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GEOLOGY
The State is mainly comprised of crystalline rocks such as charnockites,
khondalites, gneisses, and Dharwar schists of precambrian age. The charnockites
are exposed in all districts, khondalites and gneisses are concentrated in the
southern part of the State and Dharwar schists and genisses are exposed in the
northern part of the State whereas intrusive granites and dolerites occur in
some parts of the State only. Pegmatite and quartz veins cut across all the
rock formations allover the State. Late Tertiary sedimentary formations,
equivalent to the Rajahmundry sandstone form a
linear coastal outcrop to the north of Cannanore and from Kottayam to beyond Trivandrum in the south.
These, together with alluvium, cover an area of about 6000 km2. The
'sedimentaries are over 300 m thick in the widest part of the outcrop between
Sherthalai and Karunagappally. The top of the beds is 70 m below msl at Cochin, and rises to 80
m above insl south of Trivandrum. They are of
riverine origin, and comprise, from bottom to top (i) Vaikom beds of over 100 m
thickness of sandstone-clay with four lenticular granular horizons, (ii) Quilon
beds of fossiliferous limestone, marl, sand and clay of about 70 m thickness,
and (iii) Varkkallai beds of about 80 m thickness of two persistent
interconnected gritty sandstone horizons with interbedded clay apd lignite. The
Quilon beds do not persist eastward, and thin out 7 km short of the eastern
boundary of the sedimentaries. Laterites cover wide areas in Kerala. All along
the midland region it forms as a residual deposit due to weathering of either cystalline
or sedimentary' rocks. The thickness of the laterite generally varies from 5 to 8 m. Plateau laterites of greater thickness is seen
in Malappuram, Kozhikode and Cannanore
districts. Lateritic soil is predominent in the midland regions. Alluvium overlying
laterite'is 14 to 20 m thick. It extends all along the coast from Kasaragod to Trivandrum. It is
essentially sand, clay and silt. The plate shows in detail the geologic setup
of Kerala (Geological Survey of India).
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