The Gazetteers Department

By far the greater portion of the district is covered by the basaltic and amygdular lavas which have come to the surface and spread over very vast areas of the then Central India configuration of land at the commencement of the Tertiary or Cenozoic Era, nearly 60 to 100 million years ago. These lavas are spread in the form of horizontal sheets or beds. Because of their tendency to form plateaux and their dominantly basic composition, the lava flows are generally called " Plateau basalts." As these basaltic lava flows cover almost the entire Deccan region and frequently present a step-like or terraced feature on the hills, they are termed as " Deccan basalts " or more commonly the "Deccan traps."

The Deccan traps, as mentioned earlier, cover almost the whole of the district and constitute the innumerable rugged and bold, flat-topped hills, forming extensive plateaux of the entire Western Ghats. They also constitute the small hill ranges in the eastern and central parts of the district. Besides, lava flows also blanket the plains with a soil covering of varied thickness.

A remarkable feature of the traps is their horizontal disposition and considerable lateral extension over a wide area; sometimes a single flow covering nearly 300-400 square miles or even more. In places it also shows slight dip. The traps attain their maximum thickness near Mahabaleshwar and around Helvak amounting nearly 2,000 to 3,500 feet. The individual flows; vary greatly in thickness from a few feet to as much as 120 feet although the average-thickness is about 40 feet. The hills consist of several lava flows of different thinckness. In a single hill a number of lava flows, sometimes as many as 12 to 15 or even more, are seen resting nearly horizontally one above the other and these can be distinguished from a distance by the flow lines. At places these successive lava beds have thin inter-calations of a red ferruginous clay bed called " red bole." Characteristic vertical, prismatic and columnar jointings are commonly observed in the hard and compact basalts. The traps weather in concentric layers giving rise to a soft, greenish grey, friable murum leaving a hard, rounded bouldery core in the centre, known as spheroidal weathering.

Although no sedimentary inter-trappean beds have so far been reported from the district it is likely that such beds will be encountered during future detailed geological investigations. These inter-trappean beds represent the lacustrine or fresh water formations deposited during the interval of time between the outpouring of two successive lava flows and are usually represented by clay or limestone beds and occasionally, sandstones.

Petrologically, the Deccan traps are mainly basalts which are generally uniform in composition and texture. They vary from dark grey to dark greenish grey and brownish to purple in colour, and are hard, compact, tough and medium to fine grained in texture with an average specific gravity of 2.9. This type generally forms the hill and plateau tops. The vesicular and amygdular varieties occupying the lower regions are comparatively soft, like quartz, calcite, varieties of zeolites and amorphous quartz as cavity fillings. Seoreaceous, tuffaceous and breccia beds and volcanic ash are not uncommon among this type of lavas. Porphyritic trap with phenoervsts of felspars is also noticed at places.

The chief mineral constituents of the basalts are labradorite and augite, the two forming the bulk of the rock. Volcanic glass which invariably alters to palagonite, chlorophaeite. etc.. and magnetite and titaniferous iron occur in minor proportions. Olivine is present in certain varieties of trap.

Laterite occurs extensively covering almost all the plateaux of the Western Ghats- and also in the north and central portions of the district. Although laterite is noticed at different elevations there are a few notable exceptions. For instance, north of Helvak the laterite is found at 8,400 feet above sea level while the 4,177 feet high plateau of the Pandavgad fort does not exhibit any laterite formation. Malcolm Peth plateau is no less than 4,710 feel above sea level and is completely capped by laterite.

Laterite rock is ferruginous, hard, massive and generally varies in colour from dark red to yellowish and dark brown to dirty brown. A typical laterite invariably shows a red and yellow mottled appearance exhibiting a vesicular and tubular structure with a dark brown limonitic coating. A freshly cut surface of the laterite bed is usually soft but becomes very hard and tough, on exposure to atmosphere. Most of the laterite beds in the district are categorised as ferruginous laterite or a very low-grade aluminous laterite. These are considered to have been formed by the chemical alteration of the underlyine traps by a process of concentration of iron, alumina and titanium oxides with the leaching out of silica and manganese. At places, as in the Panchgani plateau, the laterite is seen to pass downwards through a zone of lithomarge into an altered trap. The laterite deposits are considered to be sub-recent in geological age.

