they left the green oasis of dunhuang at dawn , the camel-bells in a querulous jangle as the beasts reluctantly trudged away over the dune-crests , their shaggy flat feet muddling the sharp lines of the ridges which cut the sunlight into parts : the dunes like ocean waves captured in pen and ink , on one side perfectly white and on the other pure shadow , printed on the pale caramel color of the sand .