William smith was working as a surveyor in the late eighteenth century. he noticed that certain fossils were found in certain layers of sedimentary or igneous rock, that the same order of strata could be found in areas that were far apart, and these strata contained similar
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William Smith was a British engineer and geologist born in 1769. He is known for having developed the first geological map, based on his observations of the rock layers (strata).

It should be noted that during his studies of these strata, Smith realized that each stratum had certain types of fossil groups, different from the others, as it advanced vertically.

In addition, he observed that in distant places the strata showed the same characteristics and the fossils were always the same at the same level, despite the samples being kilometers far apart from each other.

From there, Smith determined a way to recognize a specific stratum (regardless of its horizontal location) by the fossil remains it contained.