File:Landscape Arch (Arches National Park, eastern Utah, USA) (16130473811).jpg

Landscape Arch, developed in the Jurassic of Utah, USA.

Rock arches are rare erosional features. Rock arches that are not formed by river or stream erosion are called "natural arches". Those that are formed by river or stream erosion are called "natural bridges". Natural bridges are rarer than natural arches. Very small erosional openings in rocks are called windows. Larger erosional openings are arches.

Arches National Park in eastern Utah has the highest concentration of natural arches on Earth. Over 2000 natural arches have been identified, in all stages of development.

The natural arches in the park are developed in the Entrada Sandstone (Middle Jurassic), which is entirely composed of siliciclastics stained red by abundant hematite (= iron oxide, Fe2O3). Most of the arches occur at & just above the contact between the lower and middle members of the Entrada Sandstone.

Traditionally, the Entrada Sandstone has been subdivided into three members: a lower Dewey Bridge Member, a middle Slickrock Member, and an upper Moab Member (a.k.a. "Moab Tongue"; a.k.a. "Moab Tongue Member")..

The lower unit, the Dewey Bridge Member of the Entrada Sandstone, consists principally of relatively soft, interbedded siltstones and fine-grained sandstones that have contorted and wavy bedding. Recent stratigraphic revision (e.g., Doelling, 2010) has changed the name to the Dewey Bridge Member of the Carmel Formation. I reject this formation-level stratigraphic assignment. The Dewey Bridge is time-equivalent to the Carmel Formation, but the bulk lithology is quite different, necessitating a different formation name, according to the code of stratigraphic nomenclature.

Overlying the Dewey Bridge is the Slickrock Member, a unit of relatively hard, massive, quartzose sandstones.

Differential weathering & erosion along the sides of relatively narrow Entrada Sandstone fins, or walls, has formed blind arches and, ultimately, natural arches. Some of the park's arches have their origin as deep potholes that have punched through.

Shown above is Landscape Arch, which is one of the longest rock arches on Earth. It is about 300 feet long and about 100 feet high. A portion of this natural arch is only 6 feet across. This spectacular feature will collapse soon (geologically). A significant partial collapse event occurred in September 1991 - broken talus is abundantly scattered beneath the arch. The detached mass was estimated to be about 60 feet long and weighed about 180 tons.

From park signage: "What caused this cataclysmic event? Water had been slowly shaping the arch for countless centuries, dissolving cement between sand grains, seeping into tiny cracks, freezing and expanding. What had finally upset the delicate balance? Unseasonably heavy rains the preceding ten days may have filled pore spaces within the sandstone. The added weight may have finally overwhelmed the rock slab in its timeless struggle with gravity."

Locality: Landscape Arch, Arches National Park, southern Grand County, eastern Utah, USA (38° 47' 26.28" North latitude, 109° 36' 26.13" West longitude)

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