Many American Indians believe that the larger boulder fragments known as the Three Maidens shelter the spirits of maidens who require offerings from visiting Indians before permitting them to quarry pipestone. During a visit here in 1836, artist George Catlin recorded his observations of such an offering and called these rocks "truly a matter of surprise for the scientific world." These huge granite rocks seem out of place in a land filled with only quartzite. During the last Ice Age, a continental glacier carried the granite down from present-day Canada. When the glacier receded, it left behind a huge boulder about 50 feet in diameter. Frosting action eventually split this glacial "erratic" into six pieces. The Three Maidens are held sacred by American Indians.