As the foreman for a cattle ranch in the far reaches of the Brazilian Amazon, Lailson Camelo da Silva was razing trees to convert rain forest into pasture when he stumbled across a bizarre arrangement of towering granite blocks.

“I had no idea that I was discovering the Amazon’s own Stonehenge,” said Mr. da Silva, 65, on a scorching October day as he gazed at the archaeological site located just north of the Equator. “It makes me wonder: What other secrets about our past are still hidden in Brazil’s jungles?”

After conducting radiocarbon testing and carrying out measurements during the winter solstice, scholars in the field of archaeoastronomy determined that an indigenous culture arranged the megaliths into an astronomical observatory about 1,000 years ago, or five centuries before the European conquest of the Americas began.