Chasma Boreale (1)

Both Martian Polar Caps show ragged outlines, with sinuous valleys and spiral-shaped troughs giving the Caps' edges a sawtooth outline. Scientists think the spirals develop as solar heating melts and evaporates the ice more strongly on the Equator-ward and West-facing sides of any slight depression. In time, these deepen into valleys and canyons.
The largest trough in the Northern Polar Cap is a huge canyon named Chasma Boreale. It runs into the ice cap for 570 Km (about 350 miles), making a broad valley that ends at a point called Tenuis Cavus, shown in this THEMIS image taken at visible wavelengths.