Most of India falls in the Indian Subcontinent bioregion of the Indomalayan realm, which covers most of India, Pakistan, Bangladesh, Nepal, Bhutan, and Sri Lanka. The Hindu Kush, Karakoram, Himalaya, and Patkai ranges bound the bioregion on the northwest, north, and northeast; these ranges were formed by the collision of the northward-drifting Indian subcontinent with Asia beginning 45 million years ago. The Hindu Kush, Karakoram, and Himalaya are a major biogeographic boundary between the subtropical and tropical flora and fauna of the Indian subcontinent and the temperate-climate Palearctic realm.