The magnetic field of Lake Michigan is dominated by a series of alternating regional maxima and minima which are, in general, directly related to gravity anomalies on the perimeter of the lake. These magnetic zones are believed to be primarily related to Penokean basement trends which extend from Wisconsin across Lake Michigan into the Southern Peninsula of Michigan. The strike of these zones changes from northeast in southern Wisconsin and east in northern Wisconsin to east and east-southeast respectively in eastern Lake Michigan and Michigan. Exceptions are found in the northern and southern extremities of the lake. In southern Lake Michigan southeast-trending magnetic anomalies are related to a regional gravity high extending across southwestern Michigan into northeastern Indiana. The positive magnetic zone in northern Lake Michigan strikes north from Grand Traverse Bay to north of Beaver Island, where it bifurcates with one limb extending north-northwest through Lake Superior to the Keweenawan basalt on Keweenaw Point. The other limb continues into the eastern portion of the Northern Peninsula of Michigan, and another segment of this branch connects to the Keweenawan flows on Mamianse Point on the eastern shore of Lake Superior. In the Grand Traverse Bay area this positive magnetic zone becomes strongly negative and connects to the south with the mid-Michigan gravity and magnetic anomaly.