This image highlights a piece of gravel that is about 1cm across. Rounded grains 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 hence at this size, scientists know the rounding occurred in water transport in a stream.
On the left you can see rounded gravel fragments up to a few cm within the rock outcrop. Erosion of the outcrop results in the gravel pile at left. The outcrop characteristics are consistent with sedimentary rock that was formed by the deposition of water. A typical Earth example of sedimentary conglomerate formed of gravel fragments in a stream is shown on the right.
source: data archive