In order to understand better the geologic history and development of
the canyon, one should also have some knowledge of the basic principles
of earth history and should be familiar with the geologic time scale
(fig. 6).
[Illustration: Fig. 6. Geologic time scale. Reproduced from
_FOSSILS: An Introduction to Prehistoric Life_, William H. Matthews
III, Barnes and Noble, Inc., 1962.]
GEOLOGIC TIME SCALE
ERA
PERIOD
EPOCH
SUCCESSION OF LIFE
CENOZOIC “RECENT LIFE”
QUATERNARY 0-1 MILLION YEARS
Recent
Pleistocene
TERTIARY 62 MILLION YEARS
Pliocene
Miocene
Oligocene
Eocene
Paleocene
MESOZOIC “MIDDLE LIFE”
CRETACEOUS 72 MILLION YEARS
JURASSIC 46 MILLION YEARS
TRIASSIC 49 MILLION YEARS
PALEOZOIC “ANCIENT LIFE”
PERMIAN 50 MILLION YEARS
CARBONIFEROUS
PENNSYLVANIAN 30 MILLION YEARS
MISSISSIPPIAN 35 MILLION YEARS
DEVONIAN 60 MILLION YEARS
SILURIAN 20 MILLION YEARS
ORDOVICIAN 75 MILLION YEARS
CAMBRIAN 100 MILLION YEARS
PRECAMBRIAN ERAS
PROTEROZOIC ERA
ARCHEOZOIC ERA
APPROXIMATE AGE OF THE EARTH MORE THAN 4 BILLION 550 MILLION YEARS
The geologist has learned that the earth’s physical features have not
always been as they are today. It is known, for example, that mountains
now occupy the sites of ancient seas. Coal is now being mined where
swamps existed many millions of years ago. Furthermore, the earth’s
plants and animals have also been subject to great change. The trend of
this organic change is, in general, toward more complex and advanced
forms of life. However, some forms have remained virtually unaltered
while others have become extinct at different points in geologic time.
In order to interpret earth history, the earth scientist gathers
evidence of the great changes in climate, geography, and life that took
place in the geologic past. He does this by studying the rock
formations, the structural relationships of these formations, and the
landforms of the area. The record of ancient events is pieced together
by studying the stony layers of the earth as one might study a giant
history book. Indeed, the sedimentary rocks are the rocky “pages” of
earth history, for in them we find the tracks and trails, and bones and
stones, which reveal the intriguing story of life long ago.
Much of the basic information which the geologist uses to reconstruct
the geologic history of a region comes from his examination and
interpretation of _bedrock outcrops_. _Bedrock_ is the solid unweathered
rock which underlies loose earth material such as soil, sand, and
gravel. An _outcrop_, or _exposure_, is a place where bedrock is exposed
at the surface.
The first chapter of earth history begins with the most ancient rocks
known. Because they were formed early in geologic time, these rocks are
normally found deeply buried beneath younger rocks which have been
deposited on top of them. It is for this reason that earth history is
read from the bottom up, for the earliest formed rock layers correspond
to the opening chapter in our earthen history book. The later chapters
are found in the upper younger rocks which are located nearer the
surface. Thus, in “reading” the geologic history of Palo Duro Canyon we
start with the oldest “chapter” which is recorded in the Quartermaster
Formation (p. 17) of Permian age, for these are the oldest rocks exposed
in the canyon.
But deciphering earth history is not as simple as it might appear. In
many areas the rock layers are not always found in the sequence in which
they were originally deposited. In places, great structural disturbances
have caused some of the rocky “pages” to become shuffled and out of
place; others may be missing completely. Many rocks have been destroyed
by weathering and erosion or greatly altered by metamorphism. As a
result, the story recorded in these particular rocks is lost forever.
These missing “pages” make the ancient story even more difficult to
interpret so the geologist must then depend on other evidence that will
permit him to “fill in the blanks.”
The record revealed in the rocks indicates that our planet is at least
4½ billion years old and that life has been present for more than 3
billion years. During this vast span of time the earth and its
inhabitants have undergone many changes.