Geologic
History
Tropical
Sea
 It
is hard to imagine that the towering limestone bluffs
where Principia College sits were once the floor of a shallow sea. Ancient
organisms such as crinoids, corals, brachiopods, trilobites and bryozoans,
now fossilized in the many layers of limestone, inhabited this flat-bottomed
sea about 350 million years ago during the early Mississippian Period. Scientists estimate that this area
was first covered by a sea over 500 million years ago
when hard-shelled marine organisms first evolved. Many transgressions and regressions of the sea occurred depositing
a variety of sedimentary rocks, most of which are now
buried below our bluffs.
Ice
Age
The
Pleistocene Epoch, commonly associated with the Ice
Age, began about 1.65 million years ago and ended approximately
10,000 years ago. During this time, the northern portions
of North America and Eurasia were periodically covered
by glaciers (ice sheets like are present in Antarctica
today).
These
continental glaciers advanced and then receded as the
climate cooled and warmed. The glacial and interglacial
periods lasted thousands of years each. Ice sheets reached
13,000 feet in thickness, causing the global sea level
to drop about 300 feet below its current level because
so much water was in the form of ice.
Glaciers
flowed southward from Hudson Bay across the northern
parts of North America between the Rocky and Appalachian
Mountains and extended as far south as southernmost
Illinois. As glaciers moved slowly over the land, the
huge amounts of ice changed the landscape, leveling
and filling valleys, creating the flat prairies that
are seen today. Thick deposits of glacial sediment,
called till, were dropped by receding glaciers. In places
along their southern margin toward the latter part of
the Ice Age, the glaciers encountered resistant rock
behind which they had scraped large depressions. These
depressions subsequently filled with meltwaters forming
the Great Lakes.
(To
learn more about the Pleistocene Glaciations in Illinois
click here)
Ice
Age Animals
Many
modern plants and animals have their origin in the Pleistocene
Epoch. Species of Pleistocene conifers and mosses are
still around today. Many species of large mammals living
in North America, Asia, and Europe during the Pleistocene
became extinct around the end of the Ice Age. These
include native horses, camels, longhorned bison, saber-toothed
cats, giant ground sloths, and our mammoth and its smaller
distant cousin the mastodon.
Mississippi
River
 The
Mississippi River downcut through the limestone bedrock
forming the great bluffs that we now see. Across the floodplain,
which is shared with the Missouri River, is another line of bluffs, also
composed of limestone. The great breadth of the floodplain
relative to the size of the Mississippi and Missouri
Rivers is due to the large volume of glacial meltwater
that flowed through this river valley in the waning
stages of the Ice Age.
Loess
Deposits
Draped
across the top of the limestone bluffs is a blanket
of wind-blown silt, called loess (pronounced "luss"),
that is over 50 feet thick in places. In the waning
days of the Ice Age, the meltwaters carried huge volumes
of sediment from the melting glaciers. This sediment
was deposited in the floodplain and became exposed in
the winter when the meltwaters receded. Winter winds
whipped up the exposed clay and silt from the floodplain
and deposited them on the bluffs. It is this loess material
in which Benny is buried.
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