The Entrada Sandstone of eastern Utah was deposited during the Jurassic Period, when dinosaurs roamed the planet and the Atlantic Ocean was beginning to open. Over the next 100 million years, this sand layer was buried by younger sediments of the Cretaceous and Tertiary Periods, slowly compacting into the rock formation that we see today.
During the Miocene and Pliocene Epochs (about 25 to 5 million years ago), the Intermountain West underwent a gradual uplift, adding five thousand feet to the elevation of the region and augmenting the erosive power of its many streams. This water erosion increased dramatically during the cool, wet climate of the Pleistocene as heavy precipitation and torrents of glacial meltwater carved the landscape. Throughout this extended period of uplift and erosion, the sediments that covered the Entrada Sandstone were carried away and the formation was dissected by numerous streams, creating the spectacular landscape of Arches National Park.
Today, the Park harbors the planet's largest collection of rock spans, one of which, Wall Arch, collapsed this past week. Gradually thinned by wind and water erosion (including rock falls from freeze-thaw cycles), the arches eventually give way, adding to the rock debris that covers the floor of this arid landscape. Over time, all of the scenic structures will disappear, the remnants of the Entrada Sandstone will be washed into the Colorado River system and older rocks will adorn the surface. Of course, new layers of volcanic debris, windblown sand or sea sediments may recoat the area during this process.
