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The toposheet number is 47 H/8 and 47 H/12. From malvan bay a chain of submerged and exposed rocky islands extend towards south. In this chain there are several islands and other small islets around sindudhurg fort are mandle rocks, malvan rocks etc. sindhudurg fort is situated around north tip.
Wilkinson (1871) gave a brief account on sketch of the geological structure of southern Konkan. Iyer(1939) gave a generalized account of the geology and structure of the area. some references to the geology and structure of this area are available in the progress reports of Hemmady l960), Konala 1963), Suryanarayana (1965) etc.
The Rivers draining in these areas originate from sahyadris with distance of 20-30 kms. The drainage observed is dendritic , they run straight and rarely meanders. No deltas are found along the coast may be due to small drainage basin, low sedimentary supply and estuarine conditions of river. Achra and gad river drainage pattern is controlled by tectonic lineament. 4
The geomorphic units of the area are classified into three categories On the basis of their form, genesis and geomorphic processes.
Units of marine origin are: beach sands, stabilized suits with sand dunes, beach ridges, and wave cut benches and platform.
Units of fluvial origin the mud flats and alluvium, channel bars and point bars, and marsh and mangrove areas represent the present flood plain.
Unit of denudation the alluvial terraces and higher valley fills represent the older flood plain. The dissected coastal upland with isolated hills has been considered as a origin.
The different litho units exposed in the area, in general trend NW-SE to E-W.
The structures noticed in the area are usually of two types viz
(a) No tectonic structure
(b) Tectonic structure.
No tectonic structure
Bedding plane is represented in the form of composition and colour bands as is noticed in Asrondi quartzite, BHC, staurolite- schist and carbonaceous rock near Ramgarh.
Tectonic or secondary structures include foliation, lineation folds, shear zones/faults and joints.
The area mapped, is covered by thick cover of laterite and soil and so it is difficult to interpret the structure, as ideal exposures for the measurements of attitudes are rare. The following observations, however, have been recorded from a few scattered outcrops exposed in valley and well sections. 5
The foliation trend of the Archaean rocks, is roughly NW-SE with steep dips ranging from 75° to 80°, mostly to the west conforming to the Dharwarian strike trend except for a few quartzite outcrops at places near Malvan, which are almost vertical.
Narrow linear strips of beach sands with width varying from 30 to 300 metres are seen developed along the coast mainly restricted to the emblements and estuary mouth. This unit is characterised by very light tones and fine texture and is devoid of vegetation. A string of linear partly submerged off shore sand bars are observed near the almost straight shoreline between Achra and Kalavale crocks.
The extensive outcrops of Meta quartzite are observed within the recent beach sands. Along the coast of malvan and nivti well developed closed spaced fractures and slicken side's surface within the conglomerates and quartzite's are not common. These oligomict conglomerates and quartzite's exhibits well developed primary sedimentary structures such as medium scale wedge and trough cross bedding, sub-horizontal plain bedding, hummocky bedding, wavy cross lamination, trochoidal ripple lamination etc.
Extensive outcrops of orthoquartzite and shales are peculiar to the west margin of the area.
A broad coastal plane of about 2 kms width and about 5 kms length is developed between malvan and karli.on this coastal plane 3 to 4 beach sand dunes ridges alternating with tidal flat clays are easily distinguished on aerial photographs, as on beach sand dune ridges heavy coconut plantation and low lying tidal flat clays exclusively paddy fields are present. 6
To the north of malvan the most striking feature of the beach is the littoral concrete or beach rock,which continues over long stretch. This littoral concrete occurs as a rocky beach either directly attached to the mainland or separated by sandy or marshy areas. The rocky beach gives protection to the coast against strong waves. These rocks are exposed only when sea recedes. They have some general composition like the loose beach sand. Its friable to well cemented rock consisting of mineral grains , fragments , calcerious debris which are cemented by CaCO3. occurrence of beach rock in inter tidal zone as well as 2-4mts above present day high water line may perhaps indicate minor fluctuations of sea level.
Beach Ridges and Dunes
They are the sets of very low sandy ridges which are parallel roughtly to present shoreline. Tthese represent continous retreat of the sea along the coast, therefore each was once a shoreline in itself.
