In geology, a graded bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Steno’s Principle of Original Horizontality explains that rock layers form in horizontal layers over an underdetermined time scale and depth. Nicholas Steno first published his hypothesis in 1669 after recognizing that fossils were preserved in layers of rock (strata). Graded beds form when a steep pile of sediment on the sea floor (or lake floor) suddenly slumps into a canyon or off a steep edge. The graded bed has a sharp base that contains large cobbles. Geology and Planetary Science HomePage. Sedimentary rock: Bedding structure. Some sandstones contain series of graded beds. The grains at the base of a graded bed are coarse and gradually become finer upward, at which point there is a sharp change to the coarse basal layer of the overlying bed.
Reconstructing earth history — By analyzing a sedimentary rock, a geologist can deduce what was happening on earth at the place and time the sediment was originally being deposited. Given the wide variety of carbonate rock types observed in the Trenton Limestones, it is no surprise that the deposition of these rock units was impacted by a variety of sedimentary processes; these include sediment production processes, transport processes, as well as depositional and erosional processes. Thus a common signature of a storm-influenced deposit, otherwise termed a tempestite, is the development of a normally graded bed. Learn the secrets of mountain geology. Sandstone is an easily identified rock made up of individual grains of sand cemented together by chemicals such as calcite, silica, or iron oxide. Graded Bedding.