Expert assesses Rock Bridge’s stability

Shards of rock litter the ground and are strewn across a crushed boardwalk shaded by towering trees at Rock Bridge Memorial State Park. The debris, located below the opening of the natural rock bridge for which the park is named, is the result of a slab that fell from the side of the geologic formation earlier this month.

The area from which the piece fell is noticeable.

“You can kind of see that fresh scar on the rocks,” said Jan Weaver, president of the Friends of Rock Bridge Memorial State Park.

Hikers must detour around this section of the trail’s boardwalk, marked by orange fencing. On Tuesday, Derek Apel, assistant professor of mining engineering at the University of Missouri-Rolla, inspected the rock bridge to assess the potential for additional rock to fall.

Park officials have been taking extra precautions since an outer piece of the rock bridge fell on June 9. Jim Gast, park superintendent, estimates the fallen rock to be 20 feet long, 10 feet wide and 1 foot thick. When the section fell, it landed on the boardwalk that leads under the rock bridge.

A park visitor called Gast after hearing a noise in the park about 8 p.m. on June 9. Gast went out the next morning to find the fallen slab. The damage is extensive enough that the Rock Bridge park’s construction crew can’t fix the boardwalk. A crew from the Missouri Department of Natural Resources came earlier to evaluate the damage to the walkway but stopped after noticing that an additional slab appeared to be in danger of falling.

“They were ready to repair it, but noticed the other piece in question and stopped,” Gast said.

Apel and three DNR officials were at the site Tuesday collecting rock samples, measurements and photographs in an effort to develop recommendations for improving the safety of the area. Apel plans to study the materials and consult colleagues before releasing a report with safety suggestions.

Rock has fallen from the natural bridge only twice in the past 14 years, Gast said. The other piece fell about seven years ago, Gast said, but it was much smaller. He said that he wasn’t sure why the rock fell in early June, but that there could have been a crack that finally gave. Park naturalist Roxie Campbell said that water dissolving limestone along cracks and the effects of freezing and thawing probably caused the slab to fall.

“It’s kind of a standard, basic geological thing that happens,” Campbell said. “It’s the kind of thing that’s a rare occurrence, but it’s difficult to predict.”

Weaver said the bridge was formed after the Ozark uplift 25 million years ago. As the swampy land in the area rose, the water table — the upper level of groundwater — dropped and streams started cutting through the rock. The water continued to flow and create a long sinkhole. The rock dropped out because of the sinkhole and formed the bridge. The same process that created the bridge is now eroding it.

“The water just wears away at a crack in the rocks,” Weaver said. “It’s slow, but it’s a constant process.”

Fred Hicks, DNR state parks risk management administrator, was on hand for the inspection.

“From a risk management standpoint, we want to see that when things reopen, it will be safe for the public to come in,” Hicks said.

The park’s staff has posted information on the park bulletin board and on signs around the area to alert visitors to the detour.

“It seems our visitors have taken it all in stride,” Campbell said.