The Narrows in Zion Canyon are a 16 mile section with some very deep parts of the gorge up to 2000 feet (600 meters) but with a width from red Navajo Sandstone cliff face to cliff face that can be as small as 20 feet (6 meters). Some parts of it are a true slot canyon.
How has the Virgin River only eroded these parts of the Zion Canyon so thin yet in other areas its water erosion has created a width of over 300 meters?
Why does the Zion National Park have so many slot canyons?
The North Fork of the Virgin River (aka "The Zion Narrows") is probably one of the most legendary canyons to hike in all of Zion National Park. The Zion Narrows is the section of the Virgin River just upstream from the Temple of Sinawava (the end of the road up the main canyon). Here, the majestic walls of the main canyon close in to form a tall and narrow canyon with beautiful dark corners and the Virgin River flowing around you. With beautiful flowing water and barely any direct sunlight reaching the bottom, this is the slot canyon that all other slot canyons are compared to.
Hiking the Zion Narrows | Joe's Guide
If any place has the power to inspire awe, it’s the Zion Narrows, southern Utah’s premier hike in Zion National Park. For 16 miles (26 kilometers), the canyon winds voluptuously through the crimson sandstone, in some spots stretching 2,000 feet (610 meters) high and narrowing to 20 feet (6 meters). Lush hanging gardens spring from the walls, stately ponderosa pines grow in nooks, and the water can turn a shade of turquoise that perfectly contrasts with the cliffs’ deep terra-cotta hues. The hike isn’t necessarily a cakewalk, however: For more than half the time, hikers walk in the Virgin River, which can be waist-high, and negotiate cobbles as large and slippery as bowling balls.
Hike the Zion Narrows, Utah | National Geographic
The Zion Subway is a part where the Zion Narrows have been eroded away by the Virgin River to look like a train underground or train subway tunnel.
Soon enough, the canyon makes a sharp turn to the right/south where you will see the dramatic lower Subway--a very short section of canyon where both walls come together very close and a larger tubular oval has been cut out by the flowing water. This is the spot where everybody gets the famous Subway photos. Enjoy the light, the flowing water in the emerald pools, and the numerous unique pothole and weeping wall formations. This is truly a unique and magical spot in the desert.
The Subway | Joe's Guide