Is there a piece of upside down foreign crust in the USA?
Most probably, he was referring to the Brunswick Magnetic Anomaly: http://www.geosociety.org/news/pr/2014/14-22.htm

This is an area of lower-than-normal magnetism present in North America in the Southwestern United States. Note that altough it COULD be a piece of Africa, it could also be caused by something else.