Geologic Time

The Cosmic Era.

 At one time the earth was a cloud of dust and
gases. Later, the materials became a molten
mass. The substances then separated according to
their densities, and the earth eventually cooled to
form a sphere with a solid surface.
 The period of millions or billions of years during
which the earth formed is called the Cosmic Era.
 The Azoic Era
 Scientists sometimes call this time in the earth’s history the
Azoic era, meaning “ without life”.
 At first the earth was probably too small to hold an
atmosphere. It would have been able to retain the gases that
were given off from its crust. These probably included
water vapor, carbon monoxide, nitrogen, and other gases.
During these period of change, the atmosphere in the crust
were also filled with water to form a primitive seas and
oceans. The higher elevations then became the continents
 The geologic timetable
 A special calendar.
 The longest division of time in the geologic timetable is
called an Era. Eras are subdivided into periods. These are
further broken down into smaller time units called Epochs,
which are shorter and less distinct time units than periods.
The end of epoch is usually marked by crustal disturbances.
These caused changes in the erosion cycle and withdrawal of
the shallow seas from the continents.
 The rocks of one era are separated from those of previous
era by evidence of gigantic geologic revolutions.
 Each era is usually characterized by different
types of plant life, flora, and animal life,
fauna, from those in the previous era.
 The rocks laid down during an entire era are
called a rock sequence. Just as the eras are
divided into periods, epochs, ages, and
stages, so are the corresponding rock
sequences of each era broken down into
systems, series, group and formation.
Time Rocks Examples

Era Sequence Paleozoic

Period System Devonian

Epoch Series Middle Devonian

Age Group Erian

Stage Formation Hamilton

 Divisions of
Geologic Time
Pre-Cambrian time.
 A large part of the geologic history of
the earth occurred during Pre-Cambrian
time. The fossil evidence in Pre-
Cambrian rocks is scanty, and for the
most part unclearly.
 1. The Archeozoic era.
 Geologists estimate that this era began more than two
billion years ago. It was characterized by widespread
volcanic activity, the formation of mountains ranges, and
the deposition of metallic ores. During this era the seas
repeatedly advanced over retreated from existing land
areas.
 Some geologists consider the term Archeozoic, meaning
“very ancient time”,. They would prefer to use an
alternate name, Archean. This means “very ancient”.
 The Laurentian-
Algoman revolution
ended the
Archeozoic era. At
this time great
diastrophic changes
occurred.
 2. The Proterozoic era.
 The general basic shapes of continents probably first
developed during this era. Also during this era, extensive
masses of igneous rocks were formed. In the Keweenawan
period rich deposits of copper and silver were formed. During
the Huronian period of this era, immense deposits of iron ore
were laid down in sedimentary layers in now the Great lakes
region. During the part of the Proterozoic era, the climate was
cold enough to maintain large glaciers.
 Some scientists prefer to call this era the Algonkian, since the
term Proterozoic means “time of earlier life”.
 The Proterozoic era closed with great crustal
upheavals and volcanic disturbances.
Sedimentary layers in what is now the great
lakes region were formed and
metamorphosed. These formed the Killarney
Mountains. This revolutions is called the
Killarney- Grand Canyon revolution.
 Paleozoic era.
 This long era lasted approximately 350 million
years.
Geologists divide it into many well-defined periods.
Each of these seems to be characterized by the
appearance of some new forms of plants and animal
life, ranging from tiny invertebrates to giant tree
ferns.
Geologists divide this era into distinct periods:
 1. The Cambrian period
 Large parts of the present North American continent
were submerged under a shallow seas. Much of the
land was low and flat. The climate was probably warm
and apparently there were no land plants and animals.
Sedimentary rocks of this period contain large
numbers of invertebrates.
 The most common animals in this period were a now-
extinct group of invertebrates known as trilobites.
 The Cambrian period ended
with the Green Mountain
disturbance. At this time
some of the folds and faults in
the present Vermont
mountains were formed.
2.The Ordovician period.
 About 60% of the North American continent
was apparently under water during the first part
of the Ordovician period. Geologists think that
these inland were shallow. the continental areas
were probably most lowlands. Extensive
limestone deposits were formed in this period.
In this period the earliest known vertebrates
made their appearance in the primitive fishes.
 The Ordovician period closed
with the Taconic disturbances.
This revealed in the folded rocks
of the Taconic region along the
Massachusetts New York
boundary, extending into
Southern Vermont.
 3. The Silurian period.
 Following the Taconic disturbance, there was a
wide-spread flooding of the central part of the North
American continent. Thick beds of salt and gypsum
were formed in the New York, Ohio, Michigan,
Pennsylvania, and West Virginia. The Silurian rocks
contain fossils of the earliest known land animals.
These were air-breathing scorpions and millipedes.
The first land plants appeared at this time.
 Other more abundant types of
fossils in these rocks are
corals, brachiopods,
nautiloids, and trilobites.
Coral appeared in this period.
However, they were much
numerous in Silurian times
and formed wide-spread coral
reefs.
4. Devonian period.
 In Devonshire, England, for which
this period is named, geologists long
ago made detailed studies of the
fossil-bearing rocks. The earliest
known large land plants made their
first appearance during the Devonian
period. Among them were huge
fernlike trees, some of which grew to
heights of 40 feet. The first simple
types of amphibian s also appeared.
 The Devonian period ended
with the Acadian
disturbance, which elevated
mountains in what is now
Northern Maine and Eastern
Canada.
 5. The Mississippian and the
Pennsylvanian period.

