Introduction

The story of the Indian subcontinent begins long before the maps we know today. The earth itself is roughly 4600 million years old, and every mountain range, plateau, and plain we see is the product of an immense, slow-motion history. Two kinds of forces have driven this change: endogenic forces, which work from within the earth (like plate movements and volcanic activity), and exogenic forces, which work on the surface (like rivers, wind, and glaciers). Together, they have sculpted both the surface and subsurface features of the land.

You have already studied plate tectonics and the movement of the earth's plates. Now apply that idea to India. Millions of years ago, the Indian plate lay to the south of the equator, and it was much larger than it is today — the Australian plate was once a part of it. Over time, this giant plate broke into several pieces. The Australian plate drifted towards the southeast, while the Indian plate moved northward. This northward journey is not a finished event; it is still continuing, and it has major consequences for the physical environment of the Indian subcontinent. Think about what those consequences might be — the collision of the Indian plate with the Eurasian plate, for instance, is what gave birth to the Himalayas and continues to shape the region's earthquakes and mountain-building.

It is the interplay of these endogenic and exogenic forces, along with the lateral movement of plates, that created the present geological structure and the geomorphologic processes active in India today. Because the geological structure and formations vary so much across the country, India can be divided into three broad geological divisions. These divisions roughly follow the physical features of the land:

• The Peninsular Block
• The Himalayas and other Peninsular Mountains
• The Indo-Ganga-Brahmaputra Plain

These three divisions form the foundation for understanding India's physiography — its structure, relief, and the drainage systems that flow across it.