Quote Yellowbird:"Has anyone heard the term, "magnetic disturbance"? Here's a bit of info from the Green Bay sectional aeronautical chart (published by the FAA), which covers the BWCA.

"Magnetic disturbance of as much as 12 degrees exists at ground level between Tower and Ely" . . . as much as 18 degrees . . . from Duluth to Grand Marias".

I've conducted magnetic and electromagnetic surveys in N. MN and the iron formations and other ore bodies definately have an influence and cause anomolies on the EM field.  That's how geologists find ore deposits.  The technology developed during WWII that was used for submarine hunters.  The classic tell tale sign is to look at USGS maps and see the deviations of the grid lines across the formations.