Tertiary/Quaternary Period

Geologic Map of Missouri, 1990, Missouri Department of Natural Resources, Division of Geology and Land Survey (now Geological Survey and Resource Assessment Division).

Tertiary/Quaternary Period (66 million years old to present aged rocks)

Most of the ground cover of the Tertiary and Quaternary Periods (Cenozoic Era) consists of alluvial (stream deposited) clays, sand and gravels (with a few poorly consolidated sandstones); glacial (ice deposited) tillites and gravels, and eolian (wind blown) clays and loess (an extremely fine "rock flour", which nonetheless forms solid masses).

Which one is which can be told rather easily by looking at the map. The large yellow area covering the Bootheel is largely alluvial. related to the Mississippi Embayment (see Cretaceous Period) and the deposits along the major rivers are both stream and wind deposited. Pleistocene till and loess is not shown--loess, clay, silt till, gravel, and glacial erratics are common over Northern Missouri anywhere north of the Missouri River, and in a few isolated "islands" south of the river near Kansas City and St. Louis.

At one time, the glacial history of North America was divided into 4 time frames: the Nebraskan, from 1.8 million to 500,000 BP (years before present); the Kansan, from 435,000 to 300,000 BP; the Illinoian, from 265,000 to 125,000 BP; and the Wisconsinian, from 75,000 to 10,000 BP.

Recent studies of ice cores, stalagmites, and other temperature/dating methods have concluded that there actually have been 30 sustained periods of frigid temperatures in the last 3 million years. Of the classical glacial periods, only two: pre-Illinoian (Nebraskan-Kansan) and Illinoian are now recognised as having left glacial deposits in Missouri. The pre-Illinoian was the most severe: amongst its legacy was the changing of the course of the Missouri River to its present location, the scouring and filling of Northern Missouri topography, and extensive outwash gravels left to the south of the present Missouri River. Although the Ozarks were not glaciated in the recent past, a cover of Pleistocene loess of varying thicknesses extends over all of the state except for the highest parts of the Ozarks.

Other Quaternary reminders in Missouri are two well-known mastodon finds, the Grundel mastadon in Holt County and a bone bed near present day Imperial in Jefferson County. The latter is preserved as Mastodon State Historic Site, where museum displays tell the story of how Missouri fossils ended up in Britain in the 1840s, how this area was preserved from being lost beneath an interstate by 4 housewives in the 1970's. Although there are a fair number of Pleistocene mammal fossil sites in the state, these two are significant because they are also archeological sites: a stone hearth in Holt County, and a Clovis point found by paleontologists in Imperial provide evidence that man may have been contemporary with mastodon in Missouri.

We are still in the Cenozoic Era; in an epoch known as the Holocene. Geology is still happening. Residuum, otherwise known as soil, clay, and rock fragments degrade from exposed (and subsurface) bedrock as you read this. Gravity and streams move this residuum, depositing it in sometimes graded layers. As long as the wind blows and the water flows rocks will continue to change and move.

I invite you to go outside and take a look around for yourself!

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