Gravity and magnetic anomalies observed over Lake Huron primarily reflect variations in the lithology of the basement rocks underlying the Phanerozoic sedimentary rocks of the Michigan Basin. Central Lake Huron is dominated by a 25-mgal gravity maximum which correlates with a complex, discontinuous belt of positive magnetic anomalies that extend southwest from Georgian Bay into Michigan. These anomalies are believed to originate from mafic gneiss intercalated with granitic gneiss such as encountered elsewhere in the Grenville Province. The western boundary of these northeasterly trending anomalies is interpreted to mark the location of the Grenville Front beneath the Lake between Killarney, Ontario, and Michigan's Saginaw Bay. To the west of the Grenville Front, the gravity and magnetic anomalies consist of alternating maxima and minima striking east-southeast which can be traced across Michigan into Wisconsin. The North Channel and the islands to the south are dominated by negative anomalies associated with lower Proterozoic sedimentary rocks. Much of the basement rock beneath Manitoulin Island and the lake to the south and west, nearly to the Michigan shoreline, is intruded by a complex of plutons having a strong magnetic signature.