Over time, these predecessors to today's North Cascades were further faulted and eroded to a nearly level plain. During the past 40 million years, heavier oceanic rocks thrust beneath the edge of this region. Intense heat at great depths caused them to melt. Some of the melt rose to the surface as fiery volcanic eruptions like Mt. Baker. The rest recrystalized at various depths to form vast bodies of granitic rock forming the core of the North Cascades. These gigantic pinnacles have pushed upward to majestic heights again, exposing the roots of the ancient collision zone. Scientists agree North Cascades geology comprises some of the most complex and least understood geology in North America.
The North Cascades are still rising, shifting and forming. Geologists believe that these mountains are a collage of terranes, distinct assemblages of rock separated by faults. Fossil and rock magnetism studies indicate that the North Cascades terranes were formed thousands of miles south in the Pacific Ocean. Attached to slowly moving plates of oceanic rock, they drifted northward merging together about 90 million years ago. Colliding with the North American Continent, the drifting rock masses were thrust upwards and faulted laterally into a jumbled array of mountains. The collision broke or sliced the terranes into north or south trending faults that are still evident today.