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Structural Geology-Introduction mity surface pr a history of depos followed by, deformation, uplift and erosion followed by subsidence and deposition Disconformity: The unconformity surface may preserve nondeposition or erosion. In a disconformity the layers above and below the unconformity surface are parallel unconformity surface Primary depositional features In the field it is often difficult to see enough of a rock sequence to establish that the principal of superposition holds, and it becomes necessary to determine the direction of younging in the outcrop. Determination of younging direction is essential in highly deformed rocks. Most often this determination relies on primary sedimentary structures Primary structures are features preserved in sedimentary rocks that display some aspect of the environment of deposition, or features that formed very early in the diagenetic Ripple marks Ripple marks are formed by current or wave action during transport and deposition of sediments Oscillation ripple marks:(symmetric) Wave induced, show bimodal current. Sharp crests, smooth troughs. Younging clearly shown Last revised August 27, 1999 Fall 1999 ( ' , ' ! # " - $ , . / $ ( # )* 0 1 ' " # 2 3