Define the following:
deflation hollows
crevasses
morain

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Explanation:

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.

Well, a and b together would be negative. b alone would also be negative.