Inner Terai Valleys of Nepal - Geology

Geology

The Himalayas were formed by the collision of the Indian sub-continent with Eurasia, which began about 50 million years ago and continues today. The oceanic crust in front of India slid under Eurasia, pushing up the Tibetan plateau. The Indian continental crust is also pushing under Tibet, but is partly compressed and thrust upward to form the Himalayan mountain range, extending for over 2400 km and rising as high as 8848 m at Mt. Everest Chomolangma.

The Himalayas have four tectonic subdivisions:

• The Indus Suture Zone, where the Indian Plate meets the Eurasian Transhimalaya or Karakoram-Lhasa Block.

• The Central Himalayan Domain, the high backbone of the Himalayas bounded on the south by a fault zone called the MCT (Main Central Thrust) that has elevated these peaks three or four thousand meters above lower ranges to the south.

• The "middle hills" below the MCT south to the Lesser Himalaya or Mahabharat Range which is bounded by another fault zone, the MBT (Main Boundary Thrust) that abruptly elevates the Mahabharats 1,000 to 2,000 meters above hills further south.

• The Subhimalaya also known as the Muree, Chure Hills or Siwaliks, which are the southernmost foothills of the Himalayan Range and mainly composed of folded and overlapping sheets of sediment from the erosion of the Himalaya. They are bounded on the south by the HFT (Himalayan Frontal Thrust) elevating them about 500 meters above the Gangetic plain.

The Inner Terai valleys lie between the Siwalik and Mahabharat ranges or sometimes between different ranges in the Siwaliks. They hold flat plains with winding rivers that shift course from time to time, running northwest or southeast along the axis of the Siwalik ranges until they find a break and flow into the Outer Terai and Gangetic plain. Usually there is little difference in elevation between the Inner Terai valley floors and the plains of the Outer Terai.