Arches National Park - The Power of Oozing Salt

Unlike the sand, silt, lime, and other materials that turn to stone under pressure, salt is an unstable substance. It becomes plastic under pressure, shifting and flowing like soft putty to places where the pressure is less. One such place was in this spot; because of fault lines in the underlying bedrock, the salt, set in motion by the weight of distant mountains, had been diverted upward here and had built to thicknesses of more than 3,000 feet. Over the course of tens of millions of years, as the weight of upper deposits increased, more and more salt was squeezed into this place. The salt bulged even farther upward, forming a long, slender dome. The land above it bulged, too, and sometimes cracked and was eroded away, so that the inequality of weight increased. Eventually the salt dome broke through the upper layers. When it breached the Entrada, the sandstone cracked in long parallel lines that were about 20 feet apart.

Meanwhile, even more massive geological forces had been at work. The continent itself had been at work. The continent itself had been moving, drifting thousands of miles westward and northward across the earth's molten mantle, and it had collided with the plate that lies beneath the Pacific Ocean. The force of the collision had fractured the western half of the continent and thrust mountain ranges upward. Wedged between them, a huge slab of land, which we now call the Colorado Plateau, began slowly to rise. It is rising still.

The Colorado Plateau includes much of Utah and parts of Colorado, New Mexico, and Arizona. It does not rise on a level but an angle, south side first, and as it began its most recent surge about 15 million years ago, waters were set in motion that stripped away layer after layer of sediment. Hence, the farther south you go on the plateau, the more sediments have been removed, and the older are the rock layers that are revealed. Arches is the northernmost of the Colorado Plateau's eight national parks, the other seven are Bryce Canyon, Canyonlands, Capitol Reef, Grand Canyon, Mesa Verde, Petrified Forest, and Zion, and here the Entrada Sandstone is now being worn away.

Slanting through the northern part of the park is Salt Valley, the sand-covered top of the huge salt dome that fractured the Entrada. Flanking it on both sides are row after row of freestanding fins, or ribbons, of Entrada Sandstone. They are the slowly vanishing walls that separate canyons 300 and 400 feet deep, canyons that erosion has gouged along the parallel cracks in the sandstone. It is within these thin fins of beige, salmon, pink, and red that the arches and other sculptured pieces are imprisoned. As the fins are worn and weathered away, nature's immense statuary remains.

The process by which nature frees the sculptured forms is called differential erosion, which simply means that hard, dense places in the rock erode more slowly than soft, crumbly places. Moisture seeps into cracks and porous spots and dissolves the cement that holds the grains of sand together. In the winter, the moisture freezes and expands within the rock, cracking it, flaking it off, and allowing even more water to enter and do its work.

Sometimes a fin is pierced straight through, a cave becomes a window that gradually widens into an arch. At times, though, the glow process of arch making begins on the top surface of a fin. Rainwater collects in a pothole and gradually works its way down into the heart of the rock. Eventually the trapped water finds its way out through the side of a fin, creating a burrowlike tunnel, which then expands into a pothole arch. The base of the fin, where moisture tends to accumulate, is worn away beneath the exit hole, while the span above remains dry and firm. Still the span is whittled ever so slowly, down to its hardest core; eventually it falls, leaving a pinnacle, butte, or buttress on either side. These, too, continue to weather away.

The fins are in all stages of deterioration, the arches that emerge from them in all stages of creation and erosion. There are miles-long sandstone slabs containing caves not yet cut through, and there are dwarfish-looking spires, once joined at the top, now standing in isolated couples on the buff-colored sandstone that lies beneath the Entrada. Eventually water, freezing temperatures, wind, and gravity will reduce them all to sand and carry them, grain by grain, away.