On sol 20, Curiosity looked back toward the blasted ground where it had landed. This is Burnside Scour, one of four spots where the descent stage's rockets blasted the ground. Burnside was the northwestern scour (the one farthest to the left in the sol 3 panorama).
The mosaic began as JPEG'ed, Bayer-encoded raw left Mastcam images from JPL (sol 20, group 0052). I used G'Mic to convert the Bayer encoded images to RGB then a PDE flow to remove most of the jpg artifacts. Those were then Mosaiced using Nip2/Vips.