Postcard from The Arches - What is an Arch?

Of all my visits to western US destinations, The Arches National Park ranks as one of my favorites as it is not only quite beautiful, but absolutely fascinating in a scientific sense . It is a stunning landscape of red rock formed in strange shapes that is wonderful to behold, even if there were no natural arches. But, the arches are the highlight of this fantastic natural show.

According to park information, there are approximately 2,000 (maybe more) known natural sandstone arches in the park. Now that is a big, big number, considering the park is not all that large and a vast portion of the park landscape couldn't possibly hold any arches. Let me explain. When you drive into the park, the first thing you do is climb up over a 1,000 feet very quickly. You emerge on a large desert plain with a lot of empty, nearly flat spaces, but also randomly spaced gigantic rock formations in different sizes and shapes. He is where you find the arches.

Even so, 2,000 seemed like a large number, so I assumed geologists had a rather liberal definition of what constitutes a real arch. Well, I was wrong, as a very patient park ranger explained to me. He showed me a computerized database and books with pictures cataloging each of the arches...some very large and some quite small.

Without getting into great detail, arches are formed by a combination of forces, mostly water, ice and gravity. Wind plays little or no part. Water erodes the sandstone slowly, breaking down the molecular structure of the sandstone...ice expands and contracts inside the rock, weakening it even further, and gravity brings down portions of the rock where the molecular bonds have been weakened beyond the ability of the rock to hold it together. As the park only gets 8"-10" of rain per year, this is obviously a very slow process.

But, what is the definition of an arch? Simply put, an arch has a front entrance and a back entrance. The front, obviously is easy to see, the rear entrance may be large or a small hole at the top or on the sides...AND the rear portion must be at least three feet wide, measured at its widest point. Check out the photos to see what I mean.

The first photo shows a less-than-average-size arch named "Pine Tree Arch," located in the "Devil's Garden" area and named for the small pine tree that grows underneath it. You can gauge the relative size of the arch by the plants growing in front of it. It is most definitely an arch as there is a front and rear entrance.

The second photo shows three huge alcoves in the rocks located near the famous "Double Arch." Even though they are very big, they are not arches as there is no rear or top opening. But, if you notice the opening above the middle alcove. Water will slowly work its way down towards the alcove and one day, long after we are gone, this will be an arch.

The third photo illustrates that size is not important when it comes to arches. This young boy is crawling through a very small opening in the arch. The back of the arch, hidden from view, is wider than three feet, so this is definitely an arch (I forgot the name). Not shown here is another arch in the making only a few feet away. It has it all, except the opening is less than three feet, so it is not an arch...yet.

In upcoming blogs, I will show you some of the largest and most spectacular arches in the park. Stay tuned.