How do mud cracks and rain prints help identify the top of an ancient rock layer?

In geochronology, identifying the original orientation of rock layers is essential for determining their relative ages, especially when geological forces have tilted or overturned the strata. Mud cracks and rain prints serve as valuable "top and bottom" indicators because they are asymmetrical features formed at the surface of sedimentary rocks.1

Specifically, the direction in which these features open indicates the original uppermost surface of the mudstone or fine-grained sediment. Because rain prints are small, shallow depressions created by the impact of raindrops on soft surfaces like mud or clay, and mud cracks form through desiccation at the surface, their physical orientation remains fixed at the time of deposition. By observing the direction toward which these features open or taper, geologists can reliably distinguish the original top of the bed from the bottom, even in complex or deformed geological settings.123