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Falling Water Level Ripple Marks
William F. Tanner (1)
Ripple marks produced in shallow water, especially when the water level is falling, are more varied, more complex, more easily interpreted, and more valuable in paleographic studies than ripple marks developed under other conditions. Shallow-water and falling water-level ripple marks are conveniently studied on sand-floored tidal flats as well as in wave tanks. Tidal flats have the advantage that a variety of wave systems, moving from different directions, can be studied, both singly and in combination.
Flat-topped ripple marks, in many different patterns, are formed when the water level drops to, or below, the ripple mark crests. When the rate of water level fall varies systematically, terraced flat-topped ripple marks are produced. Two parallel ripple mark systems, having smaller ridges centered in the troughs between larger ridges, develop as a result of the adjustment of wave orbit diameters during the fall.
Helical cell ridges ("rib-and-furrow"), windrow ridges, and other long down-current ridges are produced primarily by direct current flow, or by a vector combination of waves and currents, in shallow water. Composite ripple marks arise when the motions of two in-phase wave systems are added vectorially. Out-of-phase combination yields a wavy map pattern. Additional ripple mark types found on the tidal flat or in very shallow water have sharply-pointed troughs and gently rounded crests, or are flat-bottomed despite an abundance of sand. These types may be caused by a combination of wave action and mass flow of shallow water.
The catalog given here does not exhaust the list. New varieties are being found with some regularity. Many of these varieties have been observed in the lithified rock column, and can be interpreted with relatively great confidence.
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