Snow Rollers ( 1 of the World's 7 Strangest Winter Weather Phenomena)

Common in winter especially on hilly or mountainous terrain, snow rollers form when strong winds blow moist snow along the ground, piling it up into a cylinder-shaped ball that's hollow in the middle. When a snow roller gets too big and heavy to be blown around anymore, it stops where it lies.
Snow rollers can grow as large as a barrel or oil drum, or be as small as a roll of toilet paper. They also can leave tracks behind them. Here's a few of the conditions needed for snow rollers to form, according to the National Weather Service:

• Existing icy or crusty snow cover, so additional snow won't stick to it
• Wet, loose snow on top of the icy, crusty snow cover
• Wind strong enough to scoop up balls of snow and propel them forward

• Ground with at least some slope or incline

http://www.weather.com/science/nature/news/strangest-winter-weather-phenomena#/1