Conor Watkins' Ozark Mountain Experience

Field Trip Guide to the
Geology, Scenery, And History of The Salem Plateau of the Ozarks
–In The Buckhorn, Dixon, and Rolla Areas
For The Fall 2003 MVOR hosted by MSM Spelunkers at Shriners Club, Buckhorn, MO - Oct 3-5, 2003
By Conor Watkins, University of Missouri - Rolla

Stop 5: The Vichy Road Filled Sinks

A very common but often overlooked geologic feature in the Ozarks is the filled sink.  Although evidence indicates they are not true sinkholes at all, these are referred to as paleosinks, fossil sinks, and filled sinks.  It is thought by many geologists that these structures were once hydrothermal vents of some sort due to the presence of exotic high temperature minerals.  These structures are widespread through the Ozarks and range from the far northern Ozarks along the Missouri River to the southern Ozarks in Arkansas.  The contents of these structures tend to vary by region with Interstate 44 forming a good rough boundary for sink types.  Filled sinks north of Interstate 44 tend to be filled with clays and the region surrounding Owensville, MO once bustled with mining activity.  These clays were at one time mined for use in high temperature applications such as fire brick, dishes, pottery, and even the launch pads for the Apollo space missions to the moon.  Filled sinks to the south of Interstate 44 tend to be filled with various metallic ores of iron, lead, zinc, copper, and other minerals.  Small iron mines were common throughout the Ozarks during the 1800's with the Maramec Iron Works at present day Maramec Spring being one of the more prominent examples.  Other filled sinks are contain sandstone and coal deposits.

Excellent examples of these structures are viewable in roadcuts on I-44 and Hwy 63 north of Rolla.  One of the more accessible exposures in the state is in Rolla.  These sinks are known as the Vichy Road Filled Sinks and are viewable in the I-44 roadcut from the highway or the Vichy Road overpass.  This area is full of filled sink exposures and others remain hidden underground.  The nearby Thomas Jefferson towers (dormitories for UM-Rolla) are also built on such structures and deep drilled shafts or caissons had to be excavated through the sink filling in order to provide a stable foundation.  These features formed mostly during the Mississippian and Pennsylvanian Periods, which took place between around 360 and 286 million years ago.  The rock they formed in is mainly dolomite from the Ordovician Period, which took place between 505 and 440 million years ago.  For reference, the dinosaurs died out around 65 million years ago.

The south exposure of the Vichy Rd. Filled Sink is clearly visible here,
as it has weathered back into the hillside, allowing snow to accumulate.
The surrounding Jefferson City dolomite (buff color) remains free of snow as
its walls are steeper.  Thomas Jefferson Hall, built on another such structure,
is visible in the background.

The north and south exposures of the Vichy Road Filled Sinks are different, leading one to believe that these are two separate structures that were cut during highway construction.  A small inactive fault running through the area seems to have aided in their formation by allowing preferential fluid flow through the rock.  The north exposure contains jumbled shales, sandstones, and a small amount of purple staining from hematite, an iron ore.

The north exposure of the Vichy Rd. Filled Sink contains sandstone, shale,
and purplish staining from hematite.  Both exposures are associated with a
a small fault.

The South Tower of the UM-Rolla Thomas Jefferson Hall dormitories was built in the early 1980's atop a shale filled sink structure.  A cave was discovered during construction, indicating that the shale filling was either settling or being lost into an underlying void.  The foundation consists of reinforced concrete caissons placed through the sink filling or any caves.   These caissons were also socketed into the solid underlying bedrock to ensure their stability.  This design is very conservative and has allowed the building to stand atop weak rock with few problems.  The oversimplification of the conditions found under the building and the fact that many do not differentiate filled sinks from ordinary sinkholes has led to many myths that the dormitary is slowly sinking and falling over.  The presence of settlement cracks and a humorous article in the March 29, 1984 issue of the Missouri Miner, the student newspaper at UMR, has helped perpetuate the myth over 20 years after the building opened.

Simplified diagram of the subsurface cross section beneath TJ Hall.
Note: There are more than four caissons under TJ in reality.

There are multiple theories on how these sinks were formed and not all of them are related to standard karst processes.  Evidence indicates that these structures may not be sinkholes at all, but the remnants of hydrothermal vents and springs formed during the MVT mineralization in the Ozarks.  The sinks pervasive in the Ozarks an may number in the tens of thousands, if not more.  Although these structures are very old and filled with rocky material, they can still pose problems for those looking to build atop or drill wells through them.

Clay and shale filled sinks appear as light colored areas in most rock cuts.  They often have concentric layering that dips towards the center of the often bowl shaped structure.  Some of these are present in quarries, including Capital Quarries' Rolla Quarry located along Hwy 63 north of town in an area with a high concentration of filled sinks.

The filled sink within Capital Quarries' Rolla Quarry is filled with shales and
some sandstone.  It is very easy to see, as the face is relatively unweathered
due to its recent exposure by the quarrying.  A small mud filled cave
was exposed while quarrying nearby.

DIRECTIONS - To The Vichy Road Filled Sinks:

Return to I-44 from Conical Sink and head east towards Rolla.  Exit at Rolla exit 185 and turn right and follow to the first stoplight at Hwy 63/Bishop Ave. in Rolla.  Turn left on Hwy 63 and follow to the next stoplight at Vichy Rd.  Turn left here and left again into the gravel parking lot adjacent to a Panera/St. Louis Bread Company.  This will serve as a parking place to walk to the overpass over I-44 where the sinks may be observed.

(C) 2004 by Conor Watkins