Archaeologists have found a collection of rock art drawings in northern Sudan, which they believe date back to more than 5,000 years ago.

The team discovered 15 pieces of rock art in an arid valley known as Wadi Abu Dom, some 29 kilometers from the Nile River. Experts say the rocks were etched and picture scenes that can't be explained.

No paint was used to create the drawings and many of them were carved into the rock faces of small stream beds known as "khors" that flow into the valley, LiveScience reported.

Some of the discovered images belong to the time when Christianity was spreading in Sudan around 1,500 years ago and show crosses, a knight riding an animal with horns and a church, which archeologists say might show a nearby ancient monastery called al-Ghazali.

"One is a depiction of an armed rider, with a lance and a shield, a kind of knight depiction," said head of the archeology team Tim Karberg, suggesting that it might be an image of the legendary soldier St. George.

The mysterious part of the find, however, is a set of rock art dating back to at least 5,000 years ago bearing a mix of geometric designs.

The "oldest rock art we found are the spiral motifs," said Karberg, adding that the patterns are similar to drawings found in the Sahara Desert and so twisted up that are hard to interpret.

Some researchers connect the "spiral motifs to some astronomical or astrological forms," Karberg said. "The regularity of the spiral might be one of the earliest mathematical ideas the people developed."

Another set of geometric drawings found at the site are in Karberg's words "hard to describe," and are probably a bit younger than the spirals. They consist of "amorphous patterns which are not circular. ... It looks like an irregular-shaped net," he explained.

Some experts believe that the irregular "nets" may actually be animal hides.