Foreword
A phenomenal growth has been achieved in our understanding of the Geology of India since India's Independence through the efforts of the Geological Survey of India and several other State Surveys, exploration agencies, research organizations, universities, institutes and scientific societies. Many of our earlier concepts of geology in relation to structure and tectonics, stratigraphy and sedimentation, magmatism and metamorphism, and metallogeny and mineral deposits have undergone drastic changes with larger inflow of data, synchronising with the evolving global concepts of earth processes. New dimensions have been added to Indian stratigraphy by detailed geological mapping on the one hand and through geochronology and to a limited extent geophysics and geochemistry. The need for a comprehensive, nevertheless succinct account of the geology of India, keeping in view the above developments, has been keenly felt and a positive step has been taken by the Geological Society of India in entrusting to two of our outstanding earth scientists, Prof. R. Vaidyanadhan and Dr. M. Ramakrishnan, the task of addressing this need. Professor Vaidyanadhan hails from an educational stream from the Andhra University and is widely known for his expertise in geomorphology and a capability for incisive synthesis and review.
Dr.
M. Ramakrishnan has several decades of dedicated field work in different parts of the country to his
credit under the banner of the Geological Survey of India and in guiding
streams of younger scientists in geological mapping and interpretation.
Both the authors have published widely quoted papers of lasting value.
They served the Geological Society of India in the challenging task of
being successive editors to the Journal of the Society and have
established editorial capabilities. It goes to the credit of these two
earth scientists that they, in the present two volumes, provide an
exhaustive and commendable synthesis of Indian geology, covering the
major recent advances. They have thereby generated, perhaps, the almost
up-to-date and authentic publication on the subject now available.
Introduction and Physiography
INTRODUCTION
The Indian subcontinent is a conspicuous physical entity in a map of the Asian continent. In the north it is bordered by the Himalaya Mountains and it is surrounded by Arabian Sea in the west, Indian Ocean in the south and Bay of Bengal in the east. Within the subcontinent three major divisions will become evident. These are usually referred to as Peninsular India in the south, the Extra-Peninsular India in the north and the Indo-Gangetic Plain in between. Each one of these has distinguishable geological and geomorphological characteristics. For administrative purposes India is divided into 30 States and 5 Centrally Administrated Union Territories (Fig. 1.1). Of these, the States of Uttarakhand, Jharkhand and Chhattisgarh have been only recently (2000-2002) carved out of Uttar Pradesh, Bihar and Madhya Pradesh States respectively. Pondicherry State is now named as Puducherry. Peninsular India is made of many mountain chains, plateaus and narrow coastal plains, interspersed with deltas and estuaries. Deformed igneous,
sedimentary and metamorphic rocks are met with and they host enclaves of some of the oldest rocks of the Earth (>3300 Ma). The rocks and sediments range in age from Precambrian to the Holocene. The landscape consists of extensive high-level surfaces, some of which are dissected. Majority of the rivers flow from the west to the east into the Bay of Bengal, except for the rivers Narmada and Tapti, flowing due west. The Extra-Peninsular India is made of very high rugged mountain ranges of the Himalaya, separated by deep valleys. Majority of the rocks are sedimentary, though igneous and metamorphic rocks are also present. Most of them are Phanerozoic (<600 Ma) though Precambrian igneous and metamorphic rocks are also known. Folds and faults are clearly seen and thrust sheets can be visualised.
Perennial rivers like Indus, Ganga (Ganges), Yamuna and Brahmaputra and some of their tributaries originate from different parts of Himalaya and the neighbourhood. Some of the major rivers pass through deep gorges cutting across the ranges, before flowing down south on to the plains. The Indo-Gangetic Plain, into which the rivers mentioned above enter, is an extensive plain, sloping gently due east-southeast. The unconsolidated sediments in this plain are derived both from the Himalaya in the north and the shield in the south, though more from the north.
This is one of the greatest foredeeps in the world. Alluvial fans along the northern borders, terraces and flood plains are common features. The rivers, which meander and are braided in many places, present a variety of landforms. Though the Ganga basin might have been initiated during the Tertiary (<65 Ma), the sediments in them are essentially Quaternary in age (<2 Ma). This is mainly because the uplifted mountains became a provenance for these sediments in late Tertiary only (<20 Ma).
A brief description of some of the physical features and characteristics of the mountains, glaciers, rivers, lakes, coasts and the seas around is given here followed by an introduction to climate, soil, natural vegetation and groundwater. In India there are landforms formed practically due to all geological agents of erosion and deposition though those due to Pleistocene continental glaciation are absent or not widely recognised.
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