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Structural Geology -Introduction Prod casts: Trace of debris carried in saltation by a current Inorganic top markers Show the top of a layer if preserved as a mold. Show the bottom of a layer if preserved as a cast Mud cracks: Polygonal desiccation features. These may be preserved as molds or Rain pits: Show rain impact on a surface. (rare) Organic markers Markers that show younging direction due to the activity of organic Fossil tracks: Footprints or feeding tracks. Molds show the top of the layer, casts of a lay Algal mounds: Colonies of algae, usually blue-green. Successive layers of sediment are bound by the algae, and are domed upward Burrows: Formed by burrowing organisms. Useful only when the top of the burrow be found Geopetal structures: Show two phases of diagenesis. Infill of shell with silt or lime mud, preserving horizontal, followed by infill of remaining space by calcite spar These can easily be misinterpreted Last revised August 27, 1999 Fall 1999 $ / ' # ) . ' / 0 ' " # , # ) . ; ' ; . ( ' 4 * 9 ' ; 5 6 ' !