Local Geologic Setting

Within a driving range of four hours from Washington University, the interested geoscientist can observe an unusually wide variety of natural features ranging from the Precambrian igneous rocks of the St. Francois Mountains, through complete stratigraphic exposures of Paleozoic sedimentary rocks, to present examples of river dynamics. St. Louis is located just south of the confluence of three of our country's great rivers--the Missouri, the Illinois, and the Mississippi--with a wide variety of landforms, styles of erosion and deposition, and effects of man-made changes. Just across the Mississippi River in Illinois, opportunities exist to study fluvial sediments deposited along the ancient Michigan River. The bluffs along the major rivers, those along the nearby Meramec River, and those along lesser rivers throughout the Ozark region are esthetically pleasing as well as geologically interesting and harbor the entrances of many of Missouri's more than 5000 caves. Consequently, canoeing, caving, and rock climbing are among the most popular activities of Washington University's Outing Club.

Granite boulders at Elephant Rocks State Park in the St. Francois Mountains

The St. Francois Mountains Complex southwest of St. Louis has a core of Precambrian (~ 1.5 billion years old) granite and quartz monzonites (10 units), associated rhyolites and andesite tuffs and flows (~115 units), and mafic dikes and sills. Geological mapping of the region is incomplete and few modern petrographic, geochemical, or structural studies have yet been made. Along the edge of the St. Francois uplift is the largest lead mining district in the world. One of the magnetite deposits of the region is still mined for iron. To the south of St. Louis, along the St. Genevieve fault system, are diatremes of Devonian age.

Basalt dike in granite, St. Francois Mtns.		Unconformity - Bonne Terre dolomite over Precambrian felsite, Taum Sauk Mtn.

Most of southern Missouri is dominated by the Ozark mountains, which reach their maximum elevation in Missouri at Taum Sauk Mountain (1,772 feet or
540 m elevation), 78 south of St. Louis. A series of NW-SE trending folds of low amplitude and long wavelength cross the plains regions of northern Missouri. Washington University is within the seismically active zone of the New Madrid graben.

The Decaturville and Crooked Creek structures, which are nearby, are well known relicts of apparent meteoroid impact. An additional 20 or so circular features in the area have been identified from aerial and satellite photographs but have not yet been carefully studied from the ground.

Our outdoor laboratory is shared with the Biology Department, of course, because of the great opportunity for ecological and other natural studies. The northwestern parts of Missouri still contain patches of unbroken tall grass prairie. The eastern and Ozark regions are dense oak - hickory forest, the setting for Washington University's 2400-acre Tyson Research Center. The extreme southeastern part of the state most closely resembles the coast and delta regions of the South. The varied terrain and changing seasons give field trips in our region a diversified aspect, often challenging, always beautiful.

Field excursions to geological features in Missouri are supplemented by yearly and impromptu field trips to more distant areas of interest. Recent trips have been to the Rio Grande area of Texas, the Wichita Mountains of Oklahoma, the Precambrian of Minnesota, the Magnet Cove alkaline complex in Arkansas, the Florida Keys, the Hawaiian Islands and the Mojave Desert. Opportunities for geologic field work as part of graduate research are practically limitless.

Geologic map of Missouri.   Click on image for enlarged version.