Another Big Question

What is underneath Winneshiek County?


Beneath our feet here in Winneshiek County we have layers of soil created by thousands
of years worth of decomposing plants and weathered rocks.

Beneath the soil there are layers of sedimentary rocks. The limestone, shale, and
sandstone were deposited during the Ordovician time period, which was about 470
million years ago. At that time, Iowa was covered by a shallow ocean.

The limestone is made of the shells of the animals that died in the ocean. The shale is
made from mud and silt. The sandstone does not have many fossils in it, but sometimes
you can find red lumps. These lumps are iron nodules. They are red because they have
been exposed to oxygen in a process called oxidation. The limestone has a lot of
fractures in it. Those fractures and layers allow caves to be formed by moving water.
Sometimes the water leaves behind minerals that makes a shiny surface that looks likes
frosting. That is called calcite. Areas like ours that have caves and fractured limestone
and sinkholes have what is called karst topography.

The deeper you go under Winneshiek County, the older the rocks are. Sometimes you
can find fossils of the simple creatures that lived back then. There were no fish or any
other kind of invertebrates in the ocean during the Ordovician time period, and there were
no flowering plants on the land back then.

You cannot find fossils of dinosaurs in Winneshiek County because the layers of rock
that would have had dinosaur bones were eroded away by rain and wind and glaciers.
Sometimes you can find a rock that seems out of place because it is not limestone, shale,
or sandstone. It might be a glacial erratic. It was carried here from far away by the
glaciers and got left behind when the glaciers melted. The west half of Winneshiek
County is flat because all the low places were filled in by glacial deposits when the last
glacier was here 10,000 years ago. The east half of Winneshiek County is hilly because
the last glacier missed it.

Underneath all the layers of sedimentary rock there are really really old rocks that go
back to the Pre-cambrian time period when there was no life on Earth. As you go deeper
into our planet the temperature gets hotter and the pressure steadily increases. The thick
middle layer of the earth is called the mantle. It is made of hot soft rock that slowly
circulates around. The center of the earth is called the core. It is made mostly of the
metals iron and nickel. It has two parts- a liquid outer core and a very dense and solid
inner core. The outer layer of the earth is the cold and brittle crust or lithosphere.

Some of the common fossils around here are:
• Brachiopods- look like a clam but the ridges are vertical instead of horizontal
• Crinoids- look like little Cheerios, they were stacked up to form the stem of an
animal that looked sort of flowery
• Gastropods- look like snails
• Nautiloid Cephalopods- sort of ice cream cone shaped, they had a squid-like
animal inside
• Branching Bryozoa- can look like bones, but there were no animals with bones
back then
• Prasopora Bryozoa- look like a Chinese hat or a gumdrop
• Horn Coral- the shape is like the name
• Stromatolite- fossilized slime layers
• Receptekulites a.k.a Fisherites- looks like a sunflower but actually it is a kind of
coral
• Trilobite- these are small and hard to find in northern Winneshiek County but
abundant elsewhere