Travel Reference
In-Depth Information
the chaotic piles of rock slung across the earth's surface, first grappled with
scientific approaches to understanding the age of any rock—and the age of
the earth. They realized that if the various rock units could he dated by their
relative ages, correlations among even widely separated rocks could be es-
tablished and, from this, some order recognized in the geological chaos that
is the crust of the earth. But how could rocks be dated?
The pioneering European geologists first believed that identifying a
rock's
type
would give them a strong clue to the age of the rock formation and
that one of the most powerful clues came from the hardness of a given rock.
Specific rock types were thus assumed to have formed at characteristically
different times, the softest rocks having formed the most recently. This crude
type of dating was first used to understand the way mountains were formed.
In the mid-1700s it was thought that there were three distinct types of
mountains in Europe, each formed by a different type of rock and each cre-
ated at a different time. According to this theory, the oldest were the Alps,
which had interior cores composed of very hard, crystalline rocks (such as
granite, schist, or basalt). These mountains were called
Primitive.
Sitting on
the flanks of the Primitive mountains were younger
Secondary
mountains
composed of layered sedimentary rocks such as limestone, often rich with
fossils and intermediate in hardness. The youngest
Tertiary
mountains were
composed of softer mudstones and sandstone also rich with fossils. These
formed low hills rather than true mountains. Rock type and hardness thus es-
tablished mountain type, and rock type also became a proxy for age. How
wonderful it would have been for scientists if indeed the earth were so sim-
ply organized! Yet how disastrous for my profession! There would have been
no work for the legions of bickering geologists who, it turns out, have had to
do the real work of discerning the age of rocks.
Study soon exposed the fallacy of these early notions. It was discovered
that some of the very high mountains were composed of the softest sedi-
ments and that very hard volcanic rock was sometimes found in very low
mountains. By the early 1800s, it was understood that rock type was of little
or no help in establishing either the form or the age of a mountain and that
a rock's composition, or mineral content, is virtually independent of age.
