The Four Geologic Eras. Geology is a complex and difficult subject, especially for the novice. I recommend that beginners first concentrate on the four major divisions of geologic history: the Geologic Eras.he Precambrian Era (4.6 billion years ago to 600 million years ago) encompasses the great majority of Earth's geologic history, stretching from the formation of our planet to the appearance of shelled marine life.