Don’t Take the Sawtooth Mountains for Granite

“As we passed on, it seemed those scenes of visionary enchantment would never have an end.” -Meriwether Lewis

Named for their easily recognizable jagged peaks and U-shaped valleys, the Sawtooth Mountains encompass 678 square miles across 4 counties. The “sawtooth” formation is due to the lifting and erosion of the Idaho Batholith. 

Castle Peak, the tallest peak in the Idaho Batholith

What is a Batholith?

A Batholith is a rock formation caused by the cooling of magma deep beneath the surface. The “lobes” of cooled magma are then pushed up due to buoyancy, shifting tectonic plates, and erosion. The Sawtooth Mountains are located on the Atlantic Lobe of cooled magma, commonly known as the Idaho Batholith. Northeast of the Atlantic lobe is the Bitterroot lobe, home to the Bitterroot Mountains of Idaho and Montana, which is also part of the Idaho Batholith. The two lobes are separated by the “Salmon River Arch,” a belt of metamorphic rocks. 

The Idaho Batholith

The Idaho Batholith is made up of mostly granite and granodiorite created during the Cretaceous and Palogene Period, 43-145 million years ago. The cooled magma lobe cooled and rose to the top of the Earth’s surface, all while shifting into each other, creating the massive, jagged spears of sparkling gray granite. A section of the Idaho Batholith located in the northern end of the Sawtooth Mountains, often called the Sawtooth Batholith, is a bit younger than the rest of the Mountains. 


The Sawtooth Batholith

The Sawtooth Batholith was formed 55-33 million years ago during the Eocene Epoch. While the two ranges were formed the same way, the youth of this area is seen in the pink hue of the granite, called orthoclase. This is caused by an excess of pink feldspar, something granodiorite lacks. 

Orthoclase Granite
Thompson Peak, the tallest peak in the Sawtooth Mountain Range and the tallest Sawtooth Batholith.

Glacial Past

The U-shaped valleys that help the peaks soar is due to the glacial past of this range. Glaciers filled in between the peaks and as they slowly melted away, some evidence suggests glaciers were in the area until the end of the Little Ice Age (1850), they left the rounded valleys, moraines, and sediment. 

The beauty of these mountains did not come without age. Millions of years have meticulously formed the Sawtooth National Recreation Area.