Geologic Time and Relative Dating
In Ancient Greece, Aristotle saw that fossil seashells from rocks were similar to those found on the beach and inferred that the fossils were once part of living animals. He reasoned that the positions of land and sea had changed over long periods. Leonardo da Vinci concurred with Aristotle’s view that fossils were the remains of ancient life.
The 11th-century Persian geologist Avicenna (Ibn Sina) and the 13th-century Dominican bishop Albertus Magnus (Albert of Saxony) extended Aristotle’s explanation into a theory of a petrifying fluid. Avicenna also first proposed one of the principles underlying geologic time scales, the law of superposition of strata, while discussing the origins of mountains in The Book of Healing in 1027. The Chinese naturalist Shen Kuo (1031–1095) also recognized the concept of ‘deep time.’
Nicholas Steno later laid down the principles underlying geologic time scales in the late 17th century. Steno argued that rock layers (or strata) are laid down in succession, representing a “slice” of time. He also formulated the law of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno’s principles were simple, applying them to real rocks proved complex. Throughout the 18th century, geologists realized that:
- Sequences of strata are often eroded, distorted, tilted, or even inverted after deposition;
- Strata laid down at the same time in different areas could have entirely different appearances;
- The strata of any given area represented only part of the Earth’s long history.
The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth were made in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of the Earth’s crust into four types: Primary, Secondary, Tertiary, and Quaternary. According to the theory, each type of rock formed during a specific period in Earth’s history. It was thus possible to speak of a “Tertiary Period” and of “Tertiary Rocks.” Today “Tertiary” (now Paleocene – Pliocene) and “Quaternary” (now Pleistocene and Holocene) remained in use as names of geological periods well into the 20th century.
