This view looks west from the folded and faulted sedimentary rocks of the Methow terrane (foreground mountains) across the North Cascades Crystalline Core (snowy peaks in the middle distance) to the white, glacier-clad composite cone of Mt. Baker (vaguely visible in the far distance).

Most of the rocks in the core of the North Cascades are metamorphic and plutonic rocks that crystallized deep in the crust. These are the "crystalline" rocks of the North Cascades core-many of them sparkle with visible crystals. Most of these rocks formed at the same time as accreted terranes were being added to the area, during the Late Cretaceous orogeny. Since then the deep-crust rocks of the region have been exposed by millions of years of rapid uplift and erosion.

After the Late Cretaceous orogeny formed most of the metamorphic and intrusive rocks of the North Cascades, the rocks of the core slid along major strike-slip faults, during the early Tertiary period. In addition, during the Tertiary period, more plutonic rock was injected from below into the crust of the North Cascades, adding to the granite that makes up much of the core of the mountains.

The modern Cascade volcanic arc has been active in the region, starting in the late Tertiary. Mt. Baker, which is considered an active volcano, is the latest composite cone to add layers of lava and ash on top of the older rocks, like icing on a cake.

Larger glaciers of the Pleistocene epoch, and the smaller glaciers that remain on the high peaks today, have eroded the mountains into jagged peaks and ridges.

Jack Mountain Thrust

The highest peak in this picture is Jack Mountain, a non-volcanic peak of the North Cascades that rises over 9,000 feet above sea level. This view is from the Ross Lake overlook on Highway 20. Ross Lake, a reservoir of the Skagit River behind Ross Dam, is the body of water at the bottom of the picture. If you live in Seattle, the power of gravity pulling on this water is the source of some of your electricity. Select the image to see a larger view. Use your browser's back button to return to this page.

Jack Mountain straddles the divide between terranes of the Methow Valley and the highly metamorposed and intruded terranes of the North Cascades crystalline core. Although you can't really see it in this picture, if you were to climb up and down Jack Mountain to observe and map its geology, you would see that between the upper slopes and middle slopes of Jack Mountain is the Jack Mountain fault, a thrust fault that was active in Late Cretaceous time.

At the top of Jack Mountain, the uppermost ridge, rocky and barren, consists of the Hozameen terrane, made of slightly metamorphosed ocean-floor basalts, including pillow basalt. The thrust fault separates the Hozameen terrane at the top of Jack Mountain from its middle andn lower slopes, where the Little Jack terrane crops out. The Little Jack terrane is metamorphosed, fine-grained, ocean-floor sediments. It also includes blocks of ultramafic rock that probably came from the mantle. Perhaps the Little Jack terrane originated in setting where faulting of the ocean floor uplifted and exposed several kilometers of oceanic lithosphere, and blocks of the lithospheric mantle underwent landsliding down nearby submarine slopes into the mud.

The rock in the lower hill in the foreground is gneiss and migmatite of the the highly metamorposed and intruded Skagit Gneiss complex, part of the Chelan Mountains terrane.

Skagit Gneiss

This is a picture of part of the Skagit Gneiss. This view is about 15 feet (5 m) across, and is part of an outcrop across from the Diablo Lake overlook on the North Cascades Highway (Highway 20). Select the image to see a larger view. Use your browser's back button to return to this page.

The Skagit Gneiss composes much of the North Cascades Crystalline Core. It is a mixture of metamorphosed diorite and granodiorite (the dark rock in the picture), metamorphosed sedimentary and volcanic rocks that were probably accreted terranes, and pegmatite (the white rock in the picture). The Skagit Gneiss is a high-grade metamorphic rock that formed deep in the crust.

Polywog Agmatite

This is a type of rock that one geologist called "polywog agmatite." The polywogs are the dark splotches, which are actually inclusions of dark rock. Agmatite refers to the mixture of granitic rock (the white rock) and dark inclusions. Select the image to see a larger view. Use your browser's back button to return to this page.

This agmatite is part of the Ruby Creek Heterogeneous Plutonic Complex. In other words, it is part of a batch of mixed plutonic rocks that occur in the vicinity of Ruby Creek in the North Cascades Crystalline Core. The outcrop in the photo is located along Highway 20, between mileposts 139 and 140.

The rocks in this picture intruded along the Ross Lake Fault Zone, which is one of the strike-slip faults that was active in the North Cascades right after the Late Cretaceous orogeny, and possibly during the orogeny as well.

Rapakivi in Golden Horn Batholith Rock

This picture is a close-up of a piece of granite from the Golden Horn Batholith. The Golden Horn Batholith is Eocene in age, a younger intrusion that cuts across the Ross Lake Fault Zone. The Golden Horn granite is different from the older plutonic rocks of the North Cascades Crystalline Core - the Golden Horn rock is a true granite, with a high amount of potassium feldspar, the pink mineral in the picture. The older plutonic rocks of the Cascades core have hardly any of the pink feldspar, and many have none. Select the image to see a larger view. Use your browser's back button to return to this page.

If you have ever driven across the North Cascades Highway, Highway 20, then you have driven through the Golden Horn Batholith. At Washington Pass, the highest point on the highway, all the scenic peaks of the area have weathered to a golden hue. All of them are part of the Golden Horn Batholith, including Liberty Bell, Early Winters Spires and Silver Star Mountain, peaks that are well known to rock climbers.

The Golden Horn granite is also unusual in having a rapakivi texture, a texture that consists of pink potassium feldspar crystals surrounded by a mantle of white sodium plagioclase feldspar. You can see several of these pink cores surrounded by white rims just above the finger in the picture. The Golden Horn granite has a higher than average amount of potassium and sodium, even for a granite. The unusual composition and texture of this granite is related to its history.

The Golden Horn Batholith formed at the same time as the Challis Volcanics and other volcanic rocks erupted in various places around the Pacific Northwest in a distribution that violates the pattern of a normal volcanic arc. Also during the Eocene epoch, plutonic rocks with unusually high amounts of potassium and sodium intruded the crust of central Montana.

So, whether in terms of where the volcanoes were, or in terms of compositions of the plutonic rocks, igneous activity during the Eocene epoch in the Pacific Northwest stands out as being unusual.

Glossary terms that appear on this page: sedimentary rocks; accreted terrane; basalt; composite cone; metamorphic rock; plutonic; orogeny; intrusive; granite; lava; volcanic ash; glacier; island arc; thrust fault; gneiss; diorite; granodiorite; pegmatite; feldspar; rapakivi; plagioclase; igneous