Ozarks and Ouachitas: The Changing Mountains (JHARVNC)
By investigating these two ranges, participants will understand how mountains are formed and changed.
K - 12
Nature center lobby and aquarium room to view replicas of Ozarks and Ouachitas
Janet Huckabee Arkansas River Valley Nature Center, Fort Smith
Education Program Coordinator, 479-452-3993
Suggested Number of Participants:
25 - 30
- Understand how mountains are formed
- Understand how the Ozark Mountains were formed and have changed
- Understand how the Ouachita Mountains were formed and have changed
- Recognize the differences in geology, flora and fauna between the two mountain ranges
*See glossary for definations
The Ozarks and Ouachitas are the two dominant mountain ranges in Arkansas. In order to understand the ranges, we will discuss minerals and rocks, and then study how the Ozarks and Ouachitas were formed and have changed.
General Rock/Mineral Information
- Minerals – a naturally formed solid substance with a unique chemical and crystal makeup. Minerals are inorganic (composed of matter not from plants or animals).
- Rocks – several types of minerals joined together, forming one mass
- Igneous – rocks formed by the cooling of molten (melted) material. (“Ignis” means fire.)
- Metamorphic – rocks whose original form and mineral content have been transformed due to changes in temperature, pressure and their chemical environment. “Metamorphose” means to change.
- Sedimentary – layered rocks formed by deposited rock and mineral. “Sediment” is fragments of solid material that are deposited by water or air. It can also be separated where water evaporates.
The Ouachita Mountains
- General Description – The Ouachita Mountains rise from the heart of Arkansas, stretching into Oklahoma. The Ouachitas have long narrow ridges and wide valleys. Numerous mountain streams flow down the ridges that drain into the Ouachita, Caddo and Saline rivers. Many thermal (warm) springs bubble up from underground.
- Geology – The rocks in the Ouachitas are sedimentary: sandstone, shale and chert. Folds and faults formed the Ouachita Mountains, requiring millions of years and extensive geologic activity.
- Historical geology – Millions of years ago, the South American continental plate moved closer to the North American continent. The deep layers of sediment on the bottom of the ocean were squeezed into a smaller area. Eventually this dense ocean floor was pushed up against the crust of the North American continent. The ocean floor sediments rose up and over the existing crust. (That crust is now tens of thousands of feet below the rocks of the Ouachita Mountains.) Erosion also affected these sediments. The result is the folded Ouachita Mountains we see today
- Faulted geology – Many of the brittle sedimentary rocks in these folded mountains cracked or faulted. Heated fluids deposited quartz veins and crystals into many of the cracks during and soon after the mountain was built. Much later, fluids from molten rocks deep within the earth were injected into the Ouachita Mountains. As these molten rocks cooled and became solid, they formed many small and some larger masses of igneous rock.
- Erosion in the Ouachitas – Eventually the colliding plate of South America either bounced off North America or sank, and a shallow sea remained. This became part of the Gulf of Mexico. Water and wind eroded the newly formed mountains over millions of years, and part of the sea was gradually filled with this sediment. This filled-in area became the Arkansas Delta and West Gulf Coastal Plain to the south.
- Forested mountains and valleys – The soil on the tops of the Ouachita ridges is mostly thin and sandy. Pine forests grow well in this soil, but decayed pine needles make the soil too acidic for most other plants. Red oak and hickory grow in areas farther down the ridges. Beech and umbrella magnolia trees thrive on north-facing slopes or in sheltered coves. Mosses, ferns, vines, shrubs and wildflowers can be found in these same areas. Warmer, drier south-facing slopes have less undergrowth. The thicker, poorly drained soil of the valleys supports willow oak, water oak and sweet gum trees.
- Forest’s decline and renewal – Until the late 1920’s, there were few laws controlling the use of the Ouachita forest. Early logging and large-scale clearing destroyed much of it. As the forest began to disappear, so did the wildlife. Laws have since been passed to protect the forests and waterways. The U.S. Forest Service works with the Arkansas Game and Fish Commission and other agencies to manage the land, and pine forests are growing again on the ridges.
- Wildlife – Today the Ouachita Mountains are once again home to many animals. Black bear, white-tailed deer and muskrat are among the most familiar mammals. Many snakes and amphibians are found there. One of the least common is the Fourche Mountain salamander. Numerous birds visit or live in the forests year-round. Owls, woodpeckers, songbirds or even an American bald eagle may be spotted there. Thousands of migrating waterfowl visit the Ouachita area, and the lakes and streams are full of crappie, walleye, catfish, trout and bass.
The Ozark Plateau
- General Description – The rugged Ozark Plateau overlooks the Arkansas River Valley. It spreads from northern Arkansas into Missouri, Oklahoma, Kansas and Illinois. Flat-topped mountains, steep cliffs, long, deep valleys and winding streams are typical of the plateau. Throughout this region are numerous limestone caves carved by erosion from underground streams and springs. These caves and springs, along with sinkholes and fractures, define a “karst” landscape.
