A rock core from a 60-million-year-old slope deposit from the North Slope of Alaska. The core was taken from about 3,050 meters (10,000 feet) down into the Earth’s crust in a thick sequence of sedimentary rock that records the development of an ancient ocean continental margin. The light-colered bands are thin (less than 1-2 centimeters (0.4-0.8 inches) thick) sandstone layers, and the darker layers are organic-rich shale and mudstone.