About the time Roman legions were sacking Carthage, a Stone Age artist painted the black outline of a turtle on the wall of a cave south of what would become Billings.

Measuring nearly a foot across, the turtle adorning the wall of what’s now known as Pictograph Cave worked its magic on visitors for 2,145 years.

“It represents the earliest known painted image on the Northern Plains,” said Jennifer Lawson, marketing and communications manager for Montana State Parks.

Sometime last weekend – between closing time at 5 p.m. Friday and when a ranger arrived at 9 a.m. Saturday – a 2-to-4-inch-thick slice of sandstone bearing the ancient turtle and two other images slipped from the wall and fell into chunks on the cave floor.

“Last week there were 36 pictographs,” she said. “This week there are 33 pictographs.”

The other fallen pictographs are an abstract image in charcoal pigment that was painted above the turtle, and two pieces of a light red ochre image that was painted to the right of the turtle. The ochre painting is likely the headdress and legs of what was once a larger painting.

Rocks that fell from the ceiling were still sooty with the remnants of ancient campfires. A ranger who collected the debris said he could still smell the smoke.

“We were devastated,” said Jarret Kostrba, manager of Pictograph Cave State Park.

A ranger carefully carried the debris to the visitor center below and laid it out on tables. Amazingly, the turtle survived almost intact on a single slab of rock.

The sandstone panel hit a retaining wall in the cave, breaking apart.

“Fortunately it did not shatter,” Kostrba said.

He said the retaining wall probably prevented the rock from rolling another 10 feet over the side of the hill and breaking into too many pieces to collect.

If there is a silver lining, he said, it’s that visitors will now be able to see the fading images close up in the Visitor Center instead of from 30 feet away in the cave.

The turtle and other pieces that spalled -- that is, broke off parallel to the stone's surface -- will remain at the park and become part of a display explaining the natural forces that are slowly peeling away the cave walls.

“This has been going on for eons,” Kostrba said. “I can’t imagine what we could have done to prevent it.”

The park manager said that the three caves, used by native peoples for at least 6,000 years, are checked every morning at opening and again at closing. Because of the recent freeze-thaw cycle, park employees have been worried about sloughing of some of the images.

“We are concerned anytime we get these rapid temperature swings,” Kostrba said.

Most of the problem is a legacy of big snows in the winter of 2010-11. Water saturated the land above the cave and filtered into its porous sandstone. Moisture leaked into fissures that expanded and contracted in this winter’s cycles of freezing and thawing. Eventually the interior cave wall fractured where it meets the ceiling.

For the last few years, park staff, with the assistance of AmeriCorps volunteers, have been documenting conditions of the pictographs, including taking biweekly photographs of them.

Using special imaging techniques, they have been able to see through calcium deposits that are obscuring many of paintings in the cave, which is still held sacred by Native peoples.

“We’re going to work with experts to see what can be done,” said Chas Van Genderen, administrator of the state parks department. “We’ll do anything we can within our budget and our capacity. But it’s a very complicated problem. It’s hard to beat Mother Nature.”

Two years ago, at the recommendation of preservation specialists, a support wall was built to help wick away moisture. It covers about one quarter of the cave.

“This really used to be like the Sistine Chapel,” Kostrba said. “There were pictographs on the ceiling, on the wall panels and on the boulders in front.”

Bears, elk, beaver, shield-bearing warriors, coup sticks and rifles, many now barely visible, represented American Indian life through the centuries.

The images were first catalogued when archaeologists excavated the caves in 1937. They noted then that forces of nature were at work on the paintings. Kostrba said that when archaeologists turned over rocks that had fallen from the ceiling, they found traces of artwork that had already slipped away.

Turtles are common in regional rock art, Lawson said. Thirty-one have been recorded in Montana, and Wyoming probably has a similar number. A small bone disc with a turtle effigy was found in Pictograph Cave in the 1930s and may be associated with the painting, she said.

Pictograph Cave has been part of the Montana park system since 1969. In 1964, it was designated a National Historic Landmark. Last summer, 51,000 visitors came to the site.

This week, one of its few winter visitors was a mountain lion that left a trail of massive paw prints in the snow.

“We think there are two of them hanging around here,” Kostrba said.