3
GEOLOGY OF THE OZARKS
THE ROCKS in the Ozarks are generally older than those surrounding the area, and the oldest are the once-molten ones which form the prominent knobs, hills, and mountains in the St. Francois Mountain area. Pilot Knob, Taum Sauk Mountain, the Elephant Rocks, and the rocks at nearly all of the many shut-ins exemplify once-molten (igneous) rocks such as rhyolite, porphyry, and granite.
These rocks are from 1.2 to 1.4 billion years old, give or take a few million years. They are not the oldest rocks known on earth, but do have sufficient seniority to be assigned to the Precambrian, the oldest geologic unit found at the surface. Millions of years of erosion took place after these igneous rocks were formed, but the St. Francois Mountain area was even then a prominent topographic feature and in part formed a cluster of islands millions of years ago when seas covered the Ozarks of today.
Streams emptied their loads of mud, clay, sand, and gravel into the seas, and waves and shore currents wore away similar materials from the beaches and shoreline, sweeping debris seaward to fall to the bottom. As sea organisms died, their remains settled to the bottom to form a litter composed mainly of the shells or bony parts, and chemical precipitation added to the calcium and magnesium carbonates accumulating on the sea floor. This thick accumulation eventually hardened and was naturally cemented to produce many of the common rocks in Missouri. Mud and clay became shale; sand became sandstone; gravel became conglomerate; and the remains of sea organisms and precipitated calcium and magnesium carbonates became limestone and dolomite respectively.
The rock record shows that the Ozark area was alternately land and sea throughout various periods of geologic time. As a result, certain rock layers present in one area may be absent in another and a given rock unit may be traced and recognized as a sandstone in an area which was once a beach but became limestone in another area where the sea was deeper. The sediments deposited in the ancient seas are stratified or bedded like a huge layer cake. As shown on the accompanying map, these layers slope away from the St. Francois Mountains so that the igneous rocks exposed in these mountains are far below the surface at Rolla. For example, the rocks exposed at the top of Taum Sauk Mountain at 1772 feet above sea level are approximately 1800 feet below the surface (or 700 feet below sea level) at Rolla. This slope results from the gentle uplift of the Ozarks through a great length of geologic time with the maximum uplift in the St. Francois Mountains.
Groups of rock layers with distinctive characteristics are given proper names and called formations. The lowest formation in Phelps County is the Gasconade, a dolomite (calcium-magnesium carbonate) which forms the vertical bluffs along the Gasconade River and is the host for the major caves, springs, and sinkholes in the county, including Maramec Spring and Slaughter, Conical, and King sinks. Natural bridges and tunnels in the general area are also in the Gasconade. It is overlain by the Roubidoux, which contains conspicuous sandstone beds, as along 1-44 between Rolla and Martin Springs. This sandstone was deposited in shallow water as shown by the fossil ripple marks on display in buildings where natural slabs of the brown stone have been used for facing. The highest persistent formation in the area is the Jefferson City. It is mainly dolomite and was the source of rock for the foundations of the Phelps County Court House, the old jail west of the court house, and the older buildings on the UMR campus such as the Rolla Building, the Mining Building, and Norwood Hall. The main quarries for this building stone were west of Rolla from the School of Mines across the tracks from Bridge School road, thence northwestward to the Holiday Inn where ledges are still prominent. The three formations are about 450 million years old. After the next lesson in "Instant Geology I," you will become an expert on mineral deposits and other geologic endowments which encouraged immigration to the Ozarks in the days before brochures were plentiful (the term "plentiful" is needed because some of the early German settlements were in response to published glowing descriptions of this "Garden of Eden").
“the Elephant
Rocks… exemplify once-molten (igneous) rocks”
Elephant Rocks near
Ironton in St. Francois Mountain area.
“the Gasconade, a dolomite… which forms the vertical bluffs along the Gasconade River”
From a bridge on Highway 28.
“mainly dolomite and a source of rock for the foundations of… the older buildings on the UMR campus.”
Mining Building at the University of Missouri, Rolla.
Map of the Ozarks. (Vertical scale and slope of rocks is greatly exaggerated).