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).

Glacier Model

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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