An Exhibit of the State Historical Society of Iowa
"No one can say when the first record was made, but the story through all of
the ages is indelibly carved in the rock by the feet and forms of the
mummified dead that lie where they lived. Age after age, as the sea and the
land contended and the species struggled to live, the drama of the world was
faithfully recorded. Sometimes, to be sure, the story is partly erased,
sometimes it is lost beneath subsequent records, but at some place or other
in Iowa a fragment of each act may be found. The surface of Iowa is a
palimpsest of the ages."
John E. Briggs, The Palimpsest, 1920
EVIDENCE FROM THE PAST
Rocks and fossils provide evidence of earlier lives and environments.
Interpreting the specimens allows a geologic and zoological history of an
area to be written. You may be discover interesting Rocks and fossils as
walk the stream valleys and rock outcrops of Iowa.
A concrete sidewalk is a humanmade layer of rock. It is composed
of rock fragments and binding
material shaped by confining
borders. Simple tools mix sand,
water, and portland cement together
to form a rock-like sidewalk. A
wooden form holds the mixture in
place until it hardens.Like a fossil
in a rock, a leaf or footprint in wet
cement remains after the cement
hardens. *
Rocks are products of environmental processes in the world around us.
Observing an eruption of Mount St. Helen, a flood on the Mississippi River,
or the movement of beach sands in the Bahamas allows us to understand
how rocks containing similar features are formed. This is known as the
principle of uniformitarianism.
Limestone is formed from the compacted shells of marine
organisms or precipitates like dust as the water evaporates.
Its presence indicates and area once covered by seas.
Sand and gravel collect along seaside beaches and stream
valleys to be cemented by waterborne material. These
sandstones may preserve evidence of currents, wave actions,
and the footsteps of ancient animals.
Coal forms when large quantities of plant material
compress and decay without oxygen, a condition common
to swamps and marshes.
Shale forms when muddy bottoms of quiet lagoons, swamps or
rivers dry out, compress, and harden through time
Layers of gypsum form when calcium and sulfur concentration
increase as sea water evaporates

Cement forms when limestone and clay are burned together and then ground into
dust. Iowa is a leading producer of portland cement.
Geologic Time
We usually think of time in years, generations and civilizations. Yet
geologic time extends over billions of years. It identifies the age o rock
groups and which unit of rock is older than another. Older rock are normally
found below younger ones.
The geologic time
scale places rocks
and fossils into
sequence. The order
is established by
which rock unit is
older than another
and confirmed by
index fossils and
radioactive dating.
Iowa rocks do not
show a continuous
rock record, vast periods of time and geologic deposits are missing. Their
absence indicate periods of erosion or an environment that did not produce
rock deposits.
CENOZOIC
QUATERNARY
RECENT
PLEISTOCENE
The last 3 million years (the Pleistocene period) is
represented across Iowa by glacial deposits.
Erosion characterized Iowa during the remainder
of the Cenozoic period.
TERTIARY
PLIOCENE
MIOCENE
OLIGOCENE
EOCENE
PALEOCENE
MESOZOIC
CRETACEOUS
JURASSIC
TRIASSIC
PALEOZOIC
PERMIAN
CARBONIFEROUS
PENNSYLVANIAN
DEVONIAN
SLURIAN
ORDOVICIAN
CAMBRIAN
PRECAMBRIAN
The time from 65 to 230 million years ago. Only
portions of the Cretaceous and Jurassic periods are
represented in Iowa. These deposits are stream and
coastal sediments mixed with marine sediments of
limestone, shale, and gypsum.
The sediments were formed 230 to 600 million years
ago. Repeated episodes of marine, stream, and
coastal environments produced limestone, sandstone,
coal, and shale as characteristic deposits.
The oldest rock on earth from 600 million to 4.5
billion years old. A long and complex time of
deposition, mountain building, and erosion. Rocks
of this age are visible in Northwest Iowa.
Geologic Map of Iowa*
Iowa's bedrock is exposed
at the surface in the
ribbons of ages that cross
the state. Each color
represents deposits of a
different age.
At Burlington, Le Grand,
Sioux City, and across the
state, we can see exposed
layers rock--limestone, sandstone, dolomite, siltstone and shale. These layers
are not the same age or thickness in all parts of Iowa. Because of uneven
deposits and erosion they thicken and thin, disappear and blend across the
state.
The map colors represent rock ages and their distribution as exposed rock
beneath the glacial till. The display of the colors is effected by the angle the
rock lies within the earth, structure, and the extent of erosion of the surface
exposure. Stream valleys cut through younger rock and expose older as the
upper layers are eroded away. Erosion may leave remnants of younger or
older rock as “islands” in areas of another age.
The geologic map provides a quick way to determine what rocks are
exposed near the surface, although they may be covered by glacial deposits,
in any area of Iowa.
*The Iowa Geological Survey Bureau, Department of Natural Recourses, provides copies
of the Iowa geological map and physiological region map on their website:
http://www.igsb.uiowa.edu/browse/geoiowa/geoiowa.htm.