a close up of Summit Mountain. You are looking at billion year old rock (the oldest in GNP) on top of rock that's only 65 million years old (around the death of the dinosaurs) due to the Lewis Overthrust. You can see EXACTLY where the thrust was by the tan line in the rock. This is a fault created when a large slab of rock –300 miles north to south, 50 miles east to west– slid over the top of other, younger rock. It was the first overthrust fault recognized as such; all other faults known slid by each other, side by side.