Mars Index - Top Ten MER Images - Top Raw Images
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In this microscopic image, measuring 3 centimeters (1.2 inches) square, five thin, horizontal layers of light-colored rock covered with fine-grained sand extend from left to right, interspersed by narrow gaps.
Chocolate Layer Cake
Too bad this image is full of sand and "dirt" instead of cocoa powder -- it almost looks edible.

Image Credit: NASA/JPL/Cornell