(CNN) — From a distance they look like ridges on the side of rocks. But scientists say carvings at a dried-up lake in Nevada's Great Basin may be North America's oldest and shed light into a civilization perhaps 15,000 years old.
"We have long known of the existence of these petroglyphs throughout the world," said Dr. Eugene Hattori, an anthropologist with the Nevada State Museum.
"There are many of these carvings along Nevada's Great Basin ranging in design from human figures, riding on horseback and geometric designs. We have long wondered what the dates are," said Hattori.
"This particular set of petroglyphs at the dry Winnemucca Lake have been known for a number of decades and the designs that were carved into them stand out as very distinctive geometric designs that were deeply carved into the tufa (porous rock)."
University of Colorado geologist Dr. Larry Benson has studied other sites with petroglyphs in the area, but these, he said, are "the oldest ones so far dated in North America."
Those previously thought to be oldest are in Oregon, part of the Paisley Caves complex. The carvings there date back at least 7,630 years.
"This is speculation, but it may be that the people that occupied those caves occupied those sites (in the Great Basin) about the same time," Benson said.
How old is that petroglyph?
In order to determine the age of this set of petroglyphs in Nevada, researchers studied the calcium carbonate deposit layer on the rock's surface -- an indicator that the rocks had once been submerged in water.
Benson concluded that the incisions were made before the water levels rose during the Ice Age, and the Lake Lahontan formation. The now-dry Winnemucca Lake was a remnant of Lake Lahontan.
"Between 13,000 and 15,000 years ago, the lake levels dropped below the level of the boulders," said Hattori.
"We have other archeological finds in basketry in that area that date about 9,700 to about 11,000 years ago. Now we have these examples in carvings that are associated with that ancient culture," he said.
What do the carvings mean?
The carvings, said Hattori, "show very early ancient artistic expression of these people. We initially thought people 12,000 or 10,000 years ago were primitive, but their artistic expressions and technological expertise associated with these paints a much different picture."
But, he explained, the deeper meanings behind the carvings have yet to be unraveled.
"We do not know the reason why they carved these designs. There are others looking into deeper meanings," he said.
He added that "to the Native American people, this is a record for the Pyramid Lake Paiute Tribe. They view this as information on their ancient ancestors."