When they turn one way you see the white of their bellies. When they turn the other you see the brown of their backs. This flock is in mid-turn
Flocks of peep sandpipers (probably Western or Least) are in the marsh. When they get up and whiz around in their mega flocks you can hear the rush of their wings and the myriad peeping calls. People use the term force of nature all the time, these migrating flocks truly live up to that term.