What makes it an arch?

What does it require to be an official arch?

This is a much-debated topic.  A natural arch is a rock exposure that has a hole completely through it formed by the natural, selective removal of rock, leaving a relatively intact frame.  What that means is that any hole through rock, that was formed in a natural way, that has left solid rock all the way around it, is in theory a natural arch.  If you have hiked in areas of the Red River Gorge, you would know that you could find 1000’s of ‘arches’ that would qualify in just one small cliff line.

So how do we classify it as an arch or just a hole in Kentucky?

We use the measurement of 3 feet in any direction of the opening.  Meaning up, down, or sidewise of the smallest part of the opening of the arch.  So, it could be 3 feet tall and 1 inch wide at the smallest or most constricting part of the hole and it would be an arch. 

If the smallest part of the opening is 2.5 feet by 2.5 feet, it would not be an arch under this classification.

Why 3 feet?  That is close to the 1-meter requirement for a Minor Arch, per NABS.


Here are NABS Arch Levels:

Miniature Arch – all opening dimensions are smaller than 1 meter.

Minor Arch – one or more opening dimensions are at least 1 meter.

Significant Arch – the product of any two orthogonal opening dimensions is at least 10 square meters.

Major Arch – having a span of 50 meters or more.


Using those levels, the only major arch in Kentucky would be Mantle Rock, as it has a span of 156 feet.  There are quite a few significant arches in Kentucky, over 2600 minor arches and most likely closing in on infinite miniature arches in the state.


If you are interested in more information about what is an arch, check out NABS.

Top 5 largest spans in the database in Kentucky.

  1. Mantle Rock
  2. Rock House Natural Bridge
  3. Natural Arch
  4. Fishtrap Bridge
  5. Sheltowee Trace Arch