The Tethys Ocean closed about 50 million years ago and Indo-Australian Plate began to collide with Asia. The Himalaya is now mostly formed from the uplifted sedimentary rocks from this ocean. These sedimentary rocks along with associated igneous rocks, now often metamorphosed, from a series of East-West slivers of rock through the mountain range. From South to North there are four main tectonic sub-units within the Himalaya: the sub-Himalaya, Lesser Himalaya, Central Himalayan Domain and Indus Suture Zone.