks occur due to the eustatic changes and the prevalence of coastal hazards. Stratigraphic sequencesof shell beds alongthesoutherncoastbetweenKalametiya Kalapuwa (lagoon) and Bundala Lewaya (salt pan) clearly indicate that shells have been piled up together with stone artifacts, pieces of pottery, human bones and other animal bones. These were caused by severe storm wave action on mounds in lagoon and lake bottoms, on sand dunes and headlands. The present study shows that the shell valves of lagoon, lake and channel beds (floors of marine and brackish pools) mostly accumulated in situ consequent to the lowering of sea level between 5030 - 4390 and 3930 - 3290y B.P.


INTRODUCTION
Assemblance of bivalve and univalve mollusks occur due to eustatic changes and coastal hazards. They are used as a geoscientific tool to study former sea-level stands.' Radiometric dating of the shells in different levels and quantitative analysis of their deposition sequences are helpful for this purpose. The present study attempts to reveal the geological significance of inland marine shell beds on the southern coast of Sri Lanka at a site between Kalametiya Kalapuwa and Bundala Lewaya in the dry zone (Fig. 1). The geological significance, extension and evolution of these shell beds have not previously been investigated.

METHODS AND MATERIALS
Extension of the shell beds was mapped based on detailed field investigations. From twenty locations, shell and soil samples were collected for geologic analysis between March and December 1992. Each sample contained 1.5 22.0 kilogram of shell and shell debris. All locations and sample heights were leveled to mean sea-level (msl) using TC 1600 EDM (Electronic Distance Measurement) theodolite (Set 3 B Sokkia) by a government licensed surveyor.
Separation of the grain sizes of the soil samples was undertaken using a BS 410, Laboratory Test Sieve (Endecotts, London). Before separation of grain sizes, weighted amount of dried soil (125 -150g) was treated with 20% HCl to remove carbonates. The treated wet sample was dried in an oven at about 100°C for at least eight hours. Approximately 500 -600g shells were also cleaned by treating with 10% HC1 for identification of the shells, and about lOOg were preserved for future radiometric dating. Colour of the soil samples and shell embedded soils was determined using the revised standard soil colour charts.* The profiles of the shell layers were drawn using Harvard graphics. The laboratory work was conducted in the Department of Geography, University of Sri Jayewardenepura. Shells were identified with the help of a published ~atalogue.'~ Physical setting of the study area The study area, is situated between longitude 80'48'-81°16'and latitude 6'03'-6' 09' Ambalangoda-and Hambantota topographic sheets (1:63,360) between Kalametiya Kalapuwa and Bundala Lewaya ( Fig. 2 from a to k). Geologically the study area is underlain by the Highland complex rocks (undifferentiated rocks) on the western bank of the Walawe ganga, while the eastern bank lies on the Vijayan complex rocks (charnockite and charnockitic gneiss) of precambrian rocks.' According to a Canada -Ceylon Colombo Plan Project (Resource of the Walawe Ganga basin') the study sites consist of two types of rocks: (1) hornblende and biotite gneiss with associated pegmatite and migmatite (Vijayan complex rocks). (2) quartz0 feldspathic gneiss andgranulite (Khondalite series rocks). The zone of the quaternary deposits here are also somewhat narrow due to the extension of low hills and ridges close to the sea and lie on both Highland and Vijayan complex rocks. Broadly, the study area can be divided into three geomorphic units based on elevation and the composition of the deposits" namely: (1) flat terrain (lowland I, <30 m, shpe is 1/2O or lo (60:l or 100:l in gradient). (2) flat to slightly undulating terrain (lowland 11, c30 m). I t has l0to 3' slope or 60:l to 20:l gradient and can be designated as 'flood plain'. (3) undulating terrain (lowland III,30-150 m). Slightly undulating, undulating and rolling features appear particularly in the area between Udawalawe and Ridiyagama.
