Just took a look at the link Don posted on White Rock:

Odyssey/White Rock
http://mpfwww.jpl.nasa.gov/odyssey/gallery/martianterrain/PIA03862.html

It shows a THEMIS thermal infrared image of White Rock that I had not seen before. It shows that White Rock is anomalous in infrared spectra as well. Whatever it is, it's something colder than surrounding terrain. It could be a volcanic ash deposit as suggested by Ruff et.al., or an evaporite deposit, or perhaps there is underlying water or ice.

Because of the 3km resolution scale of TES, Ruff et.al. had to take averages to make their conclusions on White Rock:

Mars' ``White Rock'' Feature Lacks Evidence of an Aqueous Origin
http://www.lpi.usra.edu/meetings/lpsc2000/pdf/1945.pdf

THEMIS with its 100m resolution may be able to observe individual ridges without the overlap from the intervening dark surface material.

Another interesting fact, Ruff et.al. argue that White Rock is intermediate in albedo and thermal inertia. There has been a suggestion that regions intermediate in both albedo and thermal inertia on Mars consist of duricrust. Duricrusts on Earth consist of cemented sulfates or carbonates:

SPECTROSCOPIC DETECTIBILITY OF SULFATES IN DUST AND DURICRUST ON
MARS.
"Introduction: Abundant sulfur was detected in the Martian surface material by the Viking Landers, with higher concentrations occurring in crusts and clods [1,2]. Cementation by sulfate salts has been proposed to explain the higher sulfate abundance in the more cohesive materials, with magnesium as the likely
cation [1,3]. In addition, there are a number of regions on Mars with a dark red color, intermediate albedo, and
intermediate thermal inertia [4,5,6] that appear to have a unique mineral composition [7] and are thought to
consist of regionally extensive cemented material analogous to the smaller clods found at the landing sites."
http://www.planetary.brown.edu/~cooper/pubs/5Mars1999.pdf

Laboratory Studies concerning the Infrared Signatures of possible
Duri-crusts on Mars
"After the Viking mission to Mars, [Clark, 1978] and others described the mechanical properties of soils at the two Viking landing sites. Parts of the soils appear to be cemented by what is believed to be a salt component like magnesium sulfate [Clark et al., 1976]. At least two distinct types of the so called "duri-crusts" can be observed [Clark, 1978]; a fragile crust which tends to disintegrate into centimeter size fragments during the digging process of the Viking sample collector arms and a "caliche" type of crust which was excavated through the exhaust gas from the landers propulsion nozzles. The fact that also Pathfinder and Sojourner found similar types of crusts at the Ares Vallis landing site [Smith et al., 1997] is reason to suspect that the formation of duri-crusts is, or was in the past, a global process on Mars."
http://irsps.sci.unich.it/education/sc1998/abstract/meyer.abs.html

Red, White, and Somewhat Blue at the Pathfinder Site
"At a press briefing on July 22, the team showed an enhanced color image of the region in front of the rock, Yogi. (A gray-scale version of this image is seen in Figure 1). The most exciting feature in the image is a white patch of subsurface material revealed by a deep scour in the soil that was caused by the rover wheels as it turned to examine Yogi. The white patch has a visible reflectance spectrum similar to the white, crusty material exposed by wind about 2.5 m away.
[figure]
"Fig. 1. Center stage of Sojourner's activities during the first 3 weeks on Mars. In this image, obtained about 1 1/2 weeks into the mission, Sojourner can be seen with its alpha proton X ray spectrometer up against the rock, Yogi. On its way to Yogi, the rover wheels exposed white material (box) that occurs beneath the smooth patch of soil in front of Yogi. This white material is probably the same as exposed by wind further to the right, labeled "white crust." Sojourner examined the dark soil next to rock, Lamb, on its 20th Martian day, then traveled 3 m to Soufflé. Week 3 ended with the APXS placed on the rock, Souffle'. Yogi is about 1 m high and located west of the lander.
"Team member Mike Malin talked about these crusts during the first week of the mission. He interpreted them as evidence for the past presence of water that left behind dried mud and/or evaporite material."
http://www.agu.org/sci_soc/pathfinder/eosweek3.html

Pathfinder's Finds
"Britt classifies Scooby Doo and its kin as a third kind of rock that he calls "caliche-like," after a cemented mineral common in the Tucson area.
When it rains in the desert, calcium-rich salts leach out of the upper soil and begin to percolate downward. But before they've penetrated more than a foot or so, the water evaporates in the
scorching heat, concentrating them into a layer—caliche—that has many of the properties of concrete."
p. 9,
http://pr.caltech.edu/periodicals/EandS/articles/Pathfinder% 20Feature.pdf

Bob Clark