Like texture and composition, sedimentary structures and bedding are inherent in sedimentation. Both are made visible by variations in grain size and to a lesser extent by mineralogy (Fig. 4-1). Because the great majority of structures can be seen with the naked eye, their study is as old as geology itself and, therefore, most of what we know has arisen from observation of ancient sediments. However, modern sediments and flume experiments have also contributed significantly to the study and understanding of structures, as we will see in Chap. 8. Structures have been used (1) as guides to determine the agent or environment of deposition, (2) as guides to stratigraphic order, by determination of top and bottom, (3) to map paleocurrent systems, (4) as indices of flow conditions, and (5) to assess chemical changes after deposition.