The shell beds are mainly concentrated in the flat terrain (lowland 1) beyond the western a i d eastern sides of the Walawe ganga. The coastal beit which included lowland 1 has been altered by terrestrial, aeolien and marine processes, . and has formed narrow and long beaches, beach ridges with medium (3 -5 m) and somewhat high (8 -12 m) dunes. Sand spits are common features a t the estuaries of the Walawe ganga and lagoons. Salterns, salt marshes and mangrove swamps and mound topography (a hummock relief) lie behind them. Too small and low bedrock outcrops appear as erosional remnants. The stony gravel. beds and alluviam are the terrestrial deposits transported from upland and mountainous terrains.' According to K oppen classification: the southern coastal zone, from Matara to Bundala is included into 'Afwi', 'Amwi' and 'As? climates. Distribution of rainfall of the Hambantota meteorological station shows.two maximum seasons (Fig. 3) during the southwest monsoon (May to September) and convectionalcyclonic-depression (October to November).

a"
The soils of the study area have a close relationship with geologic characteristics, microrelief and the seasonal distribution of rainfall. Four main soil groups1' of the study area have been identified as follows: (1) reddish brown earths with high amount of gravel in subsoil & low humic glay soils (2) reddish brown earths & solodized solonetz (both (a) and (b) types lie on the undulating terrain) (3) alluvial soils of variable drainage and texture (flat valley bottoms, water logged areas etc. covered by these soils) (4) regosols on recent beach and dune sand. Barrier beaches, beach ridges, sand spits and dunes along the coast are formed by such materials. Rocknob plains and erosional remnants in the coastal plain are formed by granitic gneiss, quartzite, hornblende gneiss and hornblendebiotite gneissic rocks.
The coastal lowlands are covered by barren lands mainly sand dunes. Most of the dunes are covered by creeping vegetation (Spinifix littorerus; Smaharawana reula and Ipomoea pescaprae; S-mudubintamru) and scrublands. The wetlands behind the 'sand dunes are occupied by mangroves along'the estuary of the Walawe ganga and around lakes and lagoons. Salt-pans, salt and brackishwater lakes of the area are subjected to daily tidal fluctuations. Sonneratia alba (S-kililla) is the dominant mangrove species ofthe area. Among other important mangroves are Nypa fiucticans (S-ginp.ol), which extends along edges of lagoons and tidal creeks, Rhizophora (S-kadol), Bruguira (S-sirikanda) and Ceriops (S-kaduru) spp. Lowlands which are slightly above the mean high water springs level appear as freshwater marshes. Brackishwater and freshwater marshes are widely occupied by Typha angustifolia (S-hambupan) along the lower part of the Walawe basin." The undulating low ridge and valley topography (30-150 m), behind the coastal lowlands are covered by dry zone thorny forest and sparsely used croplands, homesteads and chena cultivation. The paddy lands have been concentrated in the fairly wide valley bottoms toward the inland form the coast.

~arine'mollusks as a sea-level indicator
Sea-level indicators such as raised marine deposits, coastal barrier sands, beachrock, ooids, corals and reefs, coralline algae, marine notches, submerged forests marine mollusks etc. differ widely in indicative value (accuracy). They are important for the consideration of sea-level changes within the context of the geological development of an area.12 Mollusks are a group invertebrates that are terrestrial, brackishwater or marine in habitat. The diversity and complexity of their habitats are due to winds, waves, tides, bottom features, daytime illumination, geologic origin of shoreline and ecologic conditions of the oceans and create special zones (Fig. 4) for different shell s p e~i e s .~"~ The basic nature of the mollusks was determined millions of years ago as the great land continents were drifting into their present position. As the climate (atmospheric and oceanic), changed and vacillated, all species came under new challenges to their ~urvival.'~ The least adaptable are found today only as fossil remnants locked in tertiary beds; other populations were divided; isolated and changed into new species, while more resistant oms flourished and spread from terrestrial to pelagic areas. The phylum Molluska contains six classes viz; Monoplacophora, Amphineura, Gastropoda, Scaphopoda, Bivalvia and Cephalopoda.' Habe4 states that seven classes of mollusks including Aplacophora with the above six are composed of 112,000 species.  Aplacophora dates from the Lower Cambrian; the class has 10 species. The worm-shaped creatures have no shells. The Monoplacophora (Neopilina Class) well-known from the Pdaeozoic deposits of epicontinental seas, are today only represented by those recently found from the deep sea, having a limpet-like (capshaped) shell. Scaphopoda (tusk shells), tube shaped shells which widen towards the aperture, are known from the Devonian period. The Bivalvia (Pelecypoda) are mainly found in areas close to the coast. The clams are covered with two shells. They are known from the lower Devonian. The Gastropoda are the most varied group within the molluska. They date from the Upper Cambrian. Cephalopoda (Squid and Octopus Class) are all predatory, swimming carnivores occurring in the major marine habitats. The Arnphineura (Polyplacophora -Chiton Class) are found from temperate to tropical areas and they usually live on rocky surfaces close to the shore. These date from the Ordovician to the present. These mollusks inhabit all the different regions of the sea, from cold polar regions to the warm equatorial belt, from surface water and intertidal beaches down to abyssal depths, but the warm shallow seas where reefs and rocks abound are thickly populated.
The continental shelf around Sri Lanka comprises of submergedrocks, sand banks, sandstone and coral reefs. Most of these features are situated on the mesolittoral zone, known also as the intertidal zone (it extends from the hightide mark to the normal low-tide mark). Intralittoral plane extends from the lower limit of the mesolittoral to the lowest depths at which a certain family of plants occur. The seas around Sri Lanka provide an ideal environment for a rich variety of molluscan shells. Kirtisinghe13 has pointed out that research on sea shells of Sri Lanka occur sporadically in zoological journals and the records of oceanographic surveys. He described about 530 species collected in the seas around Sri Lanka." Many of these are also found throughout the Arabian sea southward to the east of Australia, and along the coast of Australia.

Microrelief ofashell Beds
The extension of the whole shell beds along the southern coastal zone is laid in flat terrain, below 30 m. This is somewhat wider than the coastal belt which is between south of the Kelani Ganga on the west coast and the Nilwala Ganga on the south coast. Both monsoons blow parallel to the coast, rather than across it, and the waves are largely constructive southerly swells." The coastal belt from Tangalle to Bundala is formed of narrow and long barrier beaches and beach ridges. Dune bearing barrier spits are common features at the outfalls of the Walawe Ganga and the circular shaped lagoons. Garnet and ilmenite sands are found in most shore deposits along the beach. Well drained and imperfectly drained soils occupy these areas.
The wetlands are covered by lagoons and lakes, salt marshes and mangrove swamps behind them. The lagoon and lakes are known locally as 'lewayas' and these are not fed by large streams. Most of them are very saline due to the persistent winds and dry climate. These conditions have been created by rapid evaporation. Slight undulatings of the area extend as lobes sloping towards the coast. Most of them appear as low outcrops along the coast.

Geological Significance of the Shell Beds
The shells were found to be concentrated in pockets around the Kalametiya kalapuwa, Hungama, Lumnama kalapuwa, Mahasittrakala lewaya, the area between Karagan lewaya, Pallemalala, Embilikala kalapuwa and Bundala lewaya. The extension of the shell beds is shown in Figure 2. Meretrix spp. are the doininant mollusks in the whole area. Beside this, Anadara spp. .and Cerithidea spp: are also found mixed with.Meretrix spp. or separately (Plate 1). Most of these are found in paddy fields, small mounds chummocks),. former embayments and the bottoms of lagoons,'lakes and creeks. The shell beds at Hatagala extend up to Miniethiliya about 4 kilometres i d a n d from the present coast. The shell beds atkatagala -Ovitigoda yaya (paddy field) are composed of Meretrix spp. They are somewhat large (below 55 mm in size). Highly weathered pieces of elk bones and pottery fragments can be found from the mining pits (Plate 2a).
