Earth's Creation and the Concept of Deep Time
As scientists searched for an accurate age of the Earth others debated the processes that created the features we see. In 1787 Abraham Werner, a world renown mineralogist, proposed the idea of Neptunism. Neptunism was based on the assumption that the Earth originally consisted of water which contained dissolved minerals that would eventually precipitate from solution in an orderly fashion to form solid rock. Werner proposed that the oldest and hardest material, what he coined as primitive rock, precipitated first from these waters followed by transition rocks, some of which contained fossils, then secondary rocks--such as sandstone--and then finally unconsolidated alluvial rock.
Though Werner's Neptunism was originally an accepted explanation for Earth's creation, as more studies were conducted scientist began to find areas where primitive rock was located on top of younger secondary or transition rock. Finally, when igneous rock was shown to be produced from the cooling of magma and not through precipitation from water Werner's Neptunism was discredited.
While Werner developed his theory of Neptunism others, such as George Cuvier, the french paleontologist, proposed the theory of Catastrophism. This theory was based on Cuvier's observation of the fossil record and the idea that major extinctions were the result of catastrophes. Cuvier's ideas would eventually be built upon by others looking for geologic proof of biblical scripture and the major catastrophes linked to biblical catastrophes such as the great flood.
Both Werner's Neptunism and Cuvier's Catastrophism approached Earth processes and, therefore time, linearly. In other words, the world and all its features were created and time moved forward from that point. This understanding of time allowed for a geologically young Earth, however, our understanding was about to change when James Hutton observed the angular unconformity at Jedburg, Scotland.
An unconformity is a boundary within rock layers that is formed by erosion. As stated in a previous section sedimentary rock is originally deposited horizontally. A consequence of this principle is that any tilted sedimentary rock must have been uplifted by some geologic process (what we now know to be caused by plate tectonics). Before any sediment could be deposited on uplifted rock those areas would have to then be eroded to form a flat surface that would allow for deposition. Once eroded, layers beneath the erosional surface would still appear to be tilted yet any new deposition would be horizontal-- the boundary between the tilted (or angled) layers and the horizontal layers is known as an angular unconformity.
When Hutton observed the angular unconformity at Jedburg he was immediately struck by the consequences of his observation. In order to form the near vertical layers at the base of the unconformity sediment first had to be deposited horizontally for millions and millions of years. Once these sediments were deposited the entire area would have to be uplifted to form mountains--a processes that also had to take millions of years. Once the mountains were uplifted the entire mountain range was eroded flat and new deposition began to occur. The amount of time contained within this single geologic feature was unimaginable.
Not only did Hutton realize that his discovery made Earth much older than anyone could ever imagine, his discovery would show that Earth processes were cyclical not linear like most believed. For the first time Hutton saw the world features not as a constant but as part of an endless cycle where earth is uplifted to form mountains, mountains are then eroded and the material re-deposited in ocean basins where they are eventually uplifted again and the processes continues. Hutton presented his findings in 1788 and the ideas presented would eventually become the theory of Uniformitarianism--the basis of modern geology.
At the core of uniformitarianism is the knowledge that the physical processes operating on Earth have always operated in the same manner, therefore by studying these processes on the Earth today we can better understand the past. It is from this idea that Uniformitarianism is often summarized by the phrase, "the present is the key to the past."