Evidence from the fossils and magnetism found in the rocks indicates that the terranes composing the North Cascades drifted thousands of miles north until they impacted the North American Plate 90 million years ago. The collision between the rocks caused fracturing and folding as well as uplift and the terranes were further fractured into north or south trending faults. The uplifted rocks mostly eroded away; 40 million years ago the heavier basaltic rocks of the ocean floor started to push the lighter granitic rocks that are the core of the mountains upward, a process that continues. Subjected to intense heat, rocks deep underground near the collision zone became recrystallized into granitic rocks, which comprise the backbone of the highest peaks. Continued uplift and erosion and finally the action of glacial ice on the landscape during the Holocene exposed the rocks visible today.