Final answer:
In sand dunes, wind is the process that most commonly moves sediment, shaping it into patterns of ripples and dunes. Cross-bedding within dunes can reveal the direction of past wind transport, which is crucial for geological studies.
Step-by-step explanation:
The process that most commonly moves sediment in a sand dune is wind. This is because dunes are often found in deserts and some coastal environments where they are formed by the consistent movement of wind across sediment. The wind shapes the sand grains into wavy patterns, creating ripples and dunes that can indicate the direction of the wind's movement. Cross-bedding is a sedimentary structure found within dunes, showing angled layers that can reveal the direction of sediment transport due to wind. In coastal areas, although ocean currents and waves also move sediment, in the context of sand dunes, the dominant force is typically wind.
Ripple marks created by wind can be asymmetrical, with one side steeper than the other. The steep side of a dune or ripple mark is always angled downward in the direction the wind (or water) was moving. The study of these structures can aid geologists in understanding past wind patterns and environments where these sediments were deposited.