- Geology – Igneous rock lies under the Ozark Plateau. Igneous rock makes up about 95 percent of Earth’s upper crust. However, igneous rock is mostly hidden by the more recently formed metamorphic and sedimentary rock. The Ozark Plateau is mostly composed of sedimentary rocks. Some rock units contain abundant fossils. The granite igneous rocks of this region, exposed in southeast Missouri, range from 1.2 to 1.8 billion years old! Limestone and chert are two of these that are common in the Springfield Plateau. Limestone is a sedimentary rock. It is composed primarily of the mineral calcite. Chert is also a sedimentary rock, composed of sedimentary fine-grained silica. Dolostone and sandstone are common sedimentary rocks exposed in the Salem Plateau. Dolostone is composed of the mineral dolomite, and sandstone is composed of sand-sized grains, mostly quartz particles cemented by materials including calcite, silica or clay.
- Forming the Ozark Plateau – The Ozark Plateau was formed a long time ago. A plateau is a raised land formation with a flat top. Wind, rain and rivers have eroded the Ozark Plateau over millions of years, creating steep valleys. This erosional continues today. The Ozark Plateau was once a shallow continental shelf underwater.
- Uplifting an underwater shelf – The shelf that became the Ozark Plateau was alternately above or under water for millions of years. First, shells of sea organisms accumulated to form sediment layers of that became limestone and dolostone units. Later, when the area to the north was above sea level, streams flowed down the land and into the ocean, depositing layers of sediment on the shelf. These layers were made of sand, silt and clay and eventually hardened into sandstone, siltstone and shale. During this time continental plates were moving very slowly. Later (about 320 million years ago), when two plates collided to form the Ouachita Mountains, portions of the sea floor that were farther from the collision were also gently lifted above the water to form a flat plateau. This uplifted plateau is the Ozark Plateau.
- Caves and springs (karst topography) – The groundwater in the Ozark Plateau has weak acids generated by organic acids from decaying vegetation in the soils and rainwater. This acidic water filters into the limestone cracks and dolostone, dissolving the rocks and creating openings we call caves. The result is a network of underground caves, springs and sinkholes. These are known as karst features.
- Three divisions of the Ozark Plateau – The Salem Plateau, the Springfield Plateau, and the Boston Mountain Plateau make up three distinct divisions of the Ozark Plateau. The Boston Mountains are the southernmost and highest plateau. They are dominated by shale, siltstone and sandstone. The Springfield Plateau is formed by limestone and chert. The Salem Plateau is the lowest and most northern section of the Ozark Plateau, containing the oldest sedimentary rock formations of the Plateau.
- Forests – Many different kinds of plants grow on the Ozark Plateau because of its variety of soil. Fertile limestone soil is found in some regions. Hardwood forests of white and red oak and hickory do well in these soils. Many plants grow on this type of forest floor. Beech and sugar maple trees thrive in the cool, moist ravines. Pine forests grow in the thinner, rocky soils of south-facing slopes, and In the lower floodplains, elm, box elder and black walnut trees thrive. False foxglove and prairie-fringed orchids are only two of many unusual Ozark plants.
- Water – Ozark streams drain into the White, Buffalo, Little Red, Cache and Black rivers. There are many large man-made lakes here as well where trout, catfish and bass are abundant. Wastes from industry, mining and large poultry farms all affect the water. Maintaining good water quality is crucial to preserving the fragile Ozark habitat. The U.S. Fish and Wildlife Service, the Arkansas Game and Fish Commission and many other public and private partners are working to preserve this habitat.
- Wildlife – Some of the same animals that live on the Ozark Plateau live in the Ouachita Mountains. But some animals that live in the Ozarks are not found anywhere else. Several species of bats that live in Ozark caves are endangered. Many other unusual species are threatened such as the Ozark cavefish, blind grotto salamander, Ozark hellbender and Ozark snaketail dragonfly.
- Seat participants in the lobby for an introduction to mountain formations and the Ozarks and Ouachitas.
- Explain to the participants that they will be asked to visit the replica ranges and list five key characteristics for each range.
- Once all participants have their lists, gather them into the lobby to discuss the differences between the ranges and compare what they found.
- Conclude with the comparison of the differences in the two mountain ranges.
Younger participants can begin in a classroom with note cards and draw mountains.
- How was the Ozark Mountain range formed?
- How was the Ouachita Mountain range formed?
- What other ways are mountains formed?
Arkansas – state in south central United States with an area of 53,103 square miles (137,537 sq. km), capital Little Rock; also a river flowing east and southeast from central Colorado into the Mississippi in southeast Arkansas. 1450 mi. (2335 km) long
Geology – science that deals with the dynamics and physical history of the earth, its rocks and the physical, chemical and biological changes of the earth
Mountain formations – natural elevation of the Earth’s surface having considerable mass, generally steep sides and a height greater that that of a hill
Ouachitas – mountain range in west central Arkansas, southeastern Oklahoma and northeast Texas with subterranean roots that may extend as far as central Texas; the Ouachitas and the Ozark Mountains form the U.S. Interior Highlands, the only major mountainous region between the Rocky Mountains and the Appalachian Mountains
Ozarks – a physiographic, geologic and cultural highland region of the central United States covering much of the south half of Missouri, an extensive portion of northwest and north central Arkansas and extending westward into northeast Oklahoma and extreme southeast Kansas; although refered to as mountains, the region is actually a high and deeply dissected plateau