Located on state highway 61 overlooking the bay at Grand Marais
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The harbor of Grand Marais is the result of unequal weathering or erosion of two types of rock. One of these, called diabase, resulted from the cooling of molten material which was forced between two earlier lava flows. The dark, massive diabase, being very hard and resistant to wave action, has become the outer barrier to the harbor, while the lava, which was much fractured and easily eroded, was worn away to form the harbor basin.
To the west of Grand Marais, the serrated crest of the Sawtooth Range, clearly visible from the harbor breakwater, is another example of unequal erosion. Here the relatively soft basalt and the more resistant diabase have, through the process of weathering, produced the notched profile of the hills along the coast.
To the east of Grand Marais rise the hills near the mouth of the Arrowhead River, while to the north, along the Gunflint Trail are older rocks. At Saganaga Lake, the Saganaga granite, one of the oldest granites in North America, marks a core of the ancient mountains of the Laurentian Highlands.
Erected by the Geological Society of Minnesota and the Department of Highways, State of Minnesota aided by a grant from the Louis W. and Maud Hill Family Foundation. 1955
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