paperwork and bother.
Questions 15-21
The Grand Canyon, a long,
narrow gorge in Arizona, is
rich in geological history.
Its record of past plant
and animal life also makes
it an exciting and
invaluable object of study
for biologists. Using
samples from the two
hundred and seventeen
miles of the canyon’s
walls, scientists analyze
the building materials that
form the earth’ s surface
and study the natural
processes which have
affected the canyon over
Its four-billion-year
history. These processes
include the eruption of
volcanoes, which were
active as recently as one
thousand years ago, and
gradual decomposition by
erosion. Scientists also
study a great variety of
fossils that can be found
in the canyon. These
imprints of past forms of
life are preserved in rock.
The lower levels of canyon
rock contain fossils of
seashells and primitive
algae, while the upper
levels contain fossils of
such creatures as
dinosaurs and other
prehistoric land animals. All
of these discoveries
provide scientists with
information both on the
earth’s history in general
and on the area around
the Grand Canyon in
particular.