This EarthCache highlights a unique but geological feature within the Wayne National Forest’s Marietta District. This EarthCache was developed under special consideration from the Wayne National Forest. The Collection of Specimens, Either Biological, Cultural, or Mineral is Strictly Prohibited. Please stay on designated trails, see trail map of Archers Fork Hiking Trail by clicking here.
Remember as always, Leave No Trace.
Parking (included as a child waypoint) is available at the
Archers Fork Trailhead at N39 31.415, W081 10.858.
Irish Run Natural Bridge
Irish Run Natural Bridge Roof Litter
The Irish Run Natural Bridge is a breached alcove arch cut into sandstone of Permian age. It is one of the largest natural arches in the State of Ohio. It is an impressive 51’ long, 19’ wide, 16’, and 39’ high.
How do you define a natural arch?
An arch is a horizontal remnant of exposed bedrock supported at two opposing locations spanning an opening created by erosion. Not every hole in the bedrock is considered an arch. An arch must meet this measurement criterion. The span (horizontal measurement of the opening) and clearance (vertical measurement of the opening) must be greater than or equal to 6 feet with neither measurements being less than a foot. Windows are openings that are smaller than measurement criterion. They are also referred to as windows, gothic windows, lighthouses, holes, and etc…
How do you define a natural bridge?
A natural bridge is a natural arch that spans a valley formed by erosion. A natural bridge is always a natural arch, but a natural arch may not be a natural bridge.
How is a breached alcove arch formed?
As Irish Run continuously eroded away the underlying sandstone, it cut away the floor and south wall creating an alcove. An alcove is basically a small recess opening in the sandstone. Over time, the roof of the alcove collapsed leaving a portion intact forming the arch; notice that there are still roof fragments on the floor. Some examples which happen to be some of Ohio’s largest arches are Rockbridge and Irish Run Natural Bridge. However, there is another way that breached alcove arches can be formed. They can formed by joint widening or erosion. A joint refers to a fracture in rock where there has been no lateral movement in the plane of the fracture (up, down or sideways) of one side relative to the other. Examples in Ohio include Ladd Natural Bridge (no EarthCache allowed, visit by special permit only) and Greenville Falls Arch. Also of note, one of the characteristics of a breached alcove arch is the tilted front opening.
To claim a find, please email me the answers to the questions below. Also you will need to upload a picture of yourself holding your GPSr with Irish Run Natural Bridge in the background with your find log. Logs that do not follow the finding requirements will be deleted.
Question 1: How many roof fragments from the clasped roof still litter the floor? Just count the ones larger than a car wheel.
Question 2: Within a few 100 feet can you identity any other geological features? Briefly describe what you see.
Camp, Mark J.
2006 Roadside Geology of Ohio.Mountain Publishing Company, Montana.
Hansen, Michael C.
1988 Natural Bridges in Ohio, Ohio Geology.