It was not until Mr. da Silva, the former ranch foreman, came across the stones at Rego Grande while deforesting surrounding jungle in the 1990s that scholars focused greater attention on the findings. Mr. da Silva said he first stumbled on the site while hunting wild boar as a teenager in the 1960s, but had subsequently avoided the area.

“The place initially felt sacred, like we didn’t belong here,” said Mr. da Silva, who now guards the Rego Grande site as its custodian. “But it was impossible to miss it during the deforestation drive of the ’90s, when the priority was to burn down trees.”

About 10 years ago, after securing public funds to cordon off the stones, Brazilian archaeologists led by Ms. Cabral and Mr. Saldanha began excavating the site, which is shaped roughly like a circle. They soon identified a portion of a river about two miles away where the granite blocks may have been quarried.

They also found ceramic burial urns, suggesting that at least part of the Rego Grande site may have been a cemetery, while colleagues from Amapá’s Institute of Scientific and Technological Research discovered that one of the tall stones seemed to be aligned with the sun’s path during the winter solstice.

After identifying other points in the site where stones could be associated with the sun’s movement on the solstice, the researchers began piecing together a theory that Rego Grande could have served various ceremonial and astronomical functions connected to agricultural or hunting cycles.

Ms. Cabral said that Rego Grande and a series of other less elaborate megalithic sites found in Amapá may have also served as markers for hunters or fishermen on a landscape that was being transformed by Amazonian peoples a millennium ago.