Grand Canyon rocks are mostly sedimentary, and sedimentary rocks form layers or “strata,” one above the other. The study of layered rocks is called stratigraphy.
 
   The oldest layers are at the bottom. Each layer thins toward the edges of its basin. For this reason, no cross sections of strata can be seen unless erosion or faulting reveals the “insides” of the layer, as at Grand Canyon.
 
   Each rock layer, or formation, has a name, usually taken from a local geographic feature. For example, the Bright Angel Shale is named after the Bright Angel Creek in central Grand Canyon. Sometimes related formations are combined into a group such as the Supai Group.
 
   All Grand Canyon sedimentary layers formed near sea level.
 
     The Grand Canyon’s sedimentary layers are expressed as alternating cliffs and slopes. This gives the Canyon its stair-step appearance. The slopes are made of soft and easily eroded rocks: shale, mudstone and weakly cemented sandstone or siltstone. Cliffs are the more durable rocks, usually well cemented sandstone and limestone. Igneous and metamorphic rocks of the Inner Gorge are made of extremely hard rocks. They are easily hard enough to form cliffs, but their contorted structure and the easy splitting of schist often causes these rocks to break into steep rugged slopes as well as cliffs.
 
    All the Grand Canyon sedimentary layers formed near sea level (just above or just below). Any lowering of the land would cause the sea to come in. This happened many times. Any deepening of the sea would inundate the land. For example, during warming trends, the polar ice could melt and deepen the sea. Each time the sea rose or the land sank, the sea encroached and sediments accumulated, creating the various stratigraphic layers. On the other hand, each time the sea lowered or the land rose, erosion occurred. In some cases, meandering rivers brought sediment down from higher regions, making terrestrial mud layers. Other times the wind brought desert sands. The final result is the Grand Canyon as we see it--many layers, some marine and some terrestrial.