Two interpretations can apply. One proposal is that the coastal patrol’s path showed up white on the chalk cliffs that are characteristic of the English south coast (251). The other is that the Guard’s route was shown by chalk marks on stones (142). My vote is with the first interpretation. Readers may want to know that in the days before electronic communications, Coast Guardsmen patrolled the shore on foot watching for vessels in distress.
where the Coast Guard’s way was chalked