In the last few days we have been finding these rounded cm-sized nodules on eroded faces of the underlying Stimson sandstone. On Earth they can form when Fe-rich oxidising groundwaters flow through buried sediments.
Although we haven’t seen these features before with MSL, we have had a new paper about the effects of groundwater in the Sheepbed mudstone accepted for publication. In that paper we argue that groundwater flow (similar to that which might be associated with the nodules) led to dissolution of buried sulphate-rich evaporite salt layers and the creation of the pure sulphate veins that we keep seeing on our traverse.
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