Angel Falls, the world's highest uninterrupted waterfall, was formed by the unique geological structure of the Guiana Shield in Venezuela. The falls drop from the summit of Auyantepui, a massive "tepui" or table-top mountain. Geologically, these tepuis are composed of ancient Precambrian sandstone and quartzite layers that were deposited over a billion years ago. Over hundreds of millions of years, tectonic uplift and intense erosion by wind and heavy tropical rains carved the surrounding softer rock away, leaving the hardened tepuis standing as isolated plateaus. Unlike many waterfalls fed by snowmelt or large lakes, Angel Falls is primarily fed by rainwater that accumulates on the massive 700-square-kilometer plateau of Auyantepui. This water collects in fissures and rivers before plummeting 979 meters (3,212 feet) over the vertical cliff edge. The sheer height is so great that much of the water evaporates into mist or is blown away by the wind before it ever reaches the "Kerip" river at the base.