A massive megalithic earthwork featuring precisely aligned stone pillars has been identified in a remote sector of North Dakota. Preliminary analysis suggests the structure served as a solar calendar, marking the summer and winter solstices with remarkable accuracy around 3,000 BCE, long before the rise of later mound-building cultures.
The discovery is forcing historians to re-evaluate the social complexity and astronomical knowledge of Great Plains inhabitants during the Late Archaic period. The site is characterized by its concentric ring layout and central altar, which shows evidence of ceremonial use spanning several centuries of seasonal observation.