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With few exceptions, Ohio is not noted for its geologic features. The face of our state is too low and too level. However, the same basic processes which shaped the spectacular scenery of the Far West have also been at work in Ohio.
We should, therefore, expect to find some of the same results of those processes, although on a smaller scale. And we do. Among them are natural arches and bridges.
They may not be as large or abundant as those in the more rugged parts of the United States, but our geologic features are surprisingly numerous and every bit as interesting.
The arches listed here are publicly owned and can be readily visited. As noted in the descriptions, some require a free access permit.
Information on Types and Formation of Arches
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 ROCKBRIDGE
At 95 feet (29m) Rockbridge is Ohio's longest natural bridge. It is also the only one with a town named after it.
The bridge originated as a typical Hocking Hills alcove carved into the soft middle layer of Black Hand sandstone at the head of a short box canyon cut by a small tributary of the Hocking River.
Three intersecting vertical fractures in the roof of the shelter were gradually enlarged until the block they surrounded was no longer supported and fell, creating a skylight.
The bridge itself represents the front rim of the alcove. It is 3 feet (1 m) thick, varies from 6.5 to 26 feet (2-8 m) in width and crosses 40 feet (12 m) above the plunge pool of the waterfall behind it. The natural bridge at Rockbridge State Nature Preserve was acquired by the Division of Natural Areas and Preserves in 1978. It is open to public visitation.
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ROCKHOUSE
Rockhouse is unique in Ohio. It is by far the longest known natural arch in the state. It is also the longest natural tunnel and the only arch with "gothic windows" opening through its side.
This scenic treasure quickly attracted the interest of geologists and tourists alike, making it one of the first areas of Hocking Hills State Park to be protected.
Rockhouse was formed by the widening of a vertical crack which separated a large block of Black Hand sandstone from the main cliff above a small tributary of Laurel Run. This created a passage nearly 200 feet (61m) long, 40 feet (12m) high and 20 feet (6m) wide. Enlargement of secondary joints crossing the main one has led to the formation of the tunnel's distinctive "windows."
Although the existence of this impressive feature might appear to require a catastrophic explanation, the slow process of weathering is the most likely cause. If only one-half ounce of sand each day was taken out, the entire 100,000 cubic feet of sandstone which once filled Rock House could have been removed during the million year span of the Pleistocene.
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LADD NATURAL BRIDGE
This natural bridge is the only one in Ohio known to have been promoted as a commercial tourist attraction. Sometime after World War I, the Ladd family,  which has owned the arch for many years, charged visitors 10 cents to visit. Some of the braver (or more foolhardy) tourists even drove their cars across the span, making Ladd one of the few natural bridges in Ohio which has actually carried vehicular traffic.
The bridge is 40 feet (12m) long, 12 feet (4m) wide and 5 feet (1.5 m) thick--strong enough to hold a vehicle. A steady hand would be required, however, for on one side is a sheer drop of 52 feet (16m) to the bottom of the cliff.
The arch formed in sandstone of Permian age when an intermittent stream enlarged a vertical crevice intersecting the horizontal alcove formed by its waterfall. These enlarged openings eventually captured the stream which now flows under the natural bridge rather than over it.
There appears to have been a second arch of comparable size in front of the existing one, but it has long since collapsed. In 1984, the Ladd family dedicated 35 acres containing the natural bridge as a state nature preserve. A free access permit from the Division of Natural Areas and Preserves is required.
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IRISH RUN NATURAL BRIDGE
This complex structure is basically a breached alcove arch cut into sandstone of Permian age. The stream responsible has continued to cut away at the floor and south wall of the alcove, however, giving the arch a distinctively tilted front opening.
Part of the roof has collapsed to create the skylight. Several of these large blocks of sandstone still litter the floor of the alcove.
This is a massive natural bridge with a blocky span 18 feet (5.5m) wide and 22 feet (6.7m) long. Fortunately, it is part of Wayne National Forest and so is accessible to the public, although a good hike and a stiff climb are required to reach it.
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SPRING CREEK ARCH
Located in a side valley of Baker Fork Gorge, this arch is the most easily observed of the seven found there. Its opening is 18 feet (5.4 m) wide and 6 feet (1.8 m) high, and leads to an oval-shaped depression in the hillside.
This plus the ragged, "rotted" appearance of the rock forming it make this arch appear to be a remnant of Peebles dolomite which has withstood the erosion that removed weaker rock around it.
The arch is readily visible from the Gorge Trail which follows Spring Creek upstream from Baker Fork in Fort Hill State Memorial.
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 RAVEN ROCK ARCH
Although it is one of the state's smallest natural arches, Raven Rock Arch boasts the most scenic location of them all. Found high atop a cliff of Mississippian sandstone capping a 500 foot (150m) high bluff, it opens out onto a spectacular view of the Ohio River valley.
It is named for Raven Rock, a projection of the same cliff said to represent the beak of a large bird formed by the body of the bluff.
Raven Rock Arch is a thin remnant of stone 14 inches (35cm) wide at its narrowest point and 15 feet (4.6m) long. Two other natural arches (Rockgrin and Slide) are found in the cliff face below Raven Rock.
In 1996, 95 acres including all three arches and Raven Rock were dedicated as a state nature preserve and can be visited by first obtaining a permit from the Division of Natural Areas and Preserves.
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 GREENVILLE FALLS ARCH
West of the village of Covington, Greenville Creek has cut a gorge 20 to 30 feet (6-9m) deep into dolomites of Silurian age. At the head of this gorge is the impressive cascade known as Greenville Falls.
