You might take a look at "Magic on the Rocks" by Michael Furtman.

Chapter 1 - 3 give you some background about picto's and Chapter 8 focuses on picto's found in the Boundary Waters.  

For those that have interest in this subject, you might also take a look at the Fisher Maps website.  They are now offering a new "Northwoods Pictographs" series (Quetico only at this time) that was developed by a frequenter of this board.  They are full color sheets with detailed information about each picto accompanied by a series of photograph ranging from distant shots of cliff faces (for orientation) to closeups of individual images.  And since they are sold by Fisher, small map segments show a map-based reference to location.

I received a few complimentary copies from the originator of this work.  They are quite nice.

dd