At Improbable Research I found this story describing how moguls - those bumpy things on a ski slope, gradually work their way uphill. That should make sense to experience skiers, though now there is an article at Physics Today that makes it more than just anecdotal belief. The video below illustrates the movement over the course of the ski season:
The ski slope is a run call Riflesight Notch at Mary Jane ski area, Winter Park, Colorado. It looked familiar even before I read the text describing the video. I have been there many times, skiing the moguls until every muscle ached and loving every minute of it, then dinner at Katie's Beanery in Granby on the way home.