As a daytrip during our March 2002 visit to Phoeniz, AZ, we (Mom, Dad, Joanna, me & Aunt Cathy) drove up to the Grand Canyon. It was simply astounding.
It's easy to read numbers on the canyon (10 miles across, etc...), but standing there looking at its incredible vastness was unique in my experience. The most amazing part was just knowing that this little Colorado River was responsible for wearing through hundreds of thousands of years of rock to create the canyon. We had the full benefit of staying to watch the effects of the setting sun too, which really brought out the colors of the strata.
