This follows due to the fact that sedimentary rock is produced from the gradual accumulation of sediment on the surface.Therefore newer sediment is continually deposited on top of previously deposited or older sediment.As he continued his job as a surveyor, he found the same patterns across England.
Photo from Glen Canyon National Recreation Area, Utah.
Relative dating is the science of determining the relative order of past events (i.e., the age of an object in comparison to another), without necessarily determining their absolute age, (i.e. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another.
The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith.
While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers.
For purposes of relative dating this principle is used to identify faults and erosional features within the rock record.