One hundred years ago, scientists would have denied any violent shiftings of the earth's axis and its relation to climate. But modern geologists have changed their mind. The theory that variations in the Earth's orbit are responsible for triggering the advance and retreat of glaciers was developed by Yugoslavian scientist Milutin Milankovitch. Modern geology recognizes many geological cycles. There are episodic and periodic cycles.
Episodic cycles recur irregularly and periodic cycles recur at fixed intervals.
A volcanic eruption is episodic, but glacial periods, sea level changes and ice ages are claimed to be periodic. There are cyclical variations in the deposits of sediments that vary from centuries to a few million years. The main geological cycle that is considered by modern geologists to affect the surface of the earth is erosion, deposition of sediments, and folding and uplifting processes that form mountains. Another example of the gradual merging of ancient and modern views on geology relates to the age of the earth. One hundred years ago the oldest estimate for the age of the earth was about 500,000,000 years.