Geologic Time: The science of Geology didn't take shape until the discovery of geologic time (deep time). Fossils are found in rocks. What was a shark tooth doing inside a rock?

This led Steno to study how the rocks (which we today recognize as sedimentary) formed. He recognized them to be composed of lithified remains of sediment deposited in layers (or strata) and proposed a set of several principles of stratigraphy by which one could distinguish younger and older sediments. His results were published in De solido intra solidum naturaliter contento dissertationis prodromus (Preliminary discourse to a dissertation on a solid body naturally contained within a solid, mercifully referred to simply as Prodromus) in 1668.

Using these, it began to be possible to say what order the separate rock layers had formed in, provided they could be seen in association. More importantly, Steno was the first to grasp that:

Of course, Steno thought that all of this deposition had occurred during the 40 days and nights of Noah's flood. More

Rock types:

Note that by the 18th century, natural historians recognized that different rocks formed as a result of different processes. Today, we recognize three basic rock types. Although the early geologists knew them by different names, today they are called:



Prologue: Remember, Steno, a traditionalist, still thought that all of this deposition took place during the flood. In the next stage of the discovery of geologic time, natural historians took up the question of whether the Earth has a long or short history.