Sedimentary structures are patterns in sedimentary rock that form near the time of deposition before the sediments become lithified into rock. One common type of sedimentary structure is grain size patterns. Examine the above highlighted portion of the Gigapan image. Are the grain sizes uniform, variable, or do they exhibit a trend, such as gradually changing from small to large in size, or vice versa, within the layers? There are several ways to help you identify sediment grains in the outcrop and determine their size: Look for rounded to angular-edged particles resting against one another, perhaps even tightly packed together with their boundaries touching. If you can see entire particles and their edges, then they are relatively large in size (e.g., small pebbles down to sand or more generally called coarse). If you cannot make out the individual particles and their boundaries, and they tend to blend in with the matrix (the material surrounding the grains), then they are likely rather small (e.g., silt and mud or more generally called fine). Choose the interpreted sketch that best indicates the direction of fining in grain size (i.e., the direction in which grain size goes from coarse [large] to fine [small] on the highlighted portion of the rock outcrop above).
