Granite blocks are seen in Amapa, Brazil, on May 10, 2006. A grouping of 127 granite blocks along a grassy Amazon hilltop may be the vestiges of South America's oldest astronomical observatory, according to archeologists who say the find challenges long-held assumptions about the region's prehistory. [AP Photo]

A grouping of granite blocks along a grassy Amazon hilltop may be the vestiges of a centuries-old astronomical observatory, a find archaeologists say indicates early rainforest inhabitants were more sophisticated than previously believed.

The 127 blocks, some as high as 9 feet tall, are spaced at regular intervals around the hill, like a crown 100 feet in diameter.

On the shortest day of the year£¬ Dec. 21£¬ the shadow of one of the blocks disappears when the sun is directly above it.

"It is this block's alignment with the winter solstice that leads us to believe the site was once an astronomical observatory," said Mariana Petry Cabral, an archaeologist at the Amapa State Scientific and Technical Research Institute. "We may be also looking at the remnants of a sophisticated culture."

Anthropologists have long known that local indigenous populations were acute observers of the stars and sun. But the discovery of a physical structure that appears to incorporate this knowledge suggests pre-Columbian Indians in the Amazon rainforest may have been more sophisticated than previously suspected.

"Transforming this kind of knowledge into a monument; the transformation of something ephemeral into something concrete, could indicate the existence of a larger population and of a more complex social organization," Cabral said.

Cabral has been studying the site, near the village of Calcoene, just north of the equator in Amapa state in far northern Brazil, since last year. She believes it was once inhabited by the ancestors of the Palikur Indians, and while the blocks have not yet been submitted to carbon dating, she says pottery shards near the site indicate they are pre-Columbian and maybe older, as much as 2,000 years old.

Last month, archaeologists working on a hillside north of Lima, Peru, announced the discovery of the oldest astronomical observatory in the Western Hemisphere, giant stone carvings, apparently 4,200 years old, that align with sunrise and sunset on Dec. 21.

While the Incas, Mayans and Aztecs built large cities and huge rock structures, pre-Columbian Amazon societies built smaller settlements of wood and clay that quickly deteriorated in the hot, humid Amazon climate, disappearing centuries ago, archaeologists say.

Farmers and fishermen in the region around the Amazon site have long known about it, and the local press has dubbed it the "tropical Stonehenge." Archeologists got involved last year after geographers and geologists did a socio-economic survey of the area, by foot and helicopter, and noticed "the unique circular structure on top of the hill," Cabral said.

Scientists not involved in the discovery said it could prove valuable to understanding pre-Columbian societies in the Amazon.

"No one has ever described something like this before. This is an extremely novel find£¬ a one of a kind type of thing," said Michael Heckenberger of the University of Florida's Department of Anthropology.

He said that while carbon dating and further excavation must be carried out, the find adds to a growing body of thought among archaeologists that prehistory in the Amazon region was more varied than had been believed.

"Given that astronomical objects, stars, constellations etc., have a major importance in much of Amazonian mythology and cosmology, it does not in any way surprise me that such an observatory exists," said Richard Callaghan, a professor of geography, anthropology and archaeology at the University of Calgary.

Brazilian archaeologists will return in August, when the rainy season ends, to carry out carbon dating and further excavations.

"The traditional image is that some time thousands of years ago small groups of tropical forest horticulturists arrived in the area and they never changed, (that) what we see today is just like it was 3,000 years ago," Heckenberger said. "This is one more thing that suggests that through the past thousands of years, societies have changed quite a lot."