As he continued his job as a surveyor, he found the same patterns across England.
He also found that certain animals were in only certain layers and that they were in the same layers all across England.
In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock.
The principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time.
A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." The principle of intrusive relationships concerns crosscutting intrusions.
The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it.
This is because it is not possible for a younger layer to slip beneath a layer previously deposited.
Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials.
Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique.
Due to that discovery, Smith was able to recognize the order that the rocks were formed.
Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras.
These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix.