About 6-8 miles southwest of Minneapolis, Kansas is a field full of almost perfectly round rocks. They are huge! It is documented that Paul Bunyan stopped here and played marbles with these rocks on his way west. As may be seen from these pictures, it is an excellent playground for folks young and old who visit North Central Kansas.
These rock formations are known as concretions. They form under ground and when the ground is washed away they appear before your very eyes. This may take a million years or more.