A mystery reaches across the Blue Mounds, snaking through the big bluestem and the prickly pear, beneath the bur oaks and the meadowlarks. Some call it a rock wall; the more fanciful have called it Minnesota's Stonehenge.

The low jumble of rocks, little more than 1,200 feet long, lies on an east-west track so that on the first day of spring and the first day of fall, the sunrise and sunset happen at each end. Officially, that's considered a coincidence, since nothing supports the notion that its builders had astronomical intentions.

The mysteries continue, most obviously in why this immense whaleback of land just north of Luverne, Minn., is described as blue. Depending upon the season, it can be a parched brown, a lush green, a blinding white. But as settlers bumped their way westward, the grayish lichen that covers much of the purplish Sioux quartzite appeared blue. Ever in need of a landmark across the prairies, the Blue Mounds were noted, and named.

Comprising fewer than 2,000 acres, the state park itself feels like an intimate place, even as the view south on a clear day extends well into Iowa. It is home to a herd of bison, as well as some coyotes and deer. Great swaths of grassland still appear untouched, even a little mysterious.

At its southeastern corner, a craggy cliff face looks down on a natural amphitheater. The cliff, at the far end of the wall, gives rise to another theory: that the wall formed sort of a corral to help Indians drive buffalo over the edge.

There is one certainty: On Sept. 23, the sun will rise at one end of the rock wall and, 12 hours later, set at the other. What a coincidence.

If you go: For more details, visit www.Dnr.State. Mn.Us/state_parks/blue_mounds, or call 1-507-283-1307.