Standing on the rim of the Grand Canyon, gazing across this giant wound in the Earth's surface, a visitor might assume that the canyon had been caused by some ancient convulsion. In fact the events that produced the canyon, far from being sudden and cataclysmic, simply add up to the slow and orderly process of erosion. Many millions of years ago the Colorado Plateau in the Grand Canyon area contained 10, 000 more feet of rock than it does today and was relatively level. The additional material consisted of some 14-layered formations of rock. In the Grand Canyon region these layers were largely worn away over the course of millions of years. Approximately 65 million years ago the plateau's flat surface in the Grand Canyon area bulged upward from internal pressure geologists refer to this bulging action as upwarping it was followed by a general elevation of the whole Colorado Plateau, a process that is still going on. As the plateau gradually rose, shallow rivers that meandered across it began to run more swiftly and cut more definite courses. One of these rivers, located east of the upwarp, was the ancestor of the Colorado. Another river system called the Hualapai, flowing west of the upwarp, extended itself eastward by cutting back into the upwarp it eventually connected with the ancient Colorado and captured its waters. The new river then began to carve out the 277- mile-long trench that eventually became the Grand Canyon. Geologists estimate that this initial cutting action began no earlier than 10 million years go. Since then, the canyon forming has been cumulative. To the corrosive force of the river itself have been added other factors. Heat and cold, rain and snow, along with the varying resistance of the rocks, increase the opportunities for erosion. The canyon walls crumble the river acquires a cutting tool, tons of debris, rainfall running off the high plateau creates feeder streams that carve side canyons. Pushing slowly backward into the plateau, the side canyons expose new rocks, and the pattern of erosion continues. What does the passage mainly discuss?