Sources of Basaltic Sand (extra-detail mgnf n. 2; MULTISPECTRUM - credits: Lunexit)

This rocky layer may originally have been a lava flow; Martian lava flows are predominantly composed of basalt, which would account for the dark color of the sand.
The polygonal pattern of the bright upper layer may be due to repeated freezing and thawing of the soil that buries the lava flow. The tracks of Dust-Devils are clearly visible on the smooth, sandy surface but largely vanish when they cross into the polygonally fractured terrain.