Geologic Units

Quaternary alluvial deposits
Gravels and terraces on western Snake River Plain
Quaternary surficial cover, fluveolian cover on Snake River Plain, alluvial fans (Snake River Group)
Pleistocene basalt lava
Pliocene and Upper Miocene stream and lake deposits (Salt Lake Formation, Starlight Formation, Idaho Group)
Miocene felsic volcanic rocks (Idavada volcanics), includes rocks designated as Tmf (Bond, 1968) in Owyhee County and Mt. Bennett Hills
Cretaceous granite and granodiorite of the 2-mica suite (Idaho batholith)
  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.

Ada County

Ada County, Idaho's most populous county, has relatively simple geology, compared to some much more sparsely populated areas.

On the northeast is the Cretaceous Idaho batholith, home to Bogus Basin ski area. The batholith forms a mountainous area uplifted on south-dipping normal faults which form the northeast margin of the western Snake River Plain.

In the Boise foothills are a complex assemblage of sandstones and lake beds formed on the edges and within Lake Idaho in the last 10 million years. The Table Rock Sandstone, quarried since the mid 1800s belongs to these strata.

The city of Boise lies in the alluvial valley of the Boise River, which joins the Snake River on the Oregon border, south of Nyssa.

A series of northwest striking normal faults cuts Ada County, part of the western Snake River Plain graben. On the south are extensive Quaternary gravel deposits that overlie Quaternary basalt. Recent cinder cones line the Snake River near Swan Falls.

P.K. Link, 9/02

Additional Reading

Rocks Rails and Trails:
Challis Volcanic Group & Intrusive Rocks
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 here for a discussion of the Geology of the Boise Valley (PDF)

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