A Curious Phenomenon

Recently there has been a discovery of a curious phenomenon deep in the deciduous woods of Southern Illinois. In the midst of the underbrush there is a clearing revealing a circle of short wooden tree-stump-like structures. In the middle of that circle there is a post-and-lintel structure. The entire circular configuration is oriented toward the exact point at which the sun rises on the day of the summer solstice.
Who built this structure? And for what purpose? To what end? A primitive calendar? A center of worship? A lost tribe?

[Woodhenge:
A mystery that continues to cloud the American brain.]