Structures found on a ridge under the waters of Lake Huron are evidence of how early people in the Great Lakes region organized their caribou hunts around the seasons.

The V-shaped constructions and linear lines of stones are the most intricate ancient hunting constructions ever found in Great Lakes waters.

The structures are around 9,000 years old, University of Michigan researchers say, and probably helped those early people corral herds of caribou migrating over an exposed dry land corridor, now 120 feet underwater but above the lake's surface during the latest Ice Age.

"This site and its associated artifacts, along with environmental and simulation studies, suggest that Late Paleoindian/Early Archaic caribou hunters employed distinctly different seasonal approaches," anthropological archaeology Professor John O'Shea, says.

"In autumn, small groups carried out the caribou hunts, and in spring, larger groups of hunters cooperated," O'shea added.

The main feature on the ancient dry land passage that once connected northeast Michigan with southern Ontario in Canada is a lane of placed stones in two lines parallel to each other with a cul-de-sac at its end.

Also discovered were three small hunting blinds of circular shape constructed in the lines of stones.

Although the preferred caribou-hunting season would have been autumn, the lane's orientation would have been effective only if the herds were on the move toward the northwest, as would have occurred during spring migrations.

Seasonally, O'Shea said, the area "was a convergence point along different migration routes, where the landform tended to compress the animals in both the spring and autumn."

The structures and evidence of stone tools show unmistakable evidence of intentional human building and use.

They also serve as important evidence of the economic and social organization among hunters using different parts of the structure during different times.

"The larger size and multiple parts of the complex drive lanes would have necessitated a larger cooperating group of individuals involved in the hunt," O'shea said. "The smaller V-shaped hunting blinds could be operated by very small family groups relying on the natural shape of the landform to channel caribou towards them."