My reading in the subject area of rocks, minerals, and gems has been, for nearly a year, in the area of birthstones. It's a subject that says a lot about human history. But to my way of thinking is somewhat less interesting than geology. So I have gone back to studying the book Minnesota's Geology by Richard W. Ojakangas and Charles L. Matsch. The subject is glaciation. Many terms describe what was left behind when glaciers moved across Minnesota during the Quaternary (2 million years ago to the present). One interesting term is erratics. These stones can be any size from a boulder to a pebble. They were transported from one area to another by the movement of glaciers. They differ from the local bedrock in color and composition, sometimes so radically they have been mistaken for meteors. I take an interest in these because the glaciers dragged in so much material to this local area. Also, I who am not a seasoned artist sold a painting of Finnish erratics to an employee of the Guggenheim, which was a thrill and honor. Since then I've read about erratics at length. Technically, if a rock was moved due to human activity, it's an erratic. By that definition, rockhounds have created a lot of these!