Smith, William (1769-1839), an English geologist and engineer, was the first scientist to use fossils to identify and also determine the relative age of rock strata (layers). Smith discovered in the late 1700’s that each layer of rock contained its own characteristic fossils. He noticed that the fossils in the various layers were arranged in chronological sequence. Thus, Smith could tell the age of strata in relation to one another. He also noted that fossils in any layer change little from place to place. Therefore, Smith could trace and compare the strata of widely separated areas.
Smith used fossils to draw a geological map of England that showed strata of different ages underlying the entire country. This map was one of the first attempts to show the relation between rock formations in any large area. Today, geologists use fossils as a primary means of identifying rock strata.