A short distance downstream, Greenville Falls Arch opens into the north wall of the gorge. Seen from above, the arch appears as a linear opening through which the river can be glimpsed.
The cliff remnant forming the arch is 10 feet (3m) thick, 8 feet (2.4m) wide and 11 feet (3.3m) long. A permanent spring flowing out just below the top of the cliff has eroded an alcove which has been breached by the widening of a vertical crevice in its roof, leaving the remaining front portion of the alcove as an arch.
Continued erosion by the river and spring have since cut away the floor of the alcove, leaving the arch suspended over the river. It is now part of Greenville Falls State Nature Preserve, established to help protect the Stillwater State Scenic River system, and is open for public visitation.
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 TRIMMER ARCH
Of all the natural arches in Ohio, Trimmer has the most classic shape--a semi-circular opening cut through a narrow point of Greenfield dolomite of Silurian age.
This rock projection is found at the junction of two small gorges cut by tributaries of Paint Creek. Thus Trimmer is similar to many of the spectacular arches of the western canyon country which also formed in narrow "fins" of rock, although it is, of course, much smaller.
The opening has a span of 18 feet (5.5m) and a clearance of 10 feet (3m). It is now part of Paint Creek Wildlife Area.
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 MILLER SANCTUARY NATURAL BRIDGE
Located in the upper end of Rocky Fork Gorge, this natural bridge is found in the Peebles dolomite, a rock formation which contains many of western Ohio's natural arches.
It resulted when a crevice parallel to the cliff face was widened until it cut through the roof of an alcove carved out by an intermittent waterfall.
The remaining front edge of the alcove now forms the natural bridge. It has a length of 46 feet (14m) and is found in Miller Nature Sanctuary State Nature Preserve. A free access permit is required.
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 MILLER SANCTUARY ARCH
This arch may not be very large, but it makes one of the grandest appearances of any arch in Ohio. An unsuspecting visitor following the Miller Sanctuary Trail comes upon it suddenly--a great, rounded frame enclosing a cameo portrait of Rocky Fork Gorge.
This arch is also notably lop-sided. A massive lintel 13 feet (4 m) thick is attached at one end to a bold cliff and held up on the other by a thin pillar resembling a gothic flying buttress only 3 feet (.9 m) wide.
The resulting opening has a span of 9.5 feet (2.9 m) and a clearance of 5 feet (1.5 m). Although the arch appears at first to be the remnant of a collapsed cave, it was more likely formed by the weathering away of a highly-fractured section of Peebles dolomite forming the cliff.
Because this thinly bedded rock was located at the junction of a small side gorge with the larger canyon of Rocky Fork, it was eroded on both sides by the elements.
The arch is located in Miller Sanctuary State Nature Preserve which may be visited by obtaining a free access permit.
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THE KEYHOLE
This is the largest and most unusual of the seven natural arches found in the gorge of Baker Fork in Highland County. At 77 feet (23.5 m) long, it is second only to Rockhouse in size and, like it, qualifies  as a natural tunnel.
The Keyhole is also a natural bridge, providing the only outlet for tiny Bridge Creek which flows off the flanks of Jarnigan Knob behind it. Its formation appears to be intimately tied to that of the deep gorge into which it opens.
Baker Fork Gorge resulted from the advance of the Illinoisan glacier which blocked north-flowing streams and so created a large lake fed by its own meltwater. The rising waters of the lake finally reached the level of a gap in the surrounding hills and poured through it, cutting down into the Ohio shale and underlying Peebles dolomite to create Baker Fork and its gorge.
Present Bridge Creek probably existed before the glacial advance as a headwater branch of the south-flowing stream whose valley the escaping meltwater utilized and which eventually became the lower reaches of Baker Fork. As the meltwater stream deepened its gorge, Bridge Creek slowly lowered the level of its own small valley until it reached a vertical crevice which offered a shortcut to the new Baker Fork. This "stream piracy" also gave it a greater fall and thus more cutting power, enabling it to widen the crevice downward to keep up with the deepening Gorge. The Keyhole resulted.
Although only 2 feet (0.6 m) wide at its narrowest point, the tunnel is about 35 feet (10.6 m) high, large enough to allow flowstone and other features more typical of caves to form. If the Keyhole were an actual cave passage, it would be one of the largest known in Ohio.
This impressive feature is located in Fort Hill State Memorial which is open to the public. The Deer Trail goes over it, making the Keyhole a bridge as well.
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 NEEDLE'S EYE
It is appropriate that Ohio's most famous example of a sea arch should be located on Lake Erie, our inland sea.
Needle's Eye has been a landmark in the harbor at Put-in-Bay on South Bass Island for more than a century. The narrow opening is 32 inches (0.9 m) wide and 15 feet (4.5) high (including the portion below lake level) and pierces a narrow fin of thick-bedded Put-in-Bay dolomite thrusting out from the northeast end of Gibraltar Island at the entrance to the harbor.
Although it can be seen with binoculars or sharp eyes from the seaway at Perry's Victory and International Peace Memorial, it can only be viewed close-up from a boat.
According to tradition, Oliver Hazard Perry stationed a lookout on the promontory above the arch to watch for the British Fleet during the War of 1812. Later, Gibraltar Island was purchased by Jay Cooke, one of the major financiers of Union efforts during the Civil War.
Today the island with Jay Cooke's summer home and Needle's Eye is part of Stone Lab, The Ohio State University's biological field station on Lake Erie.
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