Geologic Time and Fossil Formation
Utah’s
geology has
changed over
time.
Notice where North is
in this picture.
550 Million Years Ago
Present Day
500 million years ago
Utah was near the
equator and at about
sea level.
The Early Years: Paleozoic. Utah was at the
edge of North America. The eastern part of
the state was flat and low, close to sea level.
Coral reefs became thick limestones. These
helped form the Wasatch Mountains. Shallow
seas led to deep oceans in the west.
Early Fossils
500 mya – 248 mya
Trilobites
Giant Insects
200 million years ago
Utah was no longer at
the edge of the ocean.
Early Jurassic: Utah was cut off from the ocean
winds by rising mountains. Desert sands blew
into Utah from the north. These blowing sands
formed dunes. The dunes turned into rock and
are preserved in what is now called the Navajo
Sandstone. You can see them in Zion National
Park and on the San Rafael Swell.
Fossils
206 mya – 180 mya
Allosaurus
Tanystropheidae
Captorhinus
Hot and swampy. A
great place for
dinosaurs!
Famous Dinosaurs: Late Jurassic. At this time Utah was
a hot, swampy lowland with mountains and volcanoes to
the west and northwest. Meandering rivers and lakes
abounded, while dinosaurs roamed the land. Their
fossilized bones are preserved and can be seen at
famous sites such as the Cleveland-Lloyd Dinosaur
Quarry and Dinosaur National Monument.
Late Jurassic Period Fossils
Camarasaurus
Stegosaurus
100 million years ago,
plants grew that
became coal deposits.
Late Cretaceous: Coal forms. The continent ran
into the Pacific Plate. This created high
mountains in the west. In the east, an inland sea
stretched from the Gulf of Mexico to the Arctic.
Sediment filled the sea and the basin sank. Later,
this turned into sandstone. Dinosaurs continued
to live in huge, wet forests. These forests turned
into coal deposits.
Late Cretaceous Period Fossils
Coal
Ferns
Tyrannosaurus
An inland sea formed, and
the land was pushed up.
Utah Starts to Come Up in the World: Paleocene.
Erosion wore down mountain in the west. Sediments
filled the inland sea to the east. Pressure from the
Pacific Plate pushed the ground up. This is called
uplift. It created the Uinta mountains and the
Colorado Plateau. The pressure created the swells
and depressions now found in Utah. One of these is
the San Rafael Swell. A large freshwater body,
called Lake Flagstaff, formed in southern Utah.
Paleocene Fossils
Triceratops
Ernanodon (early mammal)
Organic matter becomes
oil shale.
Oil Shale and Fossil Fish: Eocene. Utah spent nearly 500
million years near sea level. Now it rose to become
nearly a mile high. The Colorado Plateau warped even
more. It produced basins for lakes such as Lake Uinta.
Organic-rich sediments include fish fossils and oil
shales. The western mountains were nearly eroded
away.
Eocene Fossils
Fish in mudstone
Amber with insects
Oil shale with fossil leaf.
Seahorse
The land uplifted and
modern rivers began to
form.
Uplift and Volcanoes: Oligocene. Lakes on the
Colorado Plateau filled up with sediment. Wide plains
sat between mountain uplifts. Modern rivers began to
run across these plains. The land in northeastern Utah
was divided by mountains. This caused the Green
River to drain into the Mississippi River. Today, the
Green River drains into the Colorado River. The land in
western Utah began to stretch. Later, this would create
the Basin and Range.
Oligocene Fossils
Mesohippus (ancient horse)
Palaelodus
(swimming flamingo)
Basin and range forms.
Metals are deposited
in Utah’s mountains.
Precious Metals Form: Miocene. Land near what is now San
Francisco had been moving closer to Salt Lake City. Now, the
land stretched and they moved apart. This stretching left tall
mountains divided by wide plains. This formed the Basin and
Range. Volcanic activity formed three areas with metalic
minerals. They are: Park City-Oquirrh, Deer Creek-Tintic, and
Wah Wah-Tushar. The Colorado Plateau continued to rise and
tilt northeastward.
Miocene Fossils
Livyatan melvillei (toothed whale)

Australopithecine (hominid)
Finally, Lake Bonneville!
Water and Ice: Pleistocene. The geography of Utah was
very close to what it is now. Mountains, canyons, and rivers
were all well in place. The climate at this time was wetter
and colder. Glaciers formed in the mountains. They carved
canyons in the Uinta Mountains and other mountain ranges
throughout the state. A giant fresh-water body called Lake
Bonneville also formed. It stretched from the Wasatch
Mountains to Nevada and from the Utah-Idaho border
nearly to Cedar City in southern Utah.
Pleistocene Fossils
Bison latifrons
Smilodon fatalis sabertoothed cat
Mammoth
(frozen in ice)
It’s all about the past. Our
ancient geologic history
explains the present.
These are the Places: Present. Our geologic informs us about
Utah's present. We see why the rocks to the east are brightly
colored while the rocks to the west are duller. We see why
huge canyons formed on the Colorado Plateau while much of
the Basin and Range has no external drainage. It explains
why a high mountain chain, the Wasatch, runs down the
middle of the state. This history helps us understand why
settlements, industry, and recreation sites are located where
they are.
Summing Up
Land on Earth’s
surface moves over
time (plate tectonics)
Utah once sat near the
equator. Over time it
moved north.
Utah’s land has
uplifted throughout
geologic history.
Stretching and plate
movement created
ranges and basins.
Earth material and fossils
are deposited based on
the events of an era.
Older fossils are generally
found below younger
ones.
The fossil record informs
geology and geology
informs fossil dating.
Fossils are created in
different ways, depending
on location and
circumstances.