Chapter 3
The Louisiana Uplands
By Kevin Doerr
Introduction
The Louisiana Uplands are home to the highest
elevations in the state and are made of the oldest rock. They are hilly (as
hilly as Louisiana gets), unlike the flat Lowlands. The Uplands are composed of
two regions: Tertiary Hills and Terraces. This is an example of the
Louisiana Uplands, complete with pine trees.
photo by Fred and Suzi Dow
For all the Louisiana maps you could need, click here.
Foundation
The first thing one considers in construction is the foundation. The construction if land is no different. It has to sit on something, right? The hills of the Louisiana Uplands are formed on layers of sedimentary rocks that date from the Mesozoic and Cenozoic Eras. These layers get progressively younger as they approach the Gulf of Mexico and tilt downward in the same direction. This tilting is the result of a force called Isostatic Lift. Basically, the alluvial deposits from rivers in the southern half of the state are pushing the rock layers down, causing the opposite ends to lift up. This gives the hills their elevation.
Hills
Louisiana’s hills were formed when layers of rock tilted upward, as mentioned above. Some of these layers were made of very resistant rock while others were made of weak rock. If you were to travel across the surface of Louisiana you would encounter bands of resistant and weak rock, alternating as you moved towards the gulf coast. Over time streams and rivers eroded away sections of these layers. The weaker sections, of course, eroded away faster than the resistant ones, channeling the water. This process eats away the weak rock and leaves behind the resistant rock, producing hills and valleys. Louisiana is home to three series of hills, or wolds. They are: The Kisatchie Wold, The Nacogdoches Wold, and the Ouachita Hills.
Wolds
The Kisatchie Wold enters the state from the west in Vernon Parish. Parts of this wold can also be found in Natchitoches, Rapides, and LaSalle Parishes. It ends near the Ouachita River. The wold is formed from layers of the very resistant Catahoula sandstone, deposited in the Miocene and Oligocene Epochs. The western part of these hills can exceed 400 feet above sea level and is very rugged. The eastern end of the Kisatchie Wold are fragmented because of stream erosion, breaking them into The Chalk Hills and Sicily Island. These hills exceed 300 feet and are made primarily of volcanic ash and sandstone.
To the north of the Kisatchie Wold is the Nacogdoches Wold. This wold is older than the Kisatchie and has higher elevations. It enters Louisiana via Sabine Parish and passes through Natchitoches, Winn, Bienville, and Claiborne parishes, then exits into Arkansas. This wold is shaped partly by the Sabine Uplift. The Sabine Uplift is a force that has pushed up layers of rock, causing the wold to form a circle around it. The Red River crosses the wold twice, creating the Caddo Hills in the north. The crown jewel of the Nacogdoches Wold is Mt. Driskill. While not actually a mountain, Driskill is the highest point in the state of Louisiana at 535 feet above sea level.
The Ouachita Hills are the last significant string of hills in Louisiana. They are less than 300 feet above sea level. They begin just north of the eastern end of the Kisatchie Wold and stretch north to Arkansas. Under these hills is a force called The Monroe Uplift. While not visible from the surface, the uplift is important. This uplift produced one of the larger oil and gas fields in the state.
Terraces
Terraces are flat, gently sloping planes that rise above each other in a step-like fashion. They were created as part of the glaciation process that took place in the last ice age. Though glaciers never made it anywhere near Louisiana, their runoff did. As glaciers melted, their runoff formed rivers and streams, which in turn formed flood planes. These flood planes were carved out of Louisiana leaving behind terraces when the glaciers were gone. There are three major complexes of terraces in Louisiana: The Upland complex, The Intermediate Complex, and the Prairie Complex. The Dewyville Complex also exists in Louisiana but is far less significant.
The Upland Complex is made of a series of terraces ranging in elevations from 150 to 350 feet above sea level. It is not known how many Upland terraces there are, but they can be found in a belt across the state from Texas to Sicily Island, just south of the Kisatchie Wold. Also, there are some located in the Florida Parishes. One of the major characteristics of an Upland Terrace is its composition. Upland Terraces are typically composed of highly oxidized orange colored sands and gravels.
The next complex is The Intermediate Complex. The history of this complex is mostly unknown, as is its age. It is distinguishable from other complexes by its elevation; around 100 feet above sea level. The Intermediate Complex is located in the Florida Parishes in a narrow band but widens as it moves westward. It is composed of sand, silt and clay.
The last major complex is The Prairie Complex. It stands about 40 feet above the river planes of South Louisiana and is easily distinguished. It is the youngest major complex of terraces and is the closest to the gulf coast. Like the Intermediate Complex, the Prairie Complex is composed of sand, silt and clay.
Finally, the Dewyville Complex rises above the alluvial planes of the south. It exists in Louisiana only in the extreme eastern and western parts of the state. It is lower in elevation than the other complexes and has similar composition.
Loess
In the Pleistocene Epoch glacial outwash carried silt and redeposited it in nearby uplands. This silt was then picked up by the prevailing winds and blown away, to be deposited later against the sides of hills. This is called Loess. In Louisiana, loessel deposits can be found atop many Pleistocene Terraces including Bastrop Hills, Macon Ridge, Sicily Island and the Ouachita Hills. In some places the Loess has a thickness of over 100 feet. These deposits are located within sixty miles of an alluvial plane. In this case, the major deposits are found along the Mississippi alluvial plane.
Salt Domes
The last major geological formation in the Louisiana Uplands are Salt Domes. Salt Domes are masses of salt that push up through the rock layers above them. This happens because salt is less dense than rock and sediment and tries to push up through the layers of rock that are pushing down on it. Under heat and pressure salt becomes a plastic, much like lava. Unlike lava, salt moves very slowly. Most are covered entirely by rock such as limestone, gypsum or sulfur. These are formed when groundwater mixes with the salt deposit and forms other minerals. Most salt domes to not break the surface. If they do they are quickly eroded.
