When we laid out the foundation of our house we used a compass to get the correct orientation for passive solar gain. Oddly, the compass simply wouldn't work on the west end of the house. We figured it was because of the nearby heavy metal machinery and used the direction for south that the compass showed on the east wall. It was wrong by 13 degrees. It turns out that this ridge has large iron deposits and was surface mined heavily at one point in time. When we laid out the driveway we cut into one of the iron seams. One happy consequence of this is that the winter solstice sunrise comes directly and exactly through this window.
I should mention that this window was a work by the talented artist, Glenna Alderson, working in stained glass for this piece. It is a treasured gift.