North Cascades National Park guide · 1 min read
Accreted Terranes: North Cascades' Patchwork Geology
Geologists see the North Cascades as a collage of terranes, each one a separate block of rock with faults marking the seams between them.
Together these blocks form a patchwork that explains why the mountains look so jumbled.
The blocks did not start here. Evidence from fossils and the magnetism preserved in the rock points to an origin thousands of miles south, in the Pacific Ocean, where the blocks rode on slow-moving oceanic plates. About 90 million years ago they drifted north, merged, and crashed into the edge of the North American continent. The impact thrust the rock upward and sheared it sideways into a tangle of peaks.
The same collision sliced the terranes along faults that run north or south, and many of those fault lines remain evident today. Most of the bedrock is Mesozoic crystalline and metamorphic rock, which sets the park apart from the younger volcanic rock found farther south in the Cascade Range. Some of the oldest rocks here are 400 million years old and date to the Devonian period.
Over time, the rock here has been worn down, buried again, cracked and baked, and each round has scrambled the story further. Few geological records in North America are harder to untangle, and the work of reading it is far from finished. Lifting and faulting continue today, so the terrain is still changing.
Today the mountains rise steeply, and glacier-capped peaks lift almost 10,000 feet above the lakes at their feet. Glaciers here are more numerous than in any other part of the lower 48 states. Climbers are drawn to these summits, and the alpine country around them is a draw in its own right.
