The subcontinent was once part of Gondwana, the ancient southern supercontinent. Geologically, the origin of the Himalayas is the impact of the Indian tectonic plate travelling northward at 15 cm per year to slowly hit the Eurasian continent, about 40-50 million years ago. The formation of the Himalayan arc resulted as the lighter rock of the seabeds of that time were easily uplifted into mountains. An often-cited fact used to illustrate this process is that the summit of Mount Everest is made of marine limestone.