The jagged hills in the new images may be icy material thrown outward from a large impact billions of years ago, or the highly eroded remains of a large impact structure, Williams said. Each bright peak is surrounded by darker dust that appears to be slumping off the peak.

"They are continuing to erode and will eventually disappear," Klemaszewski said. One theory for an erosion process is that, as some of the ice sublimes away into vapor, it leaves behind dust that was bound in the ice. The accumulating dark material may also absorb enough heat from the Sun to warm the ice adjacent to it and keep the process going. The new images show portions of the surface where the sharp knobs have apparently eroded away, leaving a plain blanketed with dark material.

http://www.jpl.nasa.gov/releases/2001/release_2001_178.html

Interesting. Instead of plate tectonics, the 'ocean/s' underneath may be the cause(?!).

Marcia
Thanks, Tom (and Raoul for the previous moon photo). Chesley Bonnestell was THE best and very well still may be. That black and white photo of the Archamedes Crater is a good test to show his knowledge of and great sense of depth, perspective and realism. Thanks!