Opportunity is exploring in an area called Meridiani Planum on the other side of planet.
CNN: Mars rovers working overtime
Opportunity has seen similar small structures out on Mars' Meridiani plains, which the rover team nicknamed "blueberries".
BBC: Opportunity and Curiosity find similar rocks on Mars
In April, mission scientists announced Opportunity had found conclusive evidence that a salty sea once lapped the shores of Meridiani.
But some rock stacks seen from orbit at other locations in Mars' Meridiani Planum region are up to 200m thick (650ft).
BBC: NEWS | Science/Nature | Rover finds new Mars water signs
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