 These two periods together are called the

Carboniferous period. Carboniferous means,
“carbon-bearing”, and refers to the vast coal deposits
laid down during these period. During the later part
of this period the earliest known reptiles appeared.
6. The Permian period.
 The low swampy lands of the coal ages began to
rise in the Permian period. Many of the swamp
plants were no longer able to survive in this drier
climate. Two important lines of reptiles developed
during the Permian Period: (1) the root reptiles,
which were probably the ancestors of our modern
reptiles; and (2) the mammal-like reptiles whose
teeth and skulls resemble those the first mammals
probably developed.
 The Permian was the last period of the Paleozoic
era. Geologist believe that it ended with the great
Appalachian revolution. During this revolution
the ancestral Appalachian Mountains were
formed. Folds and Faults of these mountains are
still visible today. The diastrophic changes during
this revolution drained the shallow inland seas of
eastern North America. Animals such as the
trilobite became extinct after a long period of
development.
The Mesozoic era.
 This era occupies the middle zone of time in which
life has existed on earth. The name means “middle
life”. It indicates an era of transition between time
of “ancient life”(Paleozoic) and that of “recent life
“ Cenozoic). The environmental changes that took
place during the Appalachian revolution wiped out
many groups of animals and plants. But those that
survived became the ancestors of many thousands
of new forms.
 1. The Triassic period.
 During Triassic times the North America continent was very
much as it is now. The land was generally high and dry.
Desert conditions probably prevailed in some areas. This
indicated by the red color of the sedimentary formations and
the deposits of salt and gypsum.
 Many animals and plants of the early Triassic closely
resembled those of the late Paleozoic, but some new forms
appeared. In the Triassic forest grew giant cycads and
conifers resembling some of our modern evergreen. These
developed further in the Triassic, producing both dinosaurs
and the ancestors of most modern reptiles.
 The first birds, and the primitive
mammals also appeared in this
period.
 The Palisades disturbance marked
the end of the Triassic period. During
this disturbance the sills and lave
flows exposed today in the Palisades
along the Hudson were formed.
 2. The Jurassic period.
 The Jurassic is perhaps best known for its unusual
reptiles. This was the period when the dinosaurs were
dominant. Some of them were, as their name signifies
“terrible lizards”. Many were enormous creatures whose
movements were probably slow and awkward. Others
were only a foot or two long and could presumably
move about quite rapidly.
 One of the most familiar forms was Brontosaurs, whose
body length was about 65 feet. But its brain must have
weighed less than a pound.
 Another was Diplodocus, about 80 feet long, whose brain
was also extremely small for the size of its body.
 Stegosaurus, with a row of triangular body plates along
its back. These were apparently for protein against its
many enemies.
 Another large dinosaur was Allosaurus. Its daggerlike
teeth indicate that it was a flesh eating reptile or carnisaur.
 The first flying vertebrates were the pterosaurs, meaning
“winged lizards”. Their huge wings, without feathers,
were as much as 26 feet long. Although pterosaurs were
not bird as we know this group today.
 The best known type, called Archaeoperyx, was
about the size of a pigeon. Its skeleton and teeth
resembled those of reptiles. But its wings and body
were covered with feathers, so that it is generally
classified as a bird.
 In the ocean swam fishlike reptiles called
ichthyosaurs, similar in size and appearance to
modern porpoises. Another group of swimming
reptiles, called plesiosaurs, were long, slender
animals with necks which resembles snakes.
 The Jurassic period ended in the Nevadan
disturbance. At this time the Sierra
Nevada and Coast Ranges on the West
Coast were formed. Associated igneous
activity formed gold ores whose erosion
led to the placer deposits in the streams of
California. These stimulated the famous
gold rush of 1849.
Brontosaurus
Diplodocus Stegosaurus

