Near-surface Earth materials (soil, rock, debris, etc.) are subject to physical and ct Jemical degradation called weathering. We
athering produces solid particles and even ions in solution-the raw materials for sediments. These solid and dissolved products are almost always moved by gravity, water, and/or wind from where they were initially created by weathering processes. Sediment deposition begins when the transport mechanisms-gravity, wind, or water currents -- slow down enough such that the sediments can settle out the Latin sedimentum means to settle'). Graded beds are characterized by a gradual change in grain size from coarse (large) to fine (small) from bottom to top. This pattern is commonly related to changes in the energy level of the water that carries, and eventually deposits, the grains. High-energy, vigorous currents can pick up and transport large, heavy sediment particles. In contrast, stagnant water has no energy and cannot keep even fine-grained sediments suspended in the water column. As a result, very fine, mud-sized grains can be deposited. If beds exhibit a gradual change in grain size, then it follows that the current velocity that deposited them must also have changed during the depositional process. In this activity, choose from statements that describe how grain size and current velocity are connected, and how that helps us understand graded beds. Read the following sentences and select which is true.
a. Graded beds imply that only fine particles can be deposited in high velocity currents, and only coarse-grained sediments can be deposited in low velocity currents.
b. All sizes of sediments can be transported and deposited in the full range of energy levels associated with different depositional environments.
c. Graded beds are evidence of a decrease in current velocity during deposition whereby coarse sediments settle out first, followed by finer sediments as the velocity decreases over time.
d. Graded beds imply that the currents that deposited them always had very high velocities.
c. Graded beds are evidence of a decrease in current velocity during deposition whereby coarse sediments settle out first, followed by finer sediments as the velocity decreases over time.
