Northern Dunes (CTX Frame - Possible Natural Colors; credits: Dr Paolo C. Fienga - Lunexit Team)

There is a vast Region of Sand Dunes at high Northern Latitudes on Mars.
In Winter, a layer of CO2 ice covers the Dunes, and in the Spring - as the Sun warms the ice - it sublimates.
This is a very active process causing the sand to dislodge from the Dunes' Crests to cascade, often (NOT always!) forming Dark Streaks.