Earth History: Record in the Rocks
The geologic history of Earth can be
“read” from rocks if you know what
to look for. In this presentation, you
will see examples of many types of
clues, stored in rocks. Start by
reviewing how rocks are formed.
Igneous, metamorphic and
sedimentary form differently. What
are the differences?
Sedimentary rocks form
from layers of sediment
that collect in horizontal
layers. Over time, with
pressure and chemical
cementing, rocks form.
Sometimes, the layers are exposed
by erosion, like these canyon walls
in Dinosaur National Park.
These layers near Denver, Colorado show how horizontal layers can
be altered by uplift. In this case, the Rocky Mountains rose and
changed these layers. Can you tell which layer is oldest?
The layer that is on the far left is oldest.
Rockies
are over
here.
Sometimes layers of sedimentary rock are twisted by
earthquake faults.
Split Mountain, Dinosaur National Monument
The layers of sedimentary rock
no longer line up.
This picture shows Becks Spur which is in North Salt Lake City.
The cliff is the Wasatch fault it was not made by people. When the
Wasatch Fault slips the mountain rises and the valley falls. Lets
walk closer:
Scratches that formed as the rocks
slipped by each other.
Sedimentary rocks have clues to past climates and ecosystems.
These rocks were ancient sand dunes, with layers that formed at
different times, at different angles, depending on which direction
the wind was blowing.
This is called crossbedding and can be found
all over Southern Utah.
If you have gone bike riding on the Slickrock trail in Moab, you
have ridden on ancient sand dunes. What type of climate do sand
dunes suggest? Desert or dry climate
The trail is
marked with
white dots:
We can see examples of ripple
marks in the sand near a river:
Sam really likes ripple marks!
And, fossilized ripple marks
from a river long ago.
What formed these marks in an environment long ago?
Worms burrowing through soft sand or mud.
What might conditions like
these form? This is the
edge of a river in Dinosaur
National Monument.
Dinosaur footprint
Did you say dinosaur footprints or other preserved remains of
plants or animals? Fossils are direct clues to the past. This one
was part of a series of footprints near Denver, Colorado and the
circle around it is where someone attempted to cut it out of the
rock and steal it. This is selfish.
Utah has lots of dinosaur fossils. These fossil skeletons are
displayed at Dinosaur National Monument and show dinosaur legs a
head and neck and other bones. The person has been placed in for a
size comparison.
Can you find ribs, vertebrae, carbon remains from a tree trunk, a
shoulder blade, and thigh bone?
Tree outline
Vertebrae
thigh
rib
Shoulder
blade
Igneous rocks sometimes flow
upwards into cracks in other
rocks. The igneous rock in this
picture is light colored and has
intruded into darker rock. It has
a Y shape.
Igneous intrusion near Denver, CO.
Can you see igneous rock in these rocks? Sometimes as the
igneous rock cools, valuable minerals like gold and silver
collect around the edges. They are said to form veins.
Its here
An unconformity occurs when
erosion removes the top surface of
land and the surface is then
reburied. Can you find the
unconformity in this picture?
Its right here!
To interpret Earth history, a diagram called a stratigraphy is
sometimes used. It uses symbols to stand for rock types and can be
“decoded” if you know your Earth history. Take a look at this one
and see if you can see the order that events happened. Use this key
to identify the rocks:
Conglomerate
Sandstone
Limestone
Shale
The limestone formed first, then sandstone, then shale. Then an
earthquake faulted the rocks. The earthquake fault occurred last
because the rocks had to all be there before they could be offset.
Let’s try a harder one. There are
10 steps, including the events that
happened to the rocks. Do you see
the unconformity?
Conglomerate
Sandstone
Limestone
Shale
unconformity
They are conglomerate, sandstone, limestone, sandstone, shale, erosion,
limestone, sandstone, shale, erosion.
Igneous rocks “intrude” into other rocks. Because they are hot,
they affect the rocks that are near them. What type of rocks are
heated but not melted?
Metamorphic
Conglomerate
Sandstone
Limestone
Look at this stratigraphy and see if you
can list the steps. The dark lines show
contact metamorphism.
Shale
Igneous
Limestone, sandstone, igneous (volcano)
Erosion
Shale
Conglomerate
Erosion
Shale
Can you label the
geologic events in
this picture?
Landscapes can tell us about Earth. Can you walk out of your
school and see landscapes? What history do they tell?
The End.
Pictures and text by
Barbara Gentry