Geology of North Cascades National Park

On first glance, the North Cascades National Park might appear to be quite still, in fact, it doesn’t really matter how long you look at them, you probably won’t see the mountains moving, but in actual fact the North Cascades are a work in progress, they’re still rising, they’re still shifting about, they’re still forming. The North Cascades are a little of an enigma to geologists, what’s strange about them is that the North Cascades Terranes weren’t formed there at all, but in another place thousands of miles south. These were fixed to oceanic rock, slowly moving and drifting northward before joining together around 90 million years ago. Nobody really knows when they actually “came together” . . . told you it was a bit of an enigma.

As this drifting rock mass crashed into the North American Continent the rocks were thrust forcefully skywards before creating a rather jumbled bunch of mountains. This collision sliced the terrane into two pieces, the trending fault which runs north to south, still evident and crossed by Highway 20 a little to east of Marblemount.

This fault caused some serious sliding going on (not quick sliding like we’re used to, but slow, grinding sliding) and the rocks which were on the west side of the fault moved more than 100 miles further north than the rocks to the east. If you study the rocks today, the ones on the east of the fault are largely granites and gneisses, and the rocks to the west are mostly sandstones and mudstones.

Geology of North Cascades National Park

Over the last 40 million years or so, give or take a few hundred thousand, oceanic rocks thrust below the border of the region, with the great depths and intense heat causing them to melt. Not surprisingly, some of this deep burning cauldron rose up to the surface with violent volcanic eruptions creating features like the magnificent Mount Baker, the rest of the melt stayed below the surface, crystallizing at numerous depths and forming great chunks of granite. The North Cascade Mountains have grown to truly majestic heights, whilst exposing the roots of this historic “crash” zone.

North Cascades National Park might be the scene of some gigantic geological puzzle, but there’s no denying that it’s a beautiful place.