Unit
1: Geology of Iowa
C. Iowa's Sedimentary
Rocks and Environments
Most of Iowa's rocks
are sedimentary like limestone and dolomite. This
does not mean that all rocks were formed in the same environment.
Iowa's rock history shows there were different environments over time.
The types of environments include coral reefs, swampy shorelines,
streams and rivers, oceans and glaciers. Read on for more information
about each of these environments.
Oceans and coral
reefs - Over the history of the Earth, Iowa was
near the equator and under water in shallow oceans. During these
times, the limestones and dolomites were being formed! Two types of
depositions form limestone and dolomite. Deposition is the accumulation
of sediments in an area over a long time. The two types of deposition
are chemical and physical.
Physical deposition means that it formed when
seashells, skeletons and
sediments were eroded from one spot and carried to another spot
where a new rock would eventually form.
Example: The
animals whose skeletons and shells make up these
rocks live in and around coral reefs such as colonial and solitary
corals, oysters, fish, starfish, crabs, and sea urchins. After the
animals die, their skeletons and shells eventually accumulate on
the ocean floor where they can remain whole, but are usually
crushed to bits by the great pressures on the bottom. These
skeletal materials and sediments accumulate gradually over
thousands of years, and then through the pressures of
compaction, eventually bond together to become rocks. This is
why sometimes people find fossils in limestones and dolomites.
Chemical deposition
means that they precipitated from the ocean
water itself.
Example: Get
a jar with a small amount of warm water in it and
dissolve either salt or sugar in it. If the cap is left off, the
water will evaporate.
As the water evaporates, crystals of
salt or sugar form at the bottom of the jar. (This experiment
may take several days.)
The Great Salt
Lake in Utah is a perfect example of chemical
deposition. This “lake” was once a part of a larger ocean. It
was cut off from the ocean at some point in time. Without a
new source for salt water, all of the water in the lake
eventually evaporated. Once the water evaporated, only the
minerals, like salt, were left behind.
Even around the
world today, limestone and dolomite are being
deposited underwater in ocean environments, such as off the coast
of Florida, but they're not rocks yet. It will still take thousands
of years of pressures by compaction for the sediments to become
rocks.
Shorelines
- Sandstone and shale are two types of rocks
deposited along shorelines. Sandstone is usually deposited as beach
sand. The lighter and smaller sediments like clay are deposited a
little farther out from the beach. Over time, these clays form
shales. The beaches of today will become the sandstones of the
future!
These deposits
were left as the oceans rose and fell thousands of
times throughout the geologic history. Because of the cycles that
it took to form the deposits, it is called cyclical.
Swamps -
Shale and coal were formed in ancient swamps. Mud and
decaying plant matter accumulated over thousands of years to form
these deposits. Sometimes plant fossils are found in shale.
While these rocks
are not good for road building, they do have
other uses. Coal is a major source of electricity worldwide. In Iowa,
most of our electricity comes from burning coal. This means that
when a light switch is turned on, the electricity used by the light
comes from the energy created by burning coal.
Glacial deposits
- Did you ever notice how the hills roll along as you
travel through Iowa? Have you ever seen piles of rocks in farmers'
fields? Several glaciers that covered Iowa at different times left
these ancient deposits behind. These glaciers were somewhere
between several hundred to several thousand feet thick. The last
glacier left Iowa and retreated to the north about 13,000 years ago.
illustration
courtesy of Iowa Department of Natural Resources
Glacial deposits
are made up of the combination of the three basic
rock types. Do you remember what the three basic types are? (The
three basic types of rocks are sedimentary, igneous and metamorphic.)
These deposits
contain many different sizes of rocks--from the
smallest clay particle to medium-sized gravel to boulders as big as
a
house.
illustration courtesy of "Iowa's Geological Past
Three Billion Years of Change"
by Wayne I. Anderson
These
deposits are called glacial till or glacial drift. As the glaciers
melted and retreated, they left glacial till or glacial drift behind.
Occasionally the
glaciers stopped as they retreated and deposited
material in different places. New terrain was formed as a result of
the stops made by the glaciers. These terrains took on different
shapes. Sometimes the glaciers would stop for a long time and
leave
a row of hills piled up along their edges. These are called moraines.
Wherever a chunk
of the glacier was left behind after a stop, it
created a low spot called a kettle. These low spots later filled with
water and became lakes. Iowa's “Great Lakes” (Clear Lake, Okoboji
Lake and Spirit Lake) are all examples of lakes created by glaciers.
Also, rivers that flowed under the glacier deposited a single hill
called a knob, or kame.
Fossils of animals
that lived in Iowa long ago are still found in
glacial and river deposits. The fossils are bones from bison,
mammoths, giant beavers, and even camels and giant sloths.
illustration courtesy of "Landforms of Iowa"
by Jean C. Prior
River and Stream
Deposits - Earlier, deposits were discussed.
A deposit is the accumulation of sediments in an area over a long
time. River deposits are a lot like glacial deposits. They come in
many different sizes. They are made up of the three basic rock
types; remember the sedimentary, igneous and metamorphic rock
types described earlier in this unit. These types of rock are
called gravels.
There are two factors
that determine the size of rocks that a river
can move. The first factor is the amount of water in the river. The
second is how fast that water is flowing. In general, river deposits
are sorted by size. The largest particles are on the bottom. The
smallest are found on top of the larger rocks.
Have you ever walked
along a river and noticed lots of sand in one
spot in the river? This is called a sandbar.
Have you ever wondered
why a river or stream is in a particular
location? Rivers and streams are constantly changing. This is
because water flows along whatever course are easiest. As the
river flows along the course, it cuts away or erodes the material
from the outsides of the curves in the river. Then the water
moves the material and redeposits it on the insides of the curves
down river.
The reason the
outsides of the curves are eroded is because water
speeds up as it travels along the outside curve. The water then slows
down on the insides of the curves. This change in speed is what causes
the sediment the river is carrying to be redeposited on the inside of
the curve. The constant erosion and redeposition is what causes the
channel to meander and changes the river's course over
time. The older
the river, the more times it has changed its course.
When a river changes
its course, it can leave former parts of the
river cut-off. These cut-off parts are called oxbow lakes. The
Mississippi and Missouri rivers are perfect examples of rivers with
many old channels and cut off oxbow lakes.
Rivers also form
flood plains. Flood plains are the nearly
level
lowlands next to rivers. In the springtime, after the snows melt or
heavy rains fall, the areas are often covered with water. This
flooding also produces deposits. However, they are not generally
high enough quality or significant enough in size to be used in road
building.
illustration courtesy of "Landforms of Iowa"
by Jean C. Prior
TO: Unit 1 D. How Geology
Affects Iowa's Landscapes
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