The shells at Miniethiliya are mined from small mounds near the paddy fields and small mounds slightly elevated from the paddy fields ( Cerithidea spp. are well preserved and they are below 20 mm in size. Meretrix spp. of both samples are also below 55 mm in size. A thick shell layer of the mound has been mixed with stone pebbles (appear as artifacts), human bones, fragments of pottery etc. Highly weathered human skeletons were found a t same mound of location No. 6 (Fig 10). A part of a jaw with teeth, vertebral columns, rib cages and radius are found in this location. The last teeth of the jaw indicate that the skull belonged to a young human. Other conditions of the site are very similar to location No. 5. Well polished oval-shaped stone artifacts, stone balls, human bones, a head of a serpent and other animal bones as well as pottery fragments are mixed with these shell beds (Plates 2b, 4 and 5c).   Kalarnetiya (Henagahapugala) beds (Fig: 13, Location No. 9) have been deposited on arocky headland (with a thin soil cover) whichis 13.8 m above mean sea level. The shells Meretrix spp. are below 40 in size. Brokenlarge shells mixed together with brown soil (10 YR 43). Pottery fragments are also found. The shells are gathered even in rocky splits. Based on the deposition pattern there is a possible indication that shells were deposited in the area due to severe strong wave action (Plate 7b). At location no. 10, brown fine sand with clay (10 YR 4 3 ) with shell and shell fragments appear in botlrshell samples (Fig. 14). The shells belong to Meretrix spp. (below 40 mm in size other comments are same as location no. 9). Inorganic content and grain sizes of the soil sample (10.1) are shown in Figure 17. Many pieces of pottery are mixed with the shell bed in this area (Plate 7a). The thickness and the height of the shell beds a t Kalametiya (location nos. 9 & 10) are different to the other locations (Table 1). Eleven shell bearing microlayers can be identified at Kalametiya, on a former lagoon, and contain Meretrix and Cerithidea spp. This sample point is not levelled due to the physical obstacles (Fig. 18, Plates 7b ancl 8). The shells from Godawaya to Mirijjawela are deposited a s small pockets in depressions (lagoon and lake bottoms) between sandy beach and undulating terrain which gradually increase in altitude inland. The beds a t Kiula Kalapuwa consist of tiny and small to large Meretrix spp. They are below 50 mm in size. The top soil of the area is covered by dull yellowish brown medium to fine sand (10 YR 413). The soil layer which is below the shell layer contains medium to fine brown soil (10 YR 4/4).
Somewhat large shell beds are found a t Hunukotumulla on former lagoon beaches a s well as lagoon bottoms of the western bank of the Karagan Lewaya (Fig. 15, location no. ll), these can presently be seen a s mounds. They are also covered by thorny bushes, stunted trees and grass. Sample 1 in this location is formed of 7.5 YR 412 grayish brown medium to fine sand clay soil. The compact layer has weathered pebbles of iron. Grain sizes and content of inorganic matter of the sample 1 (11.1) are shown in' Fig. 19. The shells have mixed with grayish brown soil (7.5 YR 412). Meretrix spp. are below 40 mm in size. Three shells are found in position of life (30 mm below in size). Somewhat compact, soilin sample 3 is grayish yellow brown (10 YR 412) in colour). Calcareous clay patches and weathered ironstone pebbles are found. Grain sizes and content of inorganic matter of the soil sample (11.3) are shown in Figure 19. Some beds at the Location No. 12 (Fig. 16) have deposited by wave action. The shells mix with Brown soil (7.5 YR 413) and fine sand and calcareous materials. Pebbles arevery few. Plant remains are evident. Meretrix spp. (below 45 mm) and Anadara spp. (below 65 mm) are main shell types ofthe area. Grain sizes andinorganic content of the soil sample (12.1) are shown in Fig. 17.