The laterite covered plateaux which hardly support any vegetation present a very desolate and dreary appearance in the summer months. The slopes or scarps generally support some vegetation. Accumulation of big massive, dislodged masses and blocks of laterite along the slopes and foot of the hills and ridges is a common feature in the Ghat section and at other places. The laterite, ultimately yields a red to reddish-brown ferruginous soil.

Kankar is noticed at different localities in the district, especially, in the areas covered by highly decomposed traps. It usually occurs in dry streams and nala sections or in the soil mantle covering the traps. Such kankar deposits are formed by concentration of the leached out calcareous solutions from the decomposed traps in the form of tabular and rounded nodules, concretions and lumps.

The trappean country is usually characterised by a rich and fertile black soil, generally called as Regur. It is highly argillaceous and somewhat calcareous in composition and very fine grained in texture. The soil is highly plastic when wet and develops numerous deep polygonal cracks on drying. At places, the soil is also loamy. A gradual transition from a highly weathered and decomposed, greyish-green, friable murum to reddish-brown and black soil is frequently seen in the field.

Economic Geology

Bauxite.

Bauxite, the chief ore of alluminiurn metal occurs associated with laterite at a few places in the district, e.g. the Yeruli plateau north of Panchgani. The deposits, however, are small. The laterite occurring at: (1) the Plateau north of Krishna river west of Wai; (2) the ridges to the south of the Krishna river west of Wai; (3) the strip of land along the scarp from Mahabaleshwar to Helvak west of the Koyna river; (4) the group of spurs and flat-topped hills between the Koyna and Vena rivers from Mahabaleshwar to Patan; and (5) the plateau extending from the Koyna Valley below Helvak upto the boundary of the Kolhapur district may on detailed prospecting yield deposits of bauxite.

The ranges north of the Krishna comprise three main plateaux namely Raireshwar, Koleshvvar and the Yeruli of which the last named plateau has yielded some deposits of bauxite. The samples from the eastern-most capping of laterite towards the north-east of the ridge on analysis gave Al₂O₃ varying between 46.32 and 57.48 per cent, with Fe₂O₇ and SiO₂, ranging from 4.61 to 19.36 and 1.10 to 2.63 per cent., respectively. The SO₂ and P₂O₅ contents, were low. The deposits, however, are not very large.

The other ridge that is konwn to contain aluminous laterite is the Mahabaleshwar-Panchgani ridge. This is divisible into three sections, namely the Malcolm Peth, Bhilar and the Panchgani plateau. A sample from the Malcolm Peth section analysed SiO₂: 7.2, TiO₂: 4.2, Al₂O₈: 50.46, Fe₂O₃: 23.41 and H₂O: 24.99 per cent, but the deposit is not large.

Most of the laterite capped hills in the district have not been examined in detail and it is likely that detailed prospecting may yield workable deposits of bauxite in the district.

Building Materials.

Fine to medium grained, hard and compact massive traps are quite durable as building stones and are considered most suitable for construction works. The Deccan trap is also widely used as railway ballast and as an aggregate for concrete mixtures for which purposes it is best suited. These almost inexhaustible amount of traps suitable for the above purposes are available especially in the numerous hills and ridges of the district.

Zeolites and calcite derived from cavities in the weathered amygdular lava flows are hand-picked and powdered for use as Rangoli in decorating the houses and their precincts.

Limestone.

A small deposit of limestone has been recorded in the district in a small hillock near Wakhri village, five miles south-west of Phaltan. It occurs as a lenticular body resting on Deccan trap. The rock is massive and pinkish grey in colour with small pockets of earthy material as inclusions. The reserves are of an order of about 50,000 tons. The material is being profitably used for the manufacture of lime. Sporadic occurrences of dull to greyish-white, nodular kankar deposits found at a number of places in the district arc locally used for the manufacture of lime for use in constructional purposes.

Manganese-Ore.

Manganese-ore occurs in the district as concretionary noduless in the lateritic country. These concretions are formed by the chemical alteration and decomposition of the traps by which the silica and manganese contents are leached out and subsequently concentrated as concretions and nodules. The remaining constituents of trap, namely, iron, aluminium and titanium oxides together form the laterites of the area.

Small and minor concentrations of manganese-ore are found in association with laterite fragments in a number of localities near Mahabaleshwar and in the vicinity of Savitri Pass, Lingmala, Chikhli, Met-Gutad, Awakali, Bhekawli, Shindole and Takwali. The ore in all these places is of secondary origin and residual in nature and generally of low grade for any economic exploitation.

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