JoeO: the Park Service web site says Drainage systems have cut deeply through the rock, forming numerous steep-walled canyons. and It is considered one of the finest examples of arid-land erosion in the world. The Canyon, incised by the Colorado River, is immense, averaging 4,000 feet deep for its entire 277 miles.

Like JeremyC says, listen to/read what the Park is saying, not to what one book sold there says. The bookstore, BTW, is run by the Grand Canyon Association, a private organization. They also sell books about anthropomorphic coyotes and javalenas.