Chapter 2, The Ozark Plateaus

The Ozark Plateaus Physiographic Region

The first step in understanding the geology of an area is to divide it up according to similarities and differences, and to give the divisions meaningful names.

Eureka Springs is located at the southern edge of a geologic province known as the Ozark Plateaus.  The term, ‘Ozark Plateaus’, is a formal geological and geographical name.  It describes a raised, very old, and relatively flat area of sedimentary rock that spans a region of about 50,000 square miles.  The Ozark Plateaus province stretches from southeast Kansas and northeast Oklahoma, over most of southern Missouri and through parts of Northwest Arkansas, including Eureka Springs.

The Ozark Plateaus province is one of many regions, provinces, and sections into which the United States is divided for research and mapping purposes by geologists and geographers.  To learn more about these regional divisions of the country and their descriptions, you can visit the United States Geological Survey map of the regions at www.tapestry.usgs.gov   For ecological and environmental study, the part of Arkansas in which Eureka Springs lies is also called the Ozark Highlands Eco-Region.

The southern edge of the Ozarks Plateau, where Eureka Springs is located, is highly eroded.  Millions of years of sediment erosion has left the area with a deeply carved surface of highlands divided by river valleys and drainages that are 300 to 500 feet deep.  The sediments eroding off of the Ozark plateaus eventually find their way down streams and rivers flowing northward into the White River Drainage, and south and eastward from there into the Mississippi River.  The hundreds of cubic miles of sediment that have already eroded away from this ancient highland now form the more recent sediments that make up portions of the states of Louisiana and Mississippi as well as a great deal of material at the bottom of the Gulf of Mexico.

The Ozark Plateaus physiographic province is comprised of the Salem Plateau, the Springfield Plateau, and the Boston Mountains. Eureka Springs is located on the complex escarpment between the Springfield and Salem Plateaus. The base map modified here came from Wikimedia.

As suggested by the plural name, the Ozark Plateaus province is composed of more than one plateau surface.  There are two primary surfaces comprising the Ozark Plateaus, the Springfield Plateau and the older lower elevation rocks of the Salem Plateau.  At the far southern edge of the region are the Boston Mountains, the eroded remnant of what was once a third relatively flat surface.  The vertical slope separating these relatively flat surfaces is referred to as an escarpment.  The word escarpment refers to any steep slope separating two relatively flat areas of land.  What appear to be the hills and mountains of Eureka Springs are actually the complex uneven edge of this large scale drop in elevation.  The town is sitting in the ruts and valleys along the complex eroding edge of the ‘Eureka Springs Escarpment,’ the name that refers to this small section of the vast ‘Burlington Escarpment’ that extends for hundreds of miles, defining the convoluted northern edge of the Springfield Plateau in Arkansas.

Topography of the Ozark Plateaus physiographic province. Map from Wikimedia.

The top of the ridges around the town of Eureka Springs are the outer edges of the Springfield Plateau.  The flat bottoms of the steep walled valleys of the town define the inner edge of the Salem Plateau.  Each of these surfaces extends for thousands of square miles.  The Springfield Plateau spreads out to the South and West and the Salem Plateau drains to the North and East.

Karst - Caves and Sinkholes

Sinkholes are depressions resulting from slow erosion or collapse of surface limestone or dolostone around Eureka Springs into caves in the subsurface. They are fairly common. They form as water sinks into cave-riddled subsurface, carrying sediment with it, instead of running off into stream channels. In some regions, sinkholes form abruptly. This is not typically the case in the Ozarks. While one should be cautious around them, since cave ceiling collapse or unseen openings are always a possibility, most of these structures formed over thousands to millions of years, and the cave and karst structures underlying them have been forming over millions to tens of millions of years.

While they are relatively common here, parts of southern Missouri, have some of the highest sinkhole densities on earth. Several great sinkholes can be seen along the Miner’s Rock trail at Lake Leatherwood Park.

To learn more about sinkholes, you can read an excellent article from the Missouri Geological survey, at: https://dnr.mo.gov/geology/geosrv/envgeo/sinkholes.htm