Mankind's speculations about the nature of the Earth inspired much of the lore
and legend of early civilizations, but at times there were flashes of insight.
The ancient historian Herodotus, in the 5th century B.C., made one of the earliest
recorded geological observations. After finding fossil shells far inland in what
are
now parts of Egypt and Libya, he correctly inferred that the Mediterranean Sea
had
once extended much farther to the south. Few believed him, however, nor did the
idea
catch on. In the 3rd century B.C., Eratosthenes depicted a spherical Earth and
even
calculated its diameter and circumference, but the concept of a spherical Earth
was
beyond the imagination of most men. Only 500 years ago, sailors aboard the
Santa
Maria begged Columbus to turn back lest they sail off the Earth's "edge."
Similar
opinions and prejudices about the nature and age of the Earth have waxed and
waned
through the centuries. Most people, however, appear to have traditionally
believed
the Earth to be quite young--that its age might be measured in terms of thousands of
years, but certainly not in millions.
The evidence for an ancient Earth is concealed in the rocks that form the Earth's crust and surface. The rocks are not all the same age -- or even nearly so -- but, like the pages in a long and complicated history, they record the Earthshaping events and life of the past. The record, however, is incomplete. Many pages, especially in the early parts, are missing and many others are tattered, torn, and difficult to decipher. But enough of the pages are preserved to reward the reader with accounts of astounding episodes which certify that the Earth is billions of years old.
Two scales are used to date these episodes and to measure the age of the Earth: a relative time scale, based on the sequence of layering of the rocks and the evolution of life, and the radiometric time scale, based on the natural radioactivity of chemical elements in some of the rocks. An explanation of the relative scale highlights events in the growth of geologic science itself; the radiometric scale is a more recent development borrowed from the physical sciences and applied to geologic problems.