Geology started as a branch of creationism, and much of it is still comfortable ground for latter-day creationists. But some of geology is not.
Hutton the Creationist
James Hutton set modern geology on its feet in 1788 with an essay he read to the Royal Society of Edinburgh titled "Theory of the Earth; or an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land upon the Globe." He began with this statement:
"When we trace the parts of which this terrestrial system is composed, and when we view the general connection of those several parts, the whole presents a machine of a peculiar construction by which it is adapted to a certain end. We perceive a fabric, erected in wisdom, to obtain a purpose worthy of the power that is apparent in the production of it."
This was not just pious throat-clearing, but a bold yet thoroughly Christian statement. All serious thinkers in Europe accepted the Christian conceptual framework, in which God made the world in order to bring forth Man and enact the moral drama of sin and redemption laid out in the Bible. Centuries of argument centered around history and logic and philosophy had cemented that creationist framework. Hutton proposed a new line of investigation involving the "terrestrial system," the Earth rather than the world, which he treated as a machine for making soil.
The parts of his machine were the solid "central body" of rock, the water of the ocean, the continents and the atmosphere. It was obvious that, somehow, these parts interacted with gravity, heat, electricity, magnetism and so on to yield a habitable planet. "We are thus bountifully provided with the necessaries of life; we are supplied with things conducive to the growth and preservation of our animal nature, and with fit subjects to employ and to nourish our intellectual powers."
Moreover, the machine must be self-repairing, with "a reproductive operation, by which a ruined constitution may be again repaired, and a duration or stability thus procured to the machine, considered as a world sustaining plants and animals." Nowhere did Hutton shy away from the creationist's axiom that Earth is a purpose-driven planet.
The Modern (Old Earth) Creationist
The pursuit of Hutton's program yielded generations of progress in Earth science. Its purest embodiment today is the rock cycle, that familiar diagram that puts igneous rocks, sedimentary rocks and metamorphic rocks in a circle with arrows joining them. The different rock types are made by the different parts of the Earth machine, using the agents represented by the arrowserosion, mountain-building, lithification, metamorphism, magmagenesisas their mechanisms.
Creationists lag behind the mainstream of today's geology, but they still find Hutton's Earth congenial. Who would argue with this preamble to the curriculum posted at answersincreation.com by Greg Neyman: "The study of the earth and its rocks is also a study of God's creation. It is God who set the laws of nature which shape this planet's rocks into what they are today." The father of modern geology would not. But Hutton would have denounced the peculiar school of "young Earth" creationism that insists on a biblical chronology limited to approximately 6000 years.
Hutton professed, like most creationists today, that the human species is absolutely privileged over all other animals. Thus, even as the Bible was still the authority on the descent of man, fossils were legitimate indicators of a truly ancient Earth:
"The Mosaic history places [the] beginning of man at no great distance; and there has not been found, in natural history, any document by which a high antiquity might be attributed to the human race. But this is not the case with regard to the inferior species of animals, particularly those which inhabit the ocean and its shores. We find in natural history monuments which prove that those animals had long existed; and we thus procure a measure for the computation of a period of time extremely remote, though far from being precisely ascertained."
The Earth machine Hutton proposed was truly self-sustaining. It could go on forever, and there was no evidence to contradict that supposition. As he famously concluded, "we find no vestige of a beginning,no prospect of an end." The modern creationist curriculum sticks close to that machine, with sections on rocks and minerals, geologic processes, stratigraphic principles and even plate tectonics. What is missing is the real story of Earth's beginning, progress and prospects.
The Crack in Hutton's World
Scientists held Hutton's nuanced view of the Bible for a few more generations. But as research continued, human fossils joined the collections, the biblical version of Noah's Flood was refuted, and Charles Darwin devised a fruitful theory that explained the tree of life as the product of a kind of machine for creating biological diversity.
Meanwhile Hutton's tidy machine, running more or less steadily to keep Earth habitable, was eroded by growing evidence. There have been many times in geologic history when Earth was not habitable, when events outside Hutton's list of agents went on rampages and life was nearly snuffed out. There are causes and effects, in evidence today, that threaten Earth's habitability for human life.
Surely God would have taken a more straightforward route to today, and surely he would not make Earth so precarious. But that thought leads to speculation about the mind of God, not the science of Earth. Once it began to undermine Christian dogmas, natural history, rather than Hutton's natural mechanics, was the undoing of old-fashioned creationism.