Uniformitarianism (James Hutton, 1726-1797). All natural processes that affect the Earth’s crust (erosion, deposition, volcanic eruptions, faulting, glaciation etc.) operate with the same intensity and under the same set of physical constraints now as in the geologic past. This concept was paraphrased by Sir Archibald Geike (1835-1924) when he coined the term "The present is the key to the past."

Using this concept, geologists study present igneous and sedimentary conditions to infer the condition of the Earth millions of years ago. For example, it is now known that the majority of organic and inorganic production of calcium carbonate (the essential component of limestone) takes place in the shallow, tropical marine environment. Knowing this, any location that has an abundance of limestone most likely had been a shallow, tropical marine environment when the sediments were produced.