Nonetheless, it was not until the mid-17th century and the arrival of the Dane Nicolas Steno in Late
Renaissance Florence that modern geological thinking can be said to have started. Steno’s observations,
during his brief dalliance with geology, were seemingly simple by modern standards: in a normal succession
of rocks, the oldest are at the bottom and the youngest at the top; sedimentary rocks are laid down
horizontally; if they are not horizontal, then they have been folded or faulted; while fossils are the preserved
remains of ancient creatures. Yet, these notions suggested that the rock record and its fossil content
represented a chronology, effectively a book of Earth history, waiting to be read.
It was not until over 100 years later that the book of Earth history began to be read in earnest. During the Age
of Enlightenment, the Scottish intellectual James Hutton argued that from a consideration of the processes
creating rocks and their subsequent deformation, the age of the Earth had to be, by human standards,
immense (“the abyss of time” as described by Hutton’s friend and fellow intellectual, John Playfair). How
immense was uncertain, but certainly much older than might be determined from a literal interpretation of the
Bible, or other religious texts.
Geology as a stand-alone subject was born in the late 18th century with the work of Hutton and others.
Hutton was not the “Father of Geology” as he is sometimes portrayed, but he was an important catalyst in
developing inductive thinking about the age of the Earth and geological processes. Others such as Abraham
Gottlob Werner, Georges Louis Leclerc, Comte de Buffon and Peter Pallas were contemplating similar issues
in Germany, France and Russia, respectively. Around the time of Hutton, geology as a term with its current
meaning was introduced, by the Geneva-based naturalist Jean-André Deluc in 1778, although it had been
used with a broader meaning (including the study of plants and animals) since the 15th century (as the Latin
word geologia).
Werner and Hutton were on opposite sides of the controversy that existed between “Neptunists” and
“Plutonists”, which occupied geological, and indeed popular, thinking in the late 18th century. Werner had
promoted the notion that all rocks, including granites and basalts, were either deposited or precipitated out of
water (“Neptunism”), whilst Hutton favoured the plutonic view that granites and basalts were the products
of heat within the Earth creating molten magma (“Plutonism”). His observation of cross-cutting intrusions
demonstrated this. By the beginning of the 19th century, Neptunism as an explanation for crystalline rocks,
such as granite, was effectively no longer in vogue. Instead, rock classifications concentrated on the concepts
we now know as igneous, metamorphic and sedimentary.
The next major step in the history of geology was to determine that the fossil content of sedimentary rocks
could be used as a key to understanding that a given rock unit could be associated with a specific period
of Earth history. This allowed correlation to other rocks deposited during the same period — the science of
stratigraphy was born. Recognition of distinct strata permitted the mapping of these layers as they occurred
at the Earth’s surface and, equally importantly, enabled a prediction to be made of what might lie below the
surface. William Smith in England and Georges Cuvier in France pioneered this thinking at the end of the 18th
century into the beginning of the 19th century.