The shell pockets in the northwestern area of the Karagan Lewaya (Fig. 20, location no. 13), Nelumpathvila compose of dull yellowish brown soil (10 YR 51 4) with fine sandy clay and shelly sand. Meretrix spp. ofthe sample is varied from small to somewhat large (below 40 mm). The shell layer a t location no. 14 -Karagan 4 (Nelumpathvila) composed of dull yellowish brown soil (10 YR 514) with fine sandy clay and shelly sand (Fig. 21). Somewhat large Meretrix spp. are below 40 mm. Sample 1 a t the location no. 15 -Karagan 5 (Nelumpathvila) also contains brownish gray soil (10 YR 611) with calcareous sand and clay (Fig. 22). The shell layer is composed of dull yellowish brown soil (10 YR 514) with fine sandy clay and shelly sand. Meretrix spp. ofthe sample is somewhat large (below 40 mm).
Extensive shell beds a t Sippikulana, around the Maha lewaya and Koholankala (Koholankala lewaya) are found on slight undulatings (lobes). The shell layers here are thin compared to the beds a t Gurupokuna, Hungama and the Karagan lewaya (Table 1). Furthermore, many shell patches are found in the area between Maha Lewaya and the Nabadewa, Pallemulla area, on the eastern. bank of the Malala oya (Plates 5b and 9). These shell beds are composed of stone artifacts and quartzite pebbles. Based on the colour of the quartzite pebbles, it is possible to infer that these may have been fired. Some beds are more than 4 km inland from the present coast.
A considerable amount of shell beds are found.in the area between Malala lewaya and Embilikala kalapuwa (around Pallemalala, location no.16). These beds also appear on mounds and ditches of the area covered by scrublands and stunted trees. The shells at this location contain grayish brown soil (7.5 YR 412) with fine sand and clay, weathered pebbles, quartz fragments, plant remains. Meretrix spp. is below 45 mm and weathered Cerithidea spp. appear as fragments (Fig. 23). Extensive shell beds are found in many mounds along the northern beach of Embilikala kalapuwa and on the side left of the Bundala road. The shell beds around Bundala lewaya are somewhat high and thick (Table 1) compared to the shell beds a t Maha lewaya and Embilikala kalapuwa (location nos. 17,18 & 19).
Dark Brown medium sand to fine sand with clay soil (7.5 YR 313) appear a t the location no. 17 -Bundala road 1 (near 4th krn). The soil is somewhat compact.
Plant roots and other organic matter are found. Damaged Meretrix spp. found here are below 30 mm in size. Weathered Cerithidea spp. found here are below 20 mm in size (Fig 24). Dark Brown medium sand to fine sand with clay soil (7.5 YR 313) of samples 1 & 2 in location no. 18 -Bundala road 2 (near 4th km) is somewhat compact. Plant roots with other organic matter are found (Fig. 25). The shells (sample 3) have deposited with grayish brown soil (7.5 YR 4/2), medium to fine sand, calcareous, clay and feldspar pebbles (rare). Meretrix spp. found here are below 30 mm and weathered Cerithidea spp. are below 20 mm in size. Grain sizes and content of inorganic matter are shown in Figures  28(18.1) and (18.2).  The shells of the study area belong mainly to three families: Veneridae (Venus clams), Arcidae (Ark shells) and Potamididae (Horn shells). The Veneridae is a large and well-known family of hard-shelled clams (strong and glossy). The shell valves in the beds from Kalametiya Kalapuwa to Bundala Kalapuwa belong to a few species; of which the dominant species in Meretrix meretrix (Plate 1). The family Arcidae a?e heavy, squarish, porcelainous clams having a so-called taxodant hingea straight hinge with numerous small teeth, about the same size. Most common ark shell species livein warm watersin sandy or muddy areas, while a few are found near coral reefs. The shells of this family of the study area belong to Anadaragranosa orAnadara uropygmelana (Plate 1). The Potamididae included into Cerithidea cingulata (Gmelin, 1791) or Cerithidea ornata (Plate 1) is a large brackish water group with elongate, solid shells usually dirty brown in colour and. many whorled. Most columella live in mangrove and estuarine areas.
