Link outcrop of rocks on Mars (left) alongside an image of similar rocks on Earth (right), which offers a typical example of sedimentary conglomerate formed of gravel fragments in a stream. The Link region on the left features rounded gravel fragments, or clasts, up to a couple of inches within the rock outcrop.
An annotated version of the image highlights a piece of gravel that is about 0.4 inches (1 centimeter) across. It was selected as an example of coarse size and rounded shape. Rounded grains (of any size) occur by abrasion in sediment transport, by wind or water, when the grains bounce against each other. Gravel fragments are too large to be transported by wind. At this size, scientists know the rounding occurred in water transport in a stream.