GENERAL

GEOLOGY

[The section on Geology is contributed by the Geological Survey of India, Government of India, Calcutta.]

The only geological formation in the district is the deccan traps (Cretaceous-Eocene). The deccan lava flows are found usually in the form of horizontally bedded sheets. At places a gentle dip of about 5° to the west is noticed. Vertical or inclined jointings at right-angles to the bedding planes of the trap are marked at places. The flows usually form flat-topped hills so characteristic of the trappean country. The traps belong to the type called 'plateau basalt'. They are more or less uniform in composition corresponding to dolerite or basalt. These are dark grey or greenish grey in colour. These traps are distinguished into vesicular and non-vesicular varieties. The non-vesicular types are hard, tough, compact and medium to fine-grained, and break with a conchoidal fracture. The vesicular types are comparatively soft and friable and break more easily. The amygdaloidal types are characterised by vesicles filled with quartz, chalcedony, calcite and zeolite.

Some parts of the district, especially the river valley areas, are occupied by the typical black soil derived from the deccan traps. All the types, though varying in quality, are fertile on the whole. The black soil contains high alumina and carbonates of calcium and magnesium with variable amount of potash, low nitrogen and phosphorus. The soil is generally porous and swells considerably on addition of water, and dries up with cracks on losing moisture. The black soil is very fertile and does not require manuring for long periods. The broadest belt of this rich soil is found in the Krishna valley. The soil of reddish brown colour is found on the hills. This type of soil becomes fertile on proper manuring and irrigation.

There is no mineral of economic importance. The deccan traps serve as good building materials. They can also be used as road metal and railway ballasts.

The inter-trappean beds generally form aquifers. In the area comprising Tasgaon, Walwa and Shirala talukas, which is composed of deccan trap flows, the main aquifers are either the inter-trappean beds or the decomposed zones in the traps. The depth of the water-table is variable, generally being more than 6 metres. In general the deccan traps are unreliable sources of ground-water because of the sporadic distribution of their inter-trappean beds. Supplies are often exhausted owing to the limited storage areas and by leakage through natural springs. Shallow wells located on the banks of streams usually give fair supplies for household purposes. The quality of ground-water is good for all purposes, excepting where contamination results through the introduction of foul matter or by infiltration. Contamination is very common in the area and as such the water is hard, especially if drawn from the decomposed, amygdaloidal varieties of the traps, and is often brackish owing to the presence of sodium chloride and the sulphates of calcium and magnesium.

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