Gratiot Lake


One of several lakes arranged below the scarp caused by the erosion along the Keweenaw Fault, Gratiot Lake was formed in part as a great plunge pool during glacial melting about 10,000 years ago.

Gratiot Lake has a very deep portion located below the cliff exposure to the NE.  We are uncertain whether this represents a plunge pool created during glacial melting.  Its location coincides with the steepest slopes to the north of the lake, along the fault scarp trend, but does not coincide with the most likely stream channels.

Highland Copper Company is actively exploring this area.

Jacobsville Sandstone
Freda & Nonesuch
Copper Harbor Conglomerate
Portage Lake Volcanics
Gratiot Lake
932 intrusion
Deep Spot


The Gratiot Lake location allows some access to the chalcocite bearing intrusions which have been explored for decades by mineral groups interested in mining. The 932 intrusion is a small basaltic andesite body with chalcocite. There are numerous other intrusions in the oldest part of the Portage Lake Volcanics, and many contain chalcocite.