Pterosaurs Archaeoperyx Ichthyosaurs

3. The Cretaceous period.
 The Cretaceous is noted for the thick chalk deposits
formed during this period, especially those in
England and France. The name of the period is
taken from the Latin word creta, meaning “chalk”.
During this period the present Rocky Mountain area
became flooded with a shallow sea extending from
the Gulf of Mexico to Arctic Ocean.
 The mightiest dinosaur of all, Tyrannosaurus,
flourished during this period. This lizard had a skull
slightly over four feet long and a body standing 20
feet high. A carnisaur, it must have been able to kill
and eat any other animal living at that time.
 The Laramide or Rocky Mountain
revolution brought the Mesozoic era
to a close. It created the ancestral
Rocky Mountains. It also re-
elevated the ancient Appalachians,
which by this time had been worn
down to a peneplane.
 The Cenozoic era.
 The Cenozoic (recent life) era is the latest of the
great eras of the earth history. It is also called the
Age of Mammals. Some geologists consider the brief
period of human history including the present, as a
part of the Cenozoic. Other prefer to think of this
period of human existence as the first epoch of a
separate new era they call the Psychozoic, this name
is used because of the superior intellect of man, who
is dominant form of life at the present time.
 The Cenozoic is sometimes subdivided into two periods,
the Tertiary and the Quaternary. European geologist prefer
to call these periods the Paleogene and Neogene
respectively.
 The Tertiary or Paleogene period. Tertiary rocks consist
mostly of loose, incompletely consolidated marine
sediments. The salt domes of the Gulf states, containing
immense quantities of sulfur, gas, and oil, are located in
Cenozoic sediments.
 Intense diastrophic movements began in western United
States toward the close of the Miocene epoch.
 Intense diastrophic movements began in
western United States toward the close
of the Miocene epoch.
 These continued with increasing
intensity, ending in the Cascadian
revolution which closed this period.
 The San Andreas rift developed at about
this time.
1. The Paleocene epoch.
 This epoch might be regarded as
the “early morning” of the
development of mammals. The
mammals were generally small in
size. Most of them bore only slight
resemblance to modern forms.
 The first of the primates, the order
of mammals which includes man,
developed during this epoch.
2. The Eocene epoch.
 In this epoch, the climate remained similar to
that of the Paleocene. The mammals in general
became larger. Eohippus, the first horse, was
about the size of a fox terrier. It had slender
legs suited to running through subtropical
forests, and teeth fitted primarily for eating
plant food. The first whales evolved. Lemurs,
animals like monkeys but with foxlike faces,
developed at this time. They have continued
down to the present. Birds, were also numerous
in this epoch. One unusual species, Diatryma,
was a running bird seven feet tall, with a
hooked beak.
3. The Oligocene epoch.
 During this epoch diastrophic forces
produced the first folds of the European
Alps, and the beginnings of the Himalayas
in Asia. Here the climate grew cooler,
favoring the growth of grasses, conifers,
and hardwoods, which replaced the older
subtropical forests of previous epochs.
The more primitive mammals
disappeared, and the animals of this
group became more like modern forms.
 Brontops was a huge hoofed and horned
beast, somewhat like a rhinoceros but 14 feet
long.
 Mesohippus was a three-toed horse of this
epoch. It had longer legs and a larger body
than Eohippus.
 The carnivores had begun to separate into the
dogs and cat families. Primitive camels,
primitive apes, and the first elephants had
also appeared.
4. The Miocene epoch.
 Mammals flourished in the
Miocene epoch. This is often
called the Golden Age of
Mammals. Great herds of early
horses and camels roamed the
plains. Members of the deer
family, the rhinoceros family,
the pig family, and other groups
of familiar mammals left fossils
during the Miocene.
 The Pliocene epoch.
 This epoch ushered in many changes
in living things. a large number of
new genera had their start at this
time. These included other groups of
horses, camels and elephants. This
epoch also produced many
carnivores. Manlike apes lived at
The Quarternary or Neogene
period
 This period is subdivided into two epochs.
The Pleistocene epoch is also known as the
Ice Age. The Holocene epoch, which
includes contemporary life is called the
Recent epoch.
 The transition from Pliocene to Pleistocene
was accompanied by great environmental
changes
1. the Pleistocene epoch.
 The mammals of this epoch, in contrast to the
invertebrates, were quite different from present-
day forms. Unusual mammals appeared.
Boreostracon was a large armadillolike animal
with a spiked tail.Megatherium was a giant sloth
that stood 20 feet above the ground on its hind
legs and weighed several tons. Smilodon, the
saber-tooth cat or “tiger”. Another was canis diris,
or dire wolf, a six –foot animal resembling the
modern wolf.
2. The Halocene epoch.
 Today we find the world characterized by
extremes climate at different localities during
different season. The earth still has, as it did
in the past areas of deserts and swamps,
forests and grasslands, mountains and
glaciers, and salt and fresh-water seas.