Plows push snow away from the center of the road, forcing it to the edges. That left 'snow berms' that resemble retaining walls.

BEDFORD, TAKE A LOOK AT ALL THE PLOWS THEY SHOWED US AND HOW IT ALL WORKS. NOW, THIS IS A WHAT’S CALLED A WING PLOW. IT’S GOT A BLADE ON THERE THAT’S REMOVABLE AND REPLACEABLE. ONCE IT WEARS DOWN ...