This nice sample from the tonto national monument, arizona, is hard stone now, but it once was soft sediment, and it records formation and "fossilization" of mud cracks. the light was shining along the arrow, as shown, making shadows, some of which are indicated by the arrows. is this sample upside-down (you are looking at the side of this sample that was down when the sediment was soft) or right-side up (you are looking at the side of this sample that was up when the sediment was soft)?

The answer is upside-down. Mud cracks spread downward keen on soft sediment. When more sediment is eroded in, this second layer will seal the cracks underneath. Later, after the layers have toughened, the rock may be rotated upside-down and then the layers cracked. If you see edges in a mud-crack pattern, you are seeing at the side of the second layer that initially was down. You can tell that this shows edges, and not holes, through the shadow, the edges have a well-lit on one side and a shadow on the other, while holes have light and shadow on the similar side.

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