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 ago.