The Peninsular Block

The Peninsular Block is the oldest and most stable part of India, a vast tableland built of ancient rocks that has remained largely unchanged for hundreds of millions of years. To understand its structure, you first need to see where it begins and how far it extends.

Its northern boundary is not a straight line but an irregular one. It runs from Kachchh in the west, along the western flank of the Aravali Range near Delhi, and then continues roughly parallel to the Yamuna and the Ganga rivers until it reaches the Rajmahal Hills and the Ganga delta. Beyond this main block, there are extensions of the same ancient landmass in the northeast — the Karbi Anglong and the Meghalaya Plateau — and in the west, Rajasthan. The northeastern parts are separated from the Chotanagpur plateau by the Malda fault in West Bengal. In Rajasthan, the desert and desert-like features simply overlay this old block, hiding it beneath sand and arid landforms.

The Peninsula itself is formed essentially by a great complex of very ancient gneisses and granites, which make up a major part of it. Since the Cambrian period, it has stood as a rigid block, with two exceptions: some of its western coast is submerged beneath the sea, and some parts have been changed by tectonic activity without affecting the original basement rock. As part of the Indo-Australian Plate, it has been subjected to various vertical movements and block faulting. The rift valleys of the Narmada, the Tapi and the Mahanadi, along with the Satpura block mountains, are examples of this faulting.

The Peninsula mostly consists of relict and residual mountains — old, worn-down hills that have survived erosion while surrounding areas were lowered. Examples include the Aravali hills, the Nallamala hills, the Javadi hills, the Veliconda hills, the Palkonda range and the Mahendragiri hills. Because these are ancient, weathered mountains, the river valleys here are shallow with low gradients, unlike the steep, deep valleys of the Himalayas. Most of the east-flowing rivers form deltas before entering the Bay of Bengal; the deltas of the Mahanadi, the Krishna, the Kaveri and the Godavari are important examples.

The third geological division of India is the Indo-Ganga-Brahmaputra Plain, the plains formed by the Indus, the Ganga and the Brahmaputra rivers. Originally a geo-synclinal depression, it attained its maximum development during the third phase of Himalayan mountain formation, roughly 64 million years ago. Since then, it has been gradually filled by sediments brought down by both Himalayan and Peninsular rivers. The average depth of alluvial deposits in these plains ranges from 1,000 to 2,000 metres.

These significant variations in geological structure across India have far-reaching impacts on other related aspects, especially physiography and relief. The relief and physiography of India have been greatly influenced by the geological and geomorphological processes active in the subcontinent. …