Uplifting Geology of the Cascades

By Kim Brown

The Pacific Crest Trail takes a hiker through spectacular terrain, with a backstory millions of years old. If you hike the trail north of Snoqualmie Pass, you can’t miss the dramatic, dark rock rising high above. But in other areas of the same stretch of trail, the stone is strikingly lighter.

Why is that? And how did those big dark spires form? The answer is all about uplift and erosion.

The Snoqualmie Batholith. Photo by Kim Brown.

The geological transition from the South Cascades to the North Cascades begins near Snoqualmie Pass. The North Cascades have been uplifted, and eroded, more than the South Cascades. That erosion exposed older, more varied and colorful rocks.

Beginning about 35 million years ago, an oceanic tectonic plate slid under the continent, melting the rock above. Then, about 22 to 28 million years ago, huge blobs of granitic magma, called batholiths, welled up from the melting zone and solidified as the Snoqualmie Batholith, which stretches roughly from I-90 to Highway 2. The invading magma recrystallized the original rock, making it darker and more erosion-resistant than it had been before.

“A PCT hiker in North Cascades traverses the roof of an immense mass of solidified magma,” USGS Geologist Rowland Tabor says.

Rocky outcroppings in the Snoqualmie area. Photo by Kim Brown.

As they hike, Rowland explains, hikers will see different rocks—the original stone, heated and hardened stone and intrusions of batholith. Some of that rock “is much more resistant to erosion ... and stands up as craggy peaks such as Mount Thomson, Chair Peak and Guye Peak. The Kendall Katwalk, about 6 trail miles north of Snoqualmie Pass, is blasted into the lighter colored granitic rock of the batholith.”

As you move through the high country, think about how the various colors and textures of rock might be formed. Batholithic blobs of magma lie at depth, still feeding the modern volcanoes in the Cascade volcanic arc, such as Mount Rainier and Glacier Peak. Mountains are still eroding, and tectonic plates are still plunging and sliding—it’s a busy world out there!

Author's note: After researching this article,I struggled to describe a batholith without plagiarizing the many ways it has already been described. I decided on the word “blob.” I asked geologist Rowland Tabor to check my work. He did so happily and sent my document back with a note:

Kim: Please forgive me for I am a compulsive editor ... I could not resist messing with your text. He left in the word “blob.”

After I good-naturedly said I didn’t recognize much of my own writing
after his edits, he offered solace.

Kim: You have taken the editing graciously. And I did leave a good deal of your original, such as articles, prepositions, etc.

So, while Mr. Tabor contributed heavily to the text, the articles and prepositions are my own, as are the words “Snoqualmie,” “batholith”  and
“blob".