Gravity and aeromagnetic studies of the eastern portion of the Northern Peninsula of Michigan and the adjacent area indicate that the Lake Superior syncline consisting primarily of lower and middle Keweenawan mafic lava overlain by upper Keweenawan clastic sedimentary rocks extends from eastern Lake Superior into the Northern Peninsula, roughly between Grand Island in Lake Superior and the eastern margin of the peninsula. The limbs of the syncline, which are marked by positive gravity and magnetic anomalies, merge in the vicinity of the northern tip of the Southern Peninsula and mark the termination of the Lake Superior syncline. To the south, the Keweenawan volcanic rocks rapidly decrease in width and continue in a south-trending narrow belt which is reflected in the mid-Michigan gravity high of the Southern Peninsula. Two local sedimentary basins separated by a horst have been isolated within the syncline; one lies to the south and the other to the west of Whitefish Bay in Lake Superior.

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