Formation of Natural Arches of Arches National Park

If you have ever seen a Utah license plate then you know what Delicate Arch looks like. I have researched the arches in Arches National Park in Utah. This paper is designed to supply information on the geology behind the arches in Utah, specifically the area within Arches National Park. Provided first will be the geologic history of Utah and how the landscape has become the way it is. This will help explain the conditions that the arches are forming in. Secondly there will be a few paragraphs on the reasons for arch formations and the formal rules on what is considered to be an arch.

Arches National Park, which is located in the northern part of the Paradox Basin of southeastern Utah, houses thousands of natural arches. The rocks here are underlain by large salt deposits, which have been changing the landscape of this area.

Nearly 300 million years ago northwest trending faults caused the land to subside causing a basin. During this time the Uncompaghre Highlands were being elevated which bordered the north and eastern sides of the basin (Stevens and McCarrick 8). As the basin carved in it was filled with marine water from a sea that use to exist in this area. This basin soon became cut off from the sea and began to evaporate from the hot, dry climate of the area. Evaporation of this sea left gypsum, sodium, potassium and magnesium bearing sediments (Sprinkel, Chidsey and Anderson 14). The basin would occasionally fill with salt water and then become cut off again which caused another cycle of evaporation with salt left behind. According to geologic studies there has been at least 29 cycles of salt deposits. The weight of the overlying rocks, which were subsequently laid down on top of the salt deposits, exerted pressure on the underlying beds and eventually deformed them into irregular thickness with some areas having only thin layers of salt while others were very thick. This process of sediment being laid down went on for 75 million years (Sprinkle, Chidsey and Anderson 14). Because salt has a lower density than rock the salt became buoyant and squeezed its way upward. This caused areas to buckle up and create anticlines and also change the surface of the area. Water soon infiltrated the cracks on the anticline and dissolved the salt away. This caused the center of the anticlines to collapse and started the formation of valleys. This dissolution process also caused faulting to occur which led to the formation of cracks. These cracks were widened by erosion and weathering which lead to the development of fins. It is in these fins that most arches can be found. Conditions of rock thickness, hardness and cohesiveness tied into climatic conditions are all responsible for the development of an arch (Stevens and McCarrick 9).

Tectonic forces played a role in creating this landscape. There is a higher concentration of arches in landscapes that have been tectonically active in the past. Tectonic forces move the earth's crust and put it under stress which was significant to the fall of the anticlines and created joints on the sides of the anticlines which gradually would narrow out and become fins (Stevens and McCarrick 9).

Weathering and mass wasting are the biggest contributors to the formation of arches. There are some basic conditions that need to be met if an arch is going to form. First, the amount of rock an arch is to form in has to be adequate enough to allow an opening to develop. The rock needs to be strong enough to support its own weight if there were an opening to form through it. The rock must be in pretty good condition, meaning fairly free of cracks that are going to weaken the rock (Orndorff, Wieder and Futey). If there are too many cracks and breaks in the rock you are going to end up with a pile of rubble rather than a beautiful arch formation. The rocks that are weakened and fragmented by weathering are then removed by mass wasting. These processes are both important in the enlargement of an arch.

The next two effects on arch formation are wind and running water. These factors seem like they would have a major effect on the formation of an arch but in reality due little to produce an arch. Water flowing over the surface where there could be potential for an arch can pick up sediments and rocks but does little to actually modify the surface (Stevens and McCarrick 17). Wind can also play a role in removing loosened sediment but it only has negligible effects.

Some of the miscellaneous factors include the daily heating and cooling of the rock, which causes the rock to expand and contract. Freezing and thawing cycles in the winter also has a profound effect in the disintegration of rocks (Stevens and McCarrick 18). Biological communities including desert varnish can influence the rates of arch development. Even though this has not been rigorously tested, rock surfaces that have a firm coating of desert varnish seem much less prone to the usual surface weathering than those without it.

There are three basic rules for determining arches.

1. "There must be an opening through the rock, regardless of its shape, which has resulted from natural weathering and erosional processes.

2. The natural opening through the rock is always accompanied by a rock span, regardless of its size, shape or orientations. The span is typically above or to the side of the opening.

3. The opening between the rock span and any nearby rock mass must allow the passage of light along at least three feet of uninterrupted space in at least one direction (Stevens and McCarrick 22)."

These rules were given so that there would be a basis on what is verifiable an arch. If ever little opening was accepted, then there would be millions of arches and no way to document them all.

There are ten basic types of arch formations according to the comprehensive study by Dale J. Stevens & Edward McCarrick but I will only review the eight most frequently found. It should be noted that not all arches fit into just one category. Some may display characteristics of more than one category and thus are decided on which category is most like the arch being researched.

Free Standing Arch

Arches in this class "are distinguished by their relative isolation from surrounding fins, cliffs and other rock masses." These arches will develop in a fin or isolated slab of rock in the zone of weakness or in a bedding plane. The process of creating these arches normally is aided by water, at least in the initial stages. Loose material that weathers free on these arches is then carried away by mass wasting. The breaking off of pieces from the underside enlarges the arch, while the top and base of the arch experience a granular disintegration which results in a smooth top and base (Stevens and