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How do mud cracks and raindrop imprints reveal which way was originally up in a rock layer?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

Mud cracks and raindrop imprints serve as stratigraphic "top and bottom" indicators, which are essential for determining the original orientation of sedimentary rock layers that may have been tilted or overturned by geological deformation.12

These features are classified as bedding-plane structures that form specifically on the top surface of a sedimentary bed. Mud cracks develop through the desiccation of wet sediment, creating a system of cracks that widen toward the surface. Similarly, raindrop imprints are impressions made on the exposed upper surface of soft sediment.314

Because these structures originate on the surface of a deposit, they provide a clear physical record of which side of the rock layer was facing upward at the time of formation. By identifying these markings, geologists can distinguish the original top of a bed from its base, even in regions where tectonic activity has significantly altered the strata's original horizontal position.12