I have always enjoyed finding ancient examples of sedimentary structures that help a geologist determine depositional environments. The example I show here are ripple marks made by waves in a middle Miocene (about 13 million years old) lacustrine (lake) deposit in the Mint Canyon Formation, northern Los Angeles County, southern California.
This picture is a side view of the rock slab shown above. The up-current side of each ripple mark is low angle, whereas the down-current side is high angle. So, as viewed in this photograph, the wave current moved from right to left.