How does stratigraphy help scientists determine the order of sedimentary rock layers?

In the field of stratigraphy, scientists determine the relative order of sedimentary rock layers primarily through the law of superposition. Formulated by Nicolaus Steno, this principle establishes that in an undisturbed sequence of sedimentary layers, the oldest material is deposited at the base, with each subsequent layer becoming progressively younger toward the top.123

Because geological processes such as tectonic deformation can tilt or overturn rock layers, geologists must often verify the original orientation of the strata. They do this by observing asymmetric depositional features, such as:

• Ripple marks: Patterns formed by water movement that indicate the original top and bottom.2
• Cross-beds: Truncated layers in sandstone that reveal the direction of deposition.2
• Mud cracks and rain prints: Features that indicate the uppermost surface of a layer.2
• Fossil orientation: Specific patterns of fossil accumulation that help define the top side of a bed.2

By identifying these features, scientists can accurately reconstruct the chronological sequence of the rock record, even when the layers have been disturbed by Earth processes.23