Reading the history of the earth’s crust is like reading a mystery
story. The geologist must examine, or read, each rock layer, from the
youngest at the top to the oldest at the bottom, searching for clues to
its origin, age, and development from which he can reconstruct the past.
One of the clues to the past is the fossil remains of plants and animals
found in the rocks. For instance, if a certain coral is known to have
lived only during one span of time, all rocks containing that coral
fossil must have been formed within that span.
Records of wells (well logs) and rock cores or samples collected when
test holes are drilled into the bedrock also reveal much about the
sequence of rock strata beneath the surface. The kinds of rocks
encountered tell, in addition, something about ancient geography, for
some were formed on land and others were deposited in long-departed
seas.
Such knowledge is extremely important in finding and developing mineral
resources such as coal and oil.