The granite and sedimentary rock that makes up much of the North Fork John Day Wilderness reflect the Blue Mountains' history as a volcanic island archipelago in the Pacific Ocean some 250 million years ago. As the crust of the earth shifted, the archipelago collided with the advancing North American Plate approximately 200 million years ago. Later, bubbles of less dense granitic rock, or batholiths, rose through the sediment to form the granite cores of the Elkhorn and Greenhorn ranges. Columbia River basalt flows buried the region some 15 million years ago, the remains of which can be seen today in the rimrock on the benchlands in the western portion of the wilderness.