Zion National Park - Birth of a Canyon

The centerpiece of the scenery, however, inevitably remains the canyon itself. The main architect of that masterpiece in stone, the North Fork of the Virgin River, is not an especially large river, but it does flow fast, dropping as much as 160 feet per mile in some sections. And after thunderstorms, water rushing off the bare rock of the surrounding uplands can transform it into a raging torrent, one that sweeps along a load of more than a million tons of sand, grit, and even thumping boulders every year. Armed with all this water-borne debris, the Virgin has been likened to an endless belt of sandpaper that scours ceaselessly away at the rocks of the canyon floor.

But if the river is the carving tool, the rocks of Zion are the raw material from which it has created its breathtaking sculptures. The oldest in the canyon, exposed in multicolored bands along the lower levels of the cliffs, began to form some 250 million years ago. Alternating layers of limestone, sandstone, shale, volcanic ash, and other kinds of rock, they tell a tale of changing conditions over the course of the past 35 million years. Some of the rocks were deposited as sediments on the floor of a shallow sea, others on coastal plains and mud flats, in vanished lakes, and in the backwaters of bygone rivers. Fossils of clams that lived in those ancient seas, and footprints left where dinosaurs walked across streamside mud, are reminders that even then there was life in this place.

A few of the rock layers also contain chunks and pieces, even whole logs, of petrified wood. Washed in by rivers flowing from highlands far to the south long before there was a Grand Canyon to block the way, the battered trees came to rest here and were covered with silt before they had time to rot. Gradually, silica in groundwater seeping through the wood replaced the organic matter in individual cells and transformed them into stone. Today in places like Huber Wash, some of the gray stone logs have been exposed and look very much like slowly disintegrating tree trunks.

By 190 million years ago, the climate had become so dry that there was practically no precipitation at all. The entire region, in fact, became a Sahara-like sea of windblown, shifting sand dunes. Eventually piling up to a thickness of 3,000 feet or so, they would in time be transformed into the Navajo Sandstone now exposed on the upper canyon walls. The contours of the wind-shaped dunes, piled one on top of the other, are still visible in the rock.

The dunes were consolidated into sandstone when the sea once again invaded the area, about 175 million years ago. Red iron oxide and calcium carbonate in water seeping through the drowned dunes cemented the individual sand grains into a solid mass. Over the ensuing eons, as the sea came and went, layer upon layer of additional sediments, some 2,800 feet in all, was deposited on top of the Navajo Sandstone, though most of these younger rocks have since been eroded away in the Zion area.

The actual carving of Zion and the park's other canyons came about with the long, slow uplift of the Colorado Plateau, the final phase in a period of tremendous turmoil that began about 65 million years ago and continued in fits and starts that spanned millions of years. Volcanoes erupted during these periods of crustal instability, and their signatures, basaltic lava flows, are common in Zion. (Crater Hill and other well-preserved volcanic cones along the western edge of the park are products of much more recent eruptions.)

A phase of vigorous uplift, beginning some 15 million years ago, dramatically steepened the gradient of the Virgin River, and the increased water supplies during the Ice Age climate of the past million years further heightened its cutting power. During the course of the uplift, the great mass of Navajo Sandstone was riddled with networks of cracks, called joints. These formed a path of least resistance to erosion and so determined Zion's basic pattern: precipitous canyons walled in by massive blocks of sandstone.

The most dramatic of the park's erosion-enlarged fractures is the Narrows at the head of Zion Canyon. In some places along this 12-mile route, the slotlike defile is 2,000 feet deep and only 30 feet wide. Walking and wading through the remarkable gorge, which normally contains only a small stream of water, is a popular trek with park visitors, as long as weather forecasts are favorable. But beware if there is any possibility of thunderstorms: within minutes the rapid runoff of water from many square miles of bare rock can transform the usual flow into a wall-to-wall flash flood up to 30 feet deep.

The Narrows remains so deep and constricted because the river has not yet eroded its course all the way down through the Navajo Sandstone to the layer of impermeable shale at its base. Farther downstream, where the river has sliced its way down to the shale, the canyon is much wider. The shale is very soft and easily washes away, and when it washes away, the sandstone above it loses its support and caves in. The cement between individual grains in the sandstone just above the impermeable shale dissolves, and that also contributes to the undermining of the canyon walls. And so over time they have retreated as the rocks come tumbling down, sometimes in massive slabs but more often in bits and pieces that rapidly disintegrate and are washed away by the river.

Occasionally, as the rocks are undermined by erosion at their base, the entire cliff face does not collapse. Instead, only the lower section gives way while the top of the cliff remains intact. The result is a natural arch, sometimes of massive proportions. Most of the arches in Zion are so-called blind arches, alcoves in the faces of cliffs, but there are also a few freestanding arches. One of them, the Kolob Arch, spans 310 feet and is the largest freestanding arch in the world.

And so, bit by bit, Zion is falling apart: its beauty is the handiwork of forces of destruction. Occasionally we can see the changes while they are occurring, as on August 1, 1968, when 5,000 tons of rock fell on the Riverside Walk. But normally the course of change is unimaginably slow, at least in human terms. To transient visitor, the majestic walls of Zion Canyon seem immutable.