On the National Park Service"s Founders Day, we"re here on the North Rim of Grand Canyon National Park in Arizona peering out at the stunning vista. Can you see the rectangular hole in the canyon wall near the top of the image? That"s Angels Window. Brave hikers can make their way up to the trail above it, but the window itself is best viewed from various points along the North Rim. Far down below is the Colorado River. Beginning about 6 million years ago, it gradually carved downward through countless layers of sedimentary, igneous, and metamorphic rock. The Colorado and its tributary streams continue to deepen and widen the Grand Canyon even today.