The sandstone of the Colorado Plateau forms thin ridges causing some of the canyons to be deep and narrow. The most famous of these narrow slot canyons is the Zion's Narrows in Zion National Park. The entrance to the Narrows is wider and has a paved path. Very quickly the canyon narrows and progress up the canyon can only be made by walking in the Virgin River. This type of canyon exploration can be very dangerous during times of flash flood danger. A number of people have lost their lives when swept downstream by the floods.