Notice that almost all rocks in this image are either cracked or flat. The same appears to be the case in the recent images taken near the summit:

http://marsrovers.jpl.nasa.gov/gallery/all/spirit.html

This could be consistent with the hills being frost mounds through frost wedging or their being the rims of maars through thermal expansion/contraction:

re: What are the theories on how this range of hills was formed?
Posted by Robert Clark on 8/7/2004 1:16:22 AM
http://habitablezone.com/space/messages/337192.html

However, another possibility is it's a type of weathering that can occur in dry conditions:

Physical and Chemical Weathering.
"Thermal expansion, or insolation, is a very important type of physical weathering, which occurs particularly in desert type areas. It is essentially very similar to freeze-thaw weathering, without the water. The outer layer of the rock is heated greatly by the sun during the day, causing it to expand. At night, the cooling of the rock causes it to contract. After this expansion and contraction has been repeated many times, the outer ¡¥skin¡¦ of the rock peels away in a way similar to that of an onion. Exfoliation leaves rounded boulders and dome shaped rocky outcrops, which are features of many desert landscapes."
http://www.courseworkbank.co.uk/coursework/physical_chemical_weathering_821/

But we would have to explain why this dry cracking occurs in the vicinity of the hills and is more pronounced near the summit.
It may also be the case that water is required even in this type of weathering:

Desert Features.
A 2 Mechanical Weathering
"The extreme changes of temperature associated with deserts are potentially important in weathering. Daily air-temperature ranges in excess of 50° C (90° F) have been recorded in deserts. Large seasonal temperature differences also exist. Such changes can lead to insolation weathering. This occurs because rocks are not very good conductors of heat, so that the outside layers heat up more, and cool more, than the inside ones. The resulting steep temperature gradients near the rock surface lead to different rates of thermal expansion and contraction of the rock and its constituent minerals, producing fractures inside it. Because it has proved difficult to replicate this process in the laboratory, the significance of insolation weathering has been questioned. However, recent work suggests that it may in fact be a potent mechanical weathering process in these extreme environments, particularly if moisture is present in the rock, enabling chemical weathering to accentuate the process."
http://au.encarta.msn.com/encyclopedia_781533896/Desert_Features.html

Bob Clark