Archaeologists in Central China's Henan province have uncovered a sprawling layer of mulberry and cypress branches, believed to be the foundation of an ancient city wall.

Covering an area of nearly 400 square meters in the city of Shangqiu, the branches date back to the Song Dynasty (960-1279), according to the city's institute of cultural relics and archaeology.

The branches were probably used to reinforce the foundation, as they were found below the rammed-earth slope protection of the city wall and above a layer of soil, says Yue Hongbin, head of the institute.

"It indicates there was a pit of soft soil underneath when the wall was built. Without tree branches, the rammed earth of the wall would have sunk or cracked," Yue says, hailing the ancient wisdom behind the design.

Researchers say the branches are relatively well-preserved as they are buried 10 meters underground and protected by the red clay soil created by flooding of the Yellow River.

Shangqiu was the first capital of the Shang Dynasty (c.16th century-11th century BC) and later served as the capital of some states. It is known as the site of "cities above cities", as new cities continued to cover the relics of the old throughout its long history.

Archaeologists have also uncovered wells and ash pits below the branches, as well as daily utensils, such as pottery jars that date back to the Eastern Zhou Dynasty (770-256 BC).