Dune deflation hollows are where wind has removed sand down to a level where a layer of particles too heavy for the wind to move (an armoured surface) stabilises the sand and prevents the surface being lowered further.
A crevasse is a deep, wedge-shaped opening in a moving mass of ice called a glacier. Crevasses usually form in the top 50 meters (160 feet) of a glacier, where the ice is brittle.
a mass of rocks and sediment carried down and deposited by a glacier, typically as ridges at its edges or extremity.
