challenge: What happens to the sand grains during the formation process of cross-stratification?
  
  A. They are carried away by the paleocurrent, resulting in no deposition
  B. They are transported down the downstream side of the dune by the force of the depositing fluid
  C. They saltate up the upstream side of the dune, collecting at the peak
  D. They are eroded from the crest of the dune and deposited at the base
  Answer: 