Wave ripple or symmetric ripple, from Permian rocks in Nomgon, Mongolia. Note “decapatation” of ripple crests due to change in current. While wave-formed ripples are traditionally described as symmetrical, asymmetric wave ripples are common in shallow waters along sandy shores. They are produced by bottom oscillations generated by passing breaker waves, which have symmetrical ripple marks intensity in opposite directions.
Wave-formed ripples indicate an environment with weak currents where water motion is dominated by wave oscillations. Although symmetrical ripples are also called bi-directional ripples there is a difference between them. Bi-directional ripples are rarely symmetrical due to the difference in force of the two directions, where as the wave formed or oscillation ripples form from the circular water movement pattern of water molecules. These ripples form parallel to the shore line. They usually display rounded troughs and rounded crests. Ripples are relatively small, elongated ridges that form on bed surfaces perpendicular to current flow.
With continuous current flow in one direction, asymmetrical ripples form. Asymmetrical ripples contain a steeper slope downstream. With an alternation in current flow from one direction to the opposite symmetrical ripples form. Symmetrical ripples tend to have the same slope on both sides of the crest. Symmetrical ripples form as water molecules oscillate in small circles.
A particle of water within a wave does not move with the wave but rather it moves in a small circle between the wave crest and wave trough. This movement of water molecules is the same for all water molecules effected by the wave. Wikimedia Commons has media related to Ripple marks. The Changing Earth: Exploring Geology and Evolution, 2nd ed. 1997, Sea-floor geology of a part of Mamala Bay, Hawaii: Pacific Science, v. Chapter 7: A Surface Veneer: Sediments, Soils and Sedimentary Rocks.
Hummocky cross-stratification from the Book Cliffs, Grand Junction, Colorado. Hummocky cross-stratification from the Book Cliffs, Utah. Hummocky cross-stratification is a type of sedimentary structure found in sandstones. It is a form of cross-bedding usually formed by the action of large storms, such as hurricanes. The name was introduced by Harms et al. Before this time, these structures were recognized under many different names.