are small, nearly parallel sand ridges
and troughs that are also formed by moving water or
wind. They are like dunes and sand waves, but smaller.
If the water or wind flows in a single direction, the
ripple marks become asymmetrical, like miniature dunes.
In other cases, waves move back and forth in shallow
water, forming symmetrical ripple marks in bottom sand
(Fig. 7–18). Ripple marks are often preserved in sandy
sedimentary rocks