For those that like geology, Arches is a unique place. When you look around you primarily see the iconic salmon-colored sandstone that many associate with the southwest. That sandstone is Entrada sandstone. There are many other types scattered throughout the park as well.
The basis for how the arches, towers, and other unique structures scattered throughout the park starts with salt. Crazy right? The area that the park sits upon is a giant salt bed that formed ages ago after the sea that covered the area dried up. It got covered in layers and layers of debris which in turn were compressed into rock. The weight of the rock layers caused the salt layers to liquefy, shift and break, creating the domes, fins, and cavities we see. The many archways that the park is known for developed over many years as rainwater worked its way into the fractures in many of the domes and fins slowly eroding its way through the weakness. In winter, it would then freeze and expand, breaking off rock pieces and eventually creating arches and other structures. If you are interested in learning more, check out the NPS resources.