Everything I can see and read about U Mound says that it is a 1.4 billion year old porphyritic biotite granite. That may not mean much to you readers, but to me is says that magnetic problems should be highly unlikely on the rock, because granite is almost devoid of magnetic minerals such as magnetite and ilmenite.
But you can test it yourself at U Mound or anywhere else. Take an ordinary compass out with you. See if it points north as it should. See if the needle starts to swing back and forth as you lower it to the ground. I'll bet 30¢ the compass will be very stable on U Mound. You can cause a compass to go haywire just by getting up close to your metallic belt buckle or by putting a mechanical pencil on the cover glass and rotating it around a vertical axis.
Having said that, Alexander AF, I have no ready explanation for why you had compass calibration issues on U Mound, but I don't believe it was due to the rocks.
