JACKSON, Wyo. (AP) — Simeon Caskey stood beneath the steep, boulder-strewn slope that looms over Hidden Falls, pointing out a rock wall with an ominous crack a few hundred feet (91 meters) overhead.

The Grand Teton National Park ranger, who leads the park's physical sciences branch, was in an area that's been closed to the public — and he was explaining why. If the buttress were to fail, he said, chunks of it might tear through the base of the 100-foot (30.5-meter) cascade where visitors often crowd to photograph and enjoy the falls.

"It seems like a good chance that at least a percent of it would make it down to the viewing area," Caskey said.

Exactly how much of a threat the potential rockfall poses is what Teton park officials are trying to determine.

Up close, the intermittent 100-foot crack that propagates across the top of a rockwall Exum guides train their clients on is underwhelming. It's just several inches wide, and has been stable since monitoring equipment went in two weeks ago. In appearance, this split is similar to the many other fissures in the same rock face, and those found throughout the Tetons.

The difference is that this crack — which holds back a 100-foot by 20-foot (6.1-meter) block of rock that's 20-feet wide — is believed to be new.

"Our current line of thinking, based on climbing guide input, is that all 3 inches of movement has been since last fall," Caskey told the Jackson Hole News & Guide. "Did it start last fall, or this spring, or has it been incremental? We just don't have enough information to say that for sure."

In the meantime, Hidden Falls, Inspiration Point and nearby areas remain closed. Jenny Lake as a whole remains open, as does access to Cascade Canyon. It's the bustling nature of this area on the shores of Jenny Lake that compelled managers to act in the first place.

Around 2,000 people offload from Jenny Lake Boating shuttles on a typical summer day, Caskey said, and presumably most go to check out Hidden Falls.

"In our risk assessment part of the equation is the probability people will be there," he said. "In a situation where there's a trail, and people are just walking through, the probability of occupancy is much lower than at Hidden Falls, where at any given time of the day you might have 15 to 100 people that are there for 15 to 30 minutes."

The monitoring regimen, for now, requires lots of manpower: a team of two rangers who take a few hours out of their day to make sure there's been no movement. Last Thursday the stability of the past two weeks held true.

Caskey made sure of it, taping out the width of the gap in nine predetermined measuring points.

"White 1," the name of his first reading, went into the log at 9.2 centimeters wide. The day before the crack here spanned 9.3 centimeters, but that minute difference was chalked up to the margin of error inherent in a human-produced measurement.

Every day, these same measurements are logged, supplemented with data from time-lapse cameras and GPS equipment. All the while, officials are routinely talking and consulting with outside geophysical experts.

Eventually, Caskey and his Teton park colleagues plan to upgrade to a "crack meter" that's on order. The more precise readings could aid efforts to ascertain the likelihood that the fissured hunk of rock breaks free.

The other question Caskey's hunting for an answer to is, if it does go, where would the rock end up. He's consulting geophysical experts to predict how the rock would break up in a hypothetical fall.

"My objective here is to use science and robust monitoring to inform and educate our superintendent," Caskey said.

Ultimately, once a "risk assessment" is completed, Teton park Superintendent David Vela will have to make the call about what to do — open Hidden Falls, keep it closed or somehow intervene. Hidden Falls is classified as Teton park backcountry that's managed similar to designated wilderness, which means nature is usually allowed to take its natural course.

"The Park Service polices don't really mince words about that," Caskey said.

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Information from: Jackson Hole (Wyo.) News And Guide, http://www.jhnewsandguide.com