BEIJING — At one of the most treacherous and least restored stretches of China’s Great Wall, a line of pack mules halted upon emerging from the gloom of a dense forest draped in mist and dew.

Laden with 150 kg (330 pounds) of bricks each, the seven animals finally got moving in response to the coaxing and swearing of their masters, eager to gain altitude before the sun climbed high in the sky.

For more than a decade, mules have been crucial in the effort to restore Jiankou, a serpentine 20-km (12-mile) section of the wall about 70 km (44 miles) north of central Beijing that is notorious for its ridges and perilous slopes.

“The path is too steep and the mountains are too high, so the bricks can only be transported by mules,” said local mule owner Cao Xinhua, who has worked on Great Wall restoration projects in the mountains north of Beijing for 10 years.

Early morning fog covers the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. (REUTERS/Damir Sagolj)

Workers carry their tools and belongings as they climb down the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. (REUTERS/Damir Sagolj)

A man rests while working on the reconstruction of the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. (REUTERS/Damir Sagolj)

Where they could, workers used the original bricks that had broken off the wall over the centuries. When they found none, they used new bricks made to exacting specifications.

“We have to stick to the original format, the original material and the original craftsmanship, so that we can better preserve the historical and cultural values,” said Cheng Yongmao, the engineer leading Jiankou’s restoration.

Cheng, 61, who has repaired 17 km (11 miles) of the Great Wall since 2003, belongs to the 16th generation in a long line of traditional brick makers.

A government clampdown on pollution has forced the closure of almost all brick-making factories in Beijing and nearby provinces, Cheng said.

If he ran out of bricks, Cheng added, he would have to look for bricks left elsewhere or request the central government to consider reopening some brick factories.

A man walks behind a mule carrying bricks up the steep path towards the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. )REUTERS/Damir Sagolj)

People pause from working on the reconstruction of the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. (REUTERS/Damir Sagolj(

People rest after working on the reconstruction of the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. (REUTERS/Damir Sagolj)

CONCRETE PAVEMENT

Famed for its rugged beauty, Jiankou, which is Chinese for an arrow’s nock, or notch for a bowstring, was built in the twilight years of the Ming dynasty in the 1600s, but is young compared with other sections dating back two millennia.

Intensive repairs on the Jiankou section in the past year have been led by the district government keen to preserve the wall’s natural beauty and shore up its disintegrating steps.

The restoration began in 2005 and is now in its third phase, making slow progress because the uneven terrain allows use only of basic tools such as chisels, hammers, pickaxes and shovels.

People work on the reconstruction of the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. (REUTERS/Damir Sagolj)

People wait for bricks and other equipment to be delivered as they work on the reconstruction of the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. (REUTERS/Damir Sagolj)

A man walks behind a mule carrying bricks up the steep path towards the Jiankou section of the Great Wall, located in Huairou District, north of Beijing, China, June 7, 2017. (REUTERS/Damir Sagolj)

Authorities’ meticulous approach followed widespread outrage last year sparked by botched restoration efforts on some stretches.

Authorities in the northeastern province of Liaoning, home to a 700-year-old section of the wall, paved its ramparts with sand and cement, resulting in what critics said looked more like a pedestrian pavement.

Soon after, the State Administration of Cultural Heritage said it would investigate any improperly executed wall preservation projects.

Just a tenth of the wall built during the Ming dynasty has been repaired, said Dong Yaohui, vice president of the China Great Wall Association.

“In the past, we would restore the walls so that they would be visited as tourist hot spots,” he said, by contrast with today’s objective of repairing and preserving them for future generations. “This is progress.”