Castle Rock is a volcanic neck. Volcanic necks are the remnants of magma pipes that deliver molten rock to the surface. After cooling, the center of the pipe is more resistant than the surrounding rock and so after a period of erosion, it sticks up above the surrounding landscape. The diagonal line in the snow in the center right is the trail to Castle Rock. The small dot next to the trail is an emergency "tomato" hut. Mount Terror is in the background.