The principle of uniformitarianism is stated simply as "the processes affecting Earth today are the same ones that affected it in the past" - as quoted from What Stories do Rocks Tell, at ClassZone. Besides describing the principle of uniformitarianism, the website goes on to outline the other principals of geology:
- the principle of superposition
- the principle of original horizontality
- the principle of cross-cutting relationships
If geologists could not use what they see around them now, by studying geologic processes and relationships occurring and in place today, it would be difficult if not impossible to determine what had happened in the past. Any ideas, concepts, or hypotheses that a geologist might form about the past - anywhere or anywhen on earth - would be useless without the principle of uniformitarianism, simply because one could not go to the geologic record - which is also a record of events and processes that include the chemistry of the past, the physics of the past, the biology of the past, the hydrology of the past, the geochemistry of the past, the geophysics of the past, and the bio-geology of the past - and do any meaningful interpretation at all.If, for example, sedimentary rock layers of the past were laid down sideways or vertical instead of in horizontal fashion as we see them being laid down today - say, because the gravity on earth worked very differently in the past - then any interpretation of what happened to older rock layers that are now folded, faulted, and otherwise deformed, would be incorrect. Another way of looking at this would be as described at MSN Encarta:
In other words, ripple marks today indicate that water has flowed or wind has blown1 (because we also see ripple marks created in areas of blowing sand), and therefore ripple marks in rock formations of the past also indicate the same thing. By measuring and comparing ripple marks formed today under different circumstances of water or wind formation, different speeds of water current or air movement, and different environments, a geologist can then apply the data collected today to the ripple marks of the past and hypothesize an environment in which those ancient ripple marks formed.
The principle of uniformitarianism depends on the 'uniformity of laws,' which assumes that the laws of physics and chemistry have remained constant. To test uniformity of laws, geologists can examine preserved one-billion-year-old ripples that look very much like ripples on the beach today. If gravity had changed, water and sand would have interacted differently in the past, and the ripple evidence would be different.
Geologists at first sometimes took the principle of uniformitarianism to such an extreme that catastrophic events of certain kinds were almost completely ruled out, perhaps because the principle was partly formulated as a tenet opposite to the then prevailing doctrine of catastrophism. The two major types of events somewhat ruled out by geologists of the past were huge, giant floods and huge, giant volcanic eruptions. The reason for these things being ruled out or overlooked while using the principle of uniformitariansim, is that these things - huge catastrophic floods, and huge catastrophic volcanic eruptions - are not seen happening on earth today. The discovery and recognition by J. Harlan Bretz of the very large series of floods that formed the Channeled Scablands of Washington state, eventually was accepted as something that 1) really happened and 2) could happen again if circumstances similar to those that caused the floods occurred again. It's interesting to note that it was in part the identification of huge ripple marks that clinched the acceptance of the way-larger-than-usual floods. In a way, then, the principle of uniformitarianism helped confirm Harlan Bretz's ideas - if you look at the size of ripples in rivers today and compare them to the size of the ripples formed by these past floods in the Channeled Scablands, you would have to conclude that an immense volume of water was required to form them.
Likewise, the recognition of the very large volcanic calderas of Tertiary age in central Nevada and of Quaternary age in Yellowstone Park, Wyoming made geologists realize that large, "catastrophic" volcanic eruptions are part of the earth's history and therefore could happen today/soon, or sometime in the near to far future. The relative size of the eruptions at Yellowstone (Yellowstone Park and environs actually contains several - at least three - calderas) compared to that of many volcanic eruptions that we, as people, think of as large or "catastrophic" is discussed at Yellowstone Caldera. It can be good to remember that events that are considered catastrophic are on the large end of a continuum of smaller to larger events, and that the large events are often considered catastrophic only because of the effect they could have on humans. We are the ones defining normal earth processes as catastrophic.
Anyone attempting to do geology without using the principle of uniformitarianism as described above, is really not doing geology at all.
1. I am reminded of the movie Little Big Man, in which Pawnee chief, Old Lodge Skins, says "...as long as grass grow, and wind blow, and the sky is blue." One of my favorite quotes, for some reason.