Archaeologists say they've found 5,000-year-old rock carvings in northern Sudan depicting scenes they can't explain.

Researchers discovered rock art in 15 sites in an arid valley known as Wadi Abu Dom in the Bayuda Desert, about 18 miles from the Nile River, LiveScience.com reported Friday.

The drawings are carved into the rock faces of small stream beds known as "khors" that flow into the valley.

Some sites contain just a single drawing while others have as many as 30, Tim Karberg of the Westfalische Wilhelms-Universitat Munster in Germany said.

"We asked the local people about the rock art and they said that it would be very old, before their grandfathers," Karberg said.

A number of the images depict crosses and a church, suggesting they date back about 1,500 years to a period when Christianity was spreading in Sudan, the researchers said. One of the carvings may be of a nearby ancient monastery called al-Ghazali.

Another mysterious set of rock art appears to be at least 5,000 years old and shows a mix of geometric designs. The "oldest rock art we found are the spiral motifs," Karberg said.

Some researchers have linked spiral motifs to some astronomical or astrological forms, Karberg said, but he thinks it might have more to do with math.

"The regularity of the spiral might be one of the earliest mathematical ideas the people developed," he said.