The study of sedimentary facies has revealed, among other things, how sea level, relative to the shore of a continent, is constantly changing over the course of geologic time, on time scales that can vary from decades to millions of years. To give a more specific example of how facies changes record sea level change, deep in the Grand Canyon of Arizona is a sequence of three sedimentary rock formations: the Tapeats Sandstone, the Bright Angel Shale, and the Muav Limestone. Those three sedimentary formations are thought to have originated as a continuous series of sediment as sea level gradually rose, relative to the land, over a span of time that took over a million years in that area. As sea level grew deeper, the shore of the ocean moved inland, which means that the beach facies moved inland, the bay facies shifted in the same direction, and so did the reef facies. Along the low-gradient coast of the continent, as sea level rose higher, what had been a beach was covered by deeper water and became the bottom of a bay where mud accumulated on top of the sand. Then, as sea level rose higher still , the area was in deeper water farther from shore where the water was relatively clear and free of clastic sediments. This allowed a coral reef to build on top of the mud. A more geological time passed and the environment of the area changed again, the deposits of sand, mud, and carbonate sediment were buried and lithified into the sequence of sedimentary formations which, from bottom to top, are sandstone, shale, and limestone. The sequence of sediments that record a gradual sideways shift of sedimentary facies during a marine transgression is shown in the diagram below. In the diagram, the part of the continent above sea level would be on the left. As time passed (moving upward in the diagram) the shoreline was moving to the left.