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.
South America has the largest grassland of Earth.<span><span><span /></span></span>
