The Ohio Natural Arch Survey has studied over 60 of these features so far, and there are certainly more to be discovered. Not every hole in the rock qualifies as a natural arch, however. The following definitions were applied by the survey:

NATURAL ARCH--A generally horizontal remnant of bedrock supported at two opposing points and spanning an opening created by erosive processes, the sum of whose span (horizontal dimension of the opening) and clearance (vertical dimension of the opening) is equal to or greater than 6 feet (1.8 meters) with neither dimension measuring less than 1 foot (.3 meters).

NATURAL BRIDGE--A natural arch which spans a valley of erosion. A natural bridge is always a natural arch, but a natural arch may not be a natural bridge.

NATURAL TUNNEL--A natural arch whose width (horizontal dimension perpendicular to the front face of the arch) is equal to or greater than three times its span. Natural arches exist in larger numbers than might be expected because the conditions required for their formation are commonly met. The natural arches found in Ohio result from the interaction of the following five elements:

CONSOLIDATED ROCK--A natural arch can form only in rock which is solid enough to hold together against the force of gravity. The strength of the rock determines how big a natural arch may become and how long it will survive.
EROSIVE AGENCY--This is almost always water, which is capable of eroding rock through solution (dissolving minerals in the rock), abrasion (by carrying sand and grit against it) and fracture (most often by the pressure it exerts when it enters cracks and freezes).
RELIEF--Defined as the variation in height of the Earth's surface, relief is what gives water its erosive force by providing a gradient through which it can fall, thus enabling it to move. The greater the relief, the more potential erosive force exists. The amount of local relief also determines how big a natural arch may become, since it obviously cannot be any bigger than the cliff in which it forms.
PATHWAYS FOR EROSIVE ACTIVITY--These can take the form of pores, vertical fractures or horizontal bedding planes in the body of the rock. The enlargement and consolidation of these pathways by erosion is a major factor in forming the openings of natural arches.

TlME--Geological processes move forward at a pace which appears exceedingly slow by human standards. Weathering takes place grain-by-grain, sometimes molecule-by-molecule. It takes time to form a natural arch. Most of our natural arches are found in areas that were glaciated during the last Ice Age, or in features, such as gorges, which resulted from the melting of glaciers. These arches can, therefore, be no older than the last part of the Pleistocene. The natural arches of Ohio may be divided into several types according to how they were formed:

BREACHED ALCOVE--These arches result when the ceiling of a bedrock alcove is broken through in such a manner that part of it remains intact, forming a span. The breaching may be a result of joint widening, collapse of part of the roof, or erosion. This is the most common type of arch found in Ohio. Most were formed when erosion enlarged a vertical crevice intersecting a horizontal alcove. Ladd Natural Bridge and Greenville Falls Arch are examples. Some of our largest arches, including Rockbridge and Irish Run, were formed by the partial collapse of their alcove ceilings.
CAVE COLLAPSE--If a cave enlarges to the point where its roof can no longer support its own weight, collapse occurs and a sinkhole results. If two sinkholes form close together and the section of cave roof remaining between them has an open passage beneath it, a natural arch is formed. This is an important arch type in regions with well-developed cave systems, but Ohio's caves are small and rarely result in collapse arches. Only two examples, both privately owned, are known in the state.
ALCOVE ENLARGEMENT--It is possible for enlarging alcoves to form arches if they become deep enough to pierce a narrow bedrock wall or the back wall of another alcove. A variation of this type is the headward erosion arch which formed when the upper reaches of two streams cut alcoves into opposite sides of a steep-walled ridge of rock. Several of the large arches in the Red River Gorge area of Kentucky formed in this manner, but only one sizable example has been found in Ohio. Many large arches seen in western states represent another variation of the alcove enlargement arch, the meander arch, which forms when a river enlarges alcoves on both sides of a narrow fin of rock blocking the neck of a meander. The only possible example of this variation found in Ohio to date is Trimmer Arch, which formed in a fin of dolomite left at the junction of two small streams.
VERTICAL CREVICE ENLARGEMENT--This arch type differs from breached alcove arches in that the opening of the arch is formed by the widening of a vertical crevice perpendicular to the cliff rather than a horizontal bedding plane or layer of weak rock to form an alcove. These arches have strongly vertical openings which may narrow to an impassable crack or pinch out altogether at the top. The Keyhole in Fort Hill State Memorial and Rockhouse in Hocking Hills State Park are two notable examples.
BEDDING PLANE ENLARGEMENT--Some arches are a result of the enlargement of bedding planes, usually through a combination of solution and mechanical breaking. They are found in narrow bedrock projections where a massive upper layer of rock rests upon a weaker layer, often containing many closely-spaced bedding planes. The opening of the arch forms in the weaker layer of rock. Miller Sanctuary Arch is an especially picturesque example.
SEDIMENTARY GRAIN ARCHES--Because a layer of bedrock exhibits variations over its expanse, erosion may affect different parts of it in different ways. Weathering often removes weaker parts of the rock. The more resistant rock which remains may take the form of a natural arch. The ragged, irregular form and deeply pitted surface of Spring Creek Arch are typical of this type.

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