This diverse county has rolling Palouse on the west, and foothills and steep forested mountains on the east. The mountains are underlain by Mesoproterozoic Belt Supergroup, with the Emerald Creek mining district in the extreme southeastern corner of the county south of Fernwood found in metamorphic rocks of the middle-Belt Wallace Formation.
Miocene Columbia River basalts cover the low farming country in the western county and up the St. Maries river to Clarkia. Lacustrine and river deposits were deposited in valleys that had been dammed up by basalt lava flows. The world famous Clarkia fossil locality formed this way.
The St. Joe fault, and Eocene feature related to continental extension and development of metamorphic core complexes runs eastward through the northeast corner of the county, north of St. Maries. Beautiful Benewah Lake and the Coeur d'Alene river make a vacation paradise, but heavy metals in mine waste along the river raise environmental concerns.
A small area southeast of St. Maries is underlain by Eocene granodiorite.
The Coeur d'Alene Indian Reservation headquarters is at Plummer in the northwestern part of the county.
Written by P.K. Link, 9/02
Rocks Rails and Trails: page
References on Idaho Geology
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