The levels of the shell layers, types of the constituents, colour of the soil etc. are described in detail, and shown in Figures 5 to 28 and Plates 1 to 9. The types of shells, live position of the valves, rocky artifacts, animal bones, and human bones which were found from these shell beds are shown in these plates. Many layers of different thickness of constituents and deposition patterns indicate that the shell and shell fragments have been deposited by storm waves especially at Bataata -Gruupokuna (Plate 6). The shell beds at Kalametiya which appeared in many micro-layers can be identified based on the deposition pattern (Plates 7b and 8). The top shell layers contained tiny and small to large shells (below 40 mm in size) ofMeretriz spp. and Cerethedea spp. with calcareous sand which had been piled up by wave action, while the lower layers contained weathered shells and shell fragments. They are mixed with weathered organic material (10 YR 2/1) and calcareous clay. Bluish clay ofthe bottom layer indicates that the shells have been deposited on a grass biomass.

Emerged Shell Beds and Their Relationship to Sea-level Change
Katupothal~ecently indicated that the mid-Holocene sea-level was at least 1.5 m above that of the present level with three episodes as follows; Following these high sea-level episodes, .the former drainage basins were submerged forming lagoon and lakes further inland, sometimes 3 to 4 km inland from the present coast. The undulating lobes which were extended towards the coast and outcrops became headland. As a result, headland-bay-beaches were created in many areas along the southern coast. Furthermore, the corals presently being buried between Akurala andMatara thrived on such embayments where factors were suitable for the growth of coral especially on the southwestern and southern coast, while mollusks lived in intermediate and dry zone coastal embayments.
It is suggested that the beachrock,slightly above from the supratidal zone, on the west coast had developed around 3,700 y B.P. during this .stage. As evidenced from "C dating of shells embedded in emerged reef patches", " and corals ( in a position of growth) from emerged reef pat~hes~l"~ the climatic changes have occurred after the .mid-Holocene high sea-level. The lowering of sea level can be recognized between 5030-4390 y B.P. and 3930-3290 y B.P. by "C dating of shell beds (Fig. 29 and Table 2). The bulk of the shells of these beds have been piled up by severe storm wave action on mounds, in lagoon, lake bottoms and on sand dunes and headlands. Present investigations indicate further that the shell valves of lagoon, lake and channel beds (floors of marine and brackish pools) mostly accumulated as in situ consequently on the lowering of sea level. Hence, the sea level around 4700 and 3600y B.P. was at its present level or slightly below it. Further, "C datingof shell beds along the southern coast in the Hambantota district prove such changes have occurred during the late H~locene.".~' Furthermore, the deposits had been intermittently covered by vast quantities of coral andlor shelly sand and various types of debris moved by severe monsoon waves. This is shown, in Miniethiliya, Hatagala, Bataata-Gurupokuna, Kalametiya, Hunukotumulla, Nelumpathvila, Nabadewa and around Malala lewaya areas. The colour and constituents of the layers show that they are subject to local weathering conditions. Thickness ofthe top soil covered by these means varies locally and sometimes more than lm thick alluvial soil underline the shell beds. The deposition sequences of some shell patches of the mounds at Udamalala and on dune deposits help to infer that the valves have been discarded by early inhabitants and animals.

Conclusion
Stratigraphic sequences of the emerged Holocene shell bedsalong the southern coast between Kalametiya kalapuwa and Bundalalewaya clearly show that they are marine in origin. The shells of many ofthese beds have been piled up together ' with stone artifacts, pieces of pottery, human bones and other animal bones,by severe storq wave action on mounds in lagoon and lake bottoms, on sand dunes and headlands. Present investigations of theseshell beds,futther indicate that the shell valves .of lagoon, lake and channel beds (floors of marine and brackish pools) mostly accumulated in situ consequently to the lowering of sea level between 5030-4390 and 3930 -3290y B.P. The deposition sequences of some shell patches of t b mounds a t Udarmalala and on dune deposits help to infer that the valves have been discarded by early inhabitants and animals. The deposition pattern and different types of artifacts are valuable indicators in the study of geological, archaeological and palaeoenvironmental significance of these beds.