Compass Problems in Quarry? (Pearl Lake, IL)

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Hi All-

Anyone ever run into problems with compass navigation in a quarry before?  I was practicing the nav drill on shore for our OW class with my buddies but we kept getting mixed up.  The drill was to start from one location, do ten paces out, then reorient the compass to return to the original location.  One by one, each of us returned to a different spot, even with the coaching of the instructor.  ???

Then it dawned on us that the mineral content of the quarry might be throwing off magnetic north for the compass.

Anyone else experience this or can offer another explanation?

Thanks!