Context: EVA on foot heading east, following a course charted by map and GPS units in advance, using our GPS units to follow the course in the field. I also used this EVA as a chance to take geophysical measurements at our selected waypoints and several other locations.

Geological Description: We passed through sandy washes surrounded by fine-grained sedimentary layers and occasional conglomerates, all part of the Morrison Formation.

Geophysical Note: I took magnetic susceptibility and radiation measurements at each waypoint. My plan is to continue these measurements on every possible EVA in order to add to the geological understanding.

Lessons Learned: We had another fascinating experience in the scientific method. When we were almost back to the Hab, we found some fine-grained white rocks that a previous geologist had uncovered from the loose and coarse-grained sandstone above them. What were these white rocks? Were they the same type as the "Martian mushrooms" we had discovered on Monday? Were they limestone? Or were they some type of volcanic ash?

Chief Geologist Kyoichi Sasazawa demonstrated how to begin, at least, to answer our questions.

First, looking through a hand lens, he compared the grains of the two rocks and found that the "Martian mushroom" rock had coarser grains, including quartz and some feldspar, while the grains of this new white rock were smaller and more tightly compacted.

Then, he licked the rock. (Yes, he licked the rock.) The rock did not seem to absorb the moisture from his tongue, so he concluded it was not volcanic ash.

Finally, he scraped off little bits of the rock and added a 37% HCl solution to them. The rock did not fizz, so he knew it was not limestone.

The conclusion: The rock is a very fine-grained siliceous sedimentary rock, much like the other rocks in the area.