The Fossil Record 81
Dates with Rocks
We now have literally thousands of separate analyses using a wide variety of radio-
metric techniques. It is an interlocking, complex system of predictions and verified
results—not a few crackpot samples with wildly varying results, as creationists
would prefer to believe.
—Niles Eldredge, The Monkey Business
The principle that one rock layer is normally deposited on another older one (superposi-
tion) and the principle of faunal succession laid the framework for the relative geologic time
scale by the 1840s, but geologists still could not attach numerical ages to these rock units
or say whether they took thousands or millions or billions of years to deposit. It was clear
by looking at the immense thicknesses of rock and their complexity (such as in the Grand
Canyon example just discussed) that the rock record could not be explained by Noah’s flood.
But there was no reliable “clock” in the eighteenth or nineteenth century that could tell us
whether the earth was only 20 million years old (as physicist Lord Kelvin argued) or billions
of years old (as most geologists since Hutton had estimated).
Then, in 1895, the discovery of radioactivity by Henri Becquerel provided the first mech-
anism to produce reliable numerical ages (formerly called “absolute” dates) on rocks. By
1913, geologists such as Arthur Holmes had developed the radioactive decay method and
found rocks on earth that were at least 2 billion years old. Since then, numerous additional
dates have been established for older and older rocks. The oldest known rocks on earth cur-
rently date to 4.28 billion years ago, and there are individual mineral grains that give dates
of 4.3 to 4.4 billion years old (Dalrymple 2004). Rocks from the moon and meteorites are
FIGURE 3.10. Flood geology just doesn’t hold water. (Cartoon courtesy Los Angeles Times Syndicate)