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Geologic Units

Tertiary sedimentary rocks, undifferentiated
Miocene basalt (Columbia River Basalt Group)
Riggins Group and Orofino series
Eocene granite, pink granite, syenite, rhyolite dikes, and rhyolitic shallow intrusive
Eocene granodiorite, granite, diorite, and shallow dacitic intrusive
Cretaceous granite and granodiorite of the 2-mica suite (Idaho batholith)
Cretaceous orthogneiss, and foliated granodiorite and granite (includes mylonitic plutonic rocks in western Idaho suture zone)
Cretaceous tonalite and quartz diorite
Mesoproterozoic anorthosite
Upper Missoula Group.  Upper part Swauger Quartzite  (Mount Shields, Bonner, and Libby Fms.)
Lower Missoula Group.  Lower part-- Gunsight Formation  (Snowslip and Shepard) 
Piegan Group or Middle Belt carbonate, Apple Creek Formation  (includes Cobalt and Yellowjacket Formations, and Yaq argillaceous quartzite unit of Ekren)
Ravalli Group, includes Hoodoo Quartzite, Big Creek Formation, Yellowjacket Formation, lower part of Lemhi Group
Prichard Formation (Lower Belt), includes Yqcp quartzite and carbonate rocks in Prichard
Schist of the Wallace Formation and lower Missoula Group
Quartzite and schist of the Ravalli group
Quartzite and schist of the Prichard Formation
Metamorphic rocks, Elk City metamorphic sequence and related rocks, Syringa metamorphic sequence, and Priest River metamorphic complex

  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.

Clearwater County

Huge Clearwater County contains vast tracts of steep country drained by the North Fork of the Clearwater River, now impounded behind Dworshak Dam, and largely inaccessible. Pierce and Weippe occupy a relatively flat uplifted area between the North Fork Clearwater and the Lochsa River to the south. Lewis and Clark followed ridges north of the Lochsa as they crossed the continent in 1805 and 1806.

The northern part of the county is underlain by metamorphosed rocks of the Mesoproterozoic Belt Supergroup, folded and thrusted in the Cretaceous Sevier orogeny. No Paleozoic rocks are preserved. The Belt rocks are intruded by Cretaceous and Eocene plutons.

The southern part of the county is underlain by the northern expanse of the Bitterroot lobe of the Idaho batholith near Pierce and Headquarters, and by a complex of Eocene intrusive rocks cut by the Kelly Forks Fault. Eocene granite of the Bungalow Pluton occupies the center of the County.

On the southwest is the suture zone between North America and accreted terranes to the southwest, across the Clearwater river near Grangeville. The suture zone runs through the southwest part of Dworshak reservoir. Cretaceous to Permian Orofino Series rocks and Cretaceous orthogneiss underlie lava flows of the Columbia River basalts in much of the southwest part of the county. Miocene sediments were deposited in the upper portions of river valleys dammed by Columbia River basalts and crop out in numerous patches south of Elk River. Some of these Miocene sediments contain gold placers.

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.

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