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.
Sources:
Camp, Mark J.
2006 Roadside Geology of Ohio.Mountain Publishing Company,
Montana.
Hansen, Michael C.
1988 Natural Bridges in Ohio, Ohio Geology.