Arches National Park - Why Are The Arches Here?

Before nature could set about freeing this immense collection of sculpted forms, she had to imprison them in a layer of sandstone some 500 feet thick. The rock had to be a subtle blend of many textures and densities so that it would erode away, not smoothly and evenly, but in odd ways. To accumulate the grainy materials that would form this layer, called the Entrada Sandstone by today's geologists, was the labor of more than 20 million years, and it required the creation and withdrawal of an entire ocean. Another 125 million years or more went into cementing the materials into solid rock, cracking the rock in special ways, lifting it, and exposing it to the gentle tools of erosion.

The world that gave birth to the Entrada existed along the shores of a big shallow sea that spread westward from this place. During the staggering stretch of time that this world lasted, there were many variations in the lay of the land. Each variation was to invest the future rock with a different blend of densities and textures.

At first the seashore itself came and went, so that some of the sand that was to be cemented into the Entrada was deposited beneath shallow water and some along the tide-washed shore. Sometimes there were tidal marshes and brief swampy stretches dense with life, and their thick, muddy substances add to the Entrada's complexity. Back from the shore, windswept dunes moved incessantly, accumulating to depths of hundreds of fee. Over the lo9ng ages, as the sea slowly withdrew, perpetually shifting winds piled the dunes high over the vanishing shoreline in capricious patterns, reflected in the cross-grained textures of the rock. At various times and in various places, streams and rivers made their way through the dunes to the sea; their courses left pockets of firm rock.

The Entrada was deposited atop the leavings of many other worlds, deserts, swamplands, ocean beds, and verdant valleys, that had existed earlier in this same place; and it was eventually covered by other, different layers whose weight turned the sand into rock. But it was one of the deepest layers, a 300-million-year-old bed of salt, left when another ancient landlocked sea had evaporated, that was to provide the stress that would eventually result in today's awesome display.