Land in the hills and mountains is colder and recieves more precipitation. Snow will fall in these upland areas, and if the temperature is cold enough can remain through the summer. The next winter's snowfall piles up on top squeezing the lower layers that eventually turn into ice. The ice, due to the pull of gravity, begins to slide downhill and erodes the rock it passes over.

It erodes in two ways - by plucking and by abrasion.

Plucking is the ice freezing on to a jagged piece of rock and pulling it out as the ice continues to move downhill. It is similar to a dentist pulling out teeth.

Abrasion happens when ice already has fragments of rock embedded in its base and these scrape at the rock surface. This process can be compared to sandpaper smoothing wood.