Geologic Units

Quaternary alluvial deposits
Tertiary sedimentary rocks, undifferentiated
Miocene basalt (Columbia River Basalt Group)
Eocene granodiorite, granite, diorite, and shallow dacitic intrusive
Schist of the Wallace Formation and lower Missoula Group
Prichard Formation (Lower Belt), includes Yqcp quartzite and carbonate rocks in Prichard
Lower Missoula Group.  Lower part-- Gunsight Formation  (Snowslip and Shepard) 
Upper Missoula Group.  Upper part Swauger Quartzite  (Mount Shields, Bonner, and Libby Fms.)

  Map Key
Geologic units with unit designation.
Normal Fault: certain; dashed where approximately located; dotted where concealed.
Thrust Fault: certain; dashed where approximately located; dotted where concealed.
Detachment Fault: certain; dashed where approximately located; dotted where concealed.
Interstate Route.
U.S. Route.
State Route.
Location of Rockwalk rock from the county.
Feature location.


Benewah County

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

Additional Reading

Rocks Rails and Trails: page
References on Idaho Geology

Click here to see a correlation of geologic units, and the associated time scale.

Click here for a printable version of this map.

Click on the button below to search the IdahoGeologic Survey's index of Geologic Maps