A 10,000-year-old gallery
The eastern slopes of the Helan Mountains contain a vast outdoor gallery of rock art. More than 20,000 petroglyphs have been documented, pecked and carved into the dark rock faces by nomadic peoples over a period of thousands of years. The age of the art is estimated to range from 3,000 to 10,000 years old. The images provide a record of the beliefs and daily activities of cultures that inhabited the region, including the Xirong, Huns, and Tanguts.
The petroglyphs show many subjects. They include depictions of animals like tigers, leopards, horses, and sheep, alongside scenes of hunting, herding, and ritual practice. A recognizable image is a large, mask-like face with radiating lines, often identified as a "Sun God." This particular carving, along with thousands of others, is concentrated in a gorge known as Helankou, which is a natural corridor for people moving between the desert and the Ningxia Plain.
The varnish clock
Determining the precise age of these carvings long presented a challenge. A method known as varnish microlamination (VML) dating assigns ages to individual petroglyphs. The science relies on the properties of rock varnish, a thin, dark coating composed of clay minerals, manganese, and iron oxides that forms on exposed rock surfaces in arid environments. This varnish accumulates in microscopic layers over thousands of years.
The chemical composition of these layers records past climate conditions. Wetter periods result in manganese-rich, black layers, while drier periods produce iron-rich, orange layers. When a petroglyph was carved, it cut through the existing layers of varnish. Over subsequent millennia, new varnish layers formed inside the carved grooves.
Scientists can carefully scrape the varnish from inside a carving and analyze its microlayers. By comparing the sequence of layers inside the carving to the sequence on the untouched rock face, they can establish a relative age. To get a more precise date, researchers use accelerator mass spectrometry (AMS) radiocarbon dating. The varnish traps tiny amounts of organic matter as it forms. By measuring the carbon-14 in this trapped material from the lowest (oldest) layer within a carving, a minimum age for the artwork can be established.