Recent dunes very light tone in air photes and shows rise of 10-15 km islands. These dunes are stabalised by heavy growth of thick vegetation. Resent dunes in the area are cut by wave action indicating that it is now depositional in nature.
Pirwadi talashil area is a sand bar represented by narrow submergent coastal plains; they also have sandbars located at mouth of river Gad. The river got beaches on its seaward side limit and tapering end on river side.and are oblique on the sea side this indicates waver are important agent in keeping the inlet open. Wave behaviors in this area is west - south - west dominant and west North West subsidiary. On the coastal sand bars heavy coconut plantation is mainly observed.
Feldspathic gneiss has well developed mesoscopic folds exhibit various shapes. Whereas kudal kankavli and vengurla archaean rocks are tightly folded in varieties of anticline and syncline. 7
It ha been recagonized that the area has undergone three phases of folding. This was evidenced from the small scale structures within the rock types in and along the area under consideration. The three generations of folds that are recagoined are:
They are one which are tight isoclinal, their axial plane trending along NE-SW and rarely NW-SE, with steep plunge towards west. They are well preserved in BHQ band exposed near Ashiye. At place their axial trace trends North West-South East.
The second phase of deformation has resulted F2 folds, these are tight to open (asymmetrical) and at times reclined in nature. Their axial planes trends North-South to North West-South East and plunges moderately towards east.
They generally are gentle to open warps, their axial plane trends NNE-SSW to North East-South West and plunges moderately towards east. F3 folds are developed on the limbs of F2 fold.
Three major folds have been noticed near Gothane, south of Oros
Budrukh and Asrondi, whose axes trend North West -South East. The mineral assemblages of pelitic/semipelitic rocks reveal that they have suffered almandine-amphibolite facies of metamorphism, whereas ultra basic/ basic rocks have undergone green schist facies of metamorphism.
Eye folds interference of F1 and F2. Folds have given rise to development of eye folds in Pisekamte quartzite. Their long diagonal defines the F1 fold and short diagonal represents the F2 fold
Sudden difference in elevation of archaean and deccan lava flows is indicative of faulting. Galavadi has been faulted between kaladgi and deccan lava flows lava flows exhibiting horizontal and vertical joints in all directions. 8
In general, spacing of joints in granitic gneisses is more. At places, joints are filled by secondary silica and epidote.
Along the area there are abundance of cliffs. These are active cliffs, active due to present marine process. Here the rate of regression of clifed coast varies with elevation, rock resistance, geological structures and instant wave energy. Under this area retreat of cliffs are pronounced. Due to direct attack of waves, large cracks are developed in laterites. Fall of sea level might leave previous beach abrasion platform with abandoned cliffs.
Wave cut platforms
These are generally seen along the cliffed coast of the area. All along the area the cliffs and headlands are bordered by wave cut platforms. These platforms have gentle slopes. Some show thin veneer of alluvium, these terraces are distinct regression of sea level along the coast.
Rocky islands are the isolated outcrops. Generally situated 1-2Km off shore maximum by far in 6-8 km. they represent detached portions of coastal headland that are now isolated from the coast. Some islands are connected to mark land by alluvial deposits.
These units are restricted to narrow belt ranging 200m wide along the coast.55% of Indian coast is beach fringed, invariably sandy,gently gradient. These are generally assossiated with sandy beaches found at the bay heads along straight shores on the seaward side of the sandbars. 9
These are the common phenomenon seen in the coastal area under study. They are generally seen along the sea cliffs where there is protruding part of the cliff gets detached from the main mass and falls off, this happens due to clasing of the waves along the surface of these cliffs.In this case the mass below the cliff starts eroding and slowly after a span of time the mass topples down into the sea.
This is kind of similar phenomenon seen along the malvan coast but the difference here is that the mass due to lashing of the waves on the wall of cliff cracks are originated from the top, this mass then slips and falls off.
Thus topple and slips are the two associated structures seen along the coast. Where the vegetation is covers over the fallen land mass its toppling and if the vegetation is dying it's a slip. 10