This led to the theory of Uniformitarianism, which states that all ancient rocks and geological features can be explained by observing the operation of modern-day processes.
Uniformitarianism is usually referred to by the more explanatory phrase "the present is the key to the past".
In simple terms, a geological map shows the surface distribution of rocks in a particular area.
However, in order to fully understand a geological map, it is necessary to be familiar with several basic geological principles, including the laws of stratigraphy, geological age, and geological structures.
To the experienced eye, a geological map reflects the three-dimensional distribution of rocks, and also serves as a visual guide to the geological history of the area.
Sediments (mineral grains and fragments of rock) are produced by weathering at the Earth's surface.
They are removed by erosion and deposited elsewhere as layers, which thicken over time and, as the weight and pressure increases, they are eventually compressed and lithified to form sedimentary rock.
The father of modern geology was James Hutton, an eighteenth century scientist.
After observing modern rates of sediment accumulation, Hutton concluded that long periods of time were required to build up the thick layers of sedimentary rock strata seen today.
Hutton also concluded that geological processes similar to those today must have operated in the past.
Relative age refers to whether a rock layer in a sequence is older (or younger) than other layers in the sequence. According to the Law of Superposition, the sediments of sandstone, siltstone, mudstone, limestone, volcanic ash and conglomerate were deposited in a sequence.
There is, however, no implication of the true age (i.e. Sandstone is the oldest rock, and conglomerate is the youngest.
Two other principles enable the relative age of rocks and geological events to be inferred, namely: In Figure 2, because fragments of mudstone are found within the granite body (The Principle of Included Fragments), the granite intrusion must be younger than (post-date) the formation of limestone and mudstone.