Between 750 million years and 1200 million years ago, a series of marine events laid down the Grand Canyon Supergroup. So named because they are superimposed upon sections of the Inner Gorge basement rocks. There is a good section of these rocks on the South Kaibab. The North Kaibab also boasts some supergroup rocks, but they are more fragmented by the Bright Angel Fault and thus more difficult to pick out. It is within the Bass Limestone layer of the Supergroup that the hiker may see stromatolites.