In the eastern part of Utah, near the town of Moab, lies Arches National Park, where nature's rock sculpting has produced some unbelievable shapes, most particularly an abundance of geological "arches" or "windows". In fact, Arches NP contains the greatest density of natural arches in the world.
Arches is located in the high desert of the American southwest, with elevations from 4,085 to 5,650 feet above sea level and very little rainfall. The climate ranges from very hot summers to cold winters and temperatures may fluctuate as much as 50 degrees (Fahrenheit) in one day.
Geological arches and windows start from narrow fractures in the rock. These fractures are widened by "frost wedging," a process whereby water seeps into the cracks during the day, then freezes at night. As water freezes it expands by almost 10%, thus forcing the cracks wider and wider. (This same process creates the potholes in roads that plague drivers in many parts of the country.) When this frost wedging process occurs in a narrow "fin" of rock, an arch or window may develop.
For more information about U.S. National Parks in general, visit the web site of the National Park Service.