image

Geology Resources

Arkansas is divided topographically into two major provinces along a northeast-southwest trending “fall line”. This imaginary line divides resistant sedimentary Paleozoic rocks of the Interior Highlands from unconsolidated sediments of the Gulf Coastal Plain. Each province is further divided into five physiographic provinces of Arkansas.

image
Physiographic regions of Arkansas

View details »

image
Map of the Physiographic Provinces of Arkansas

View details »

image
Physiographic Provinces of Arkansas Poster

View details »

Arkansas River Valley
Arkansas River Valley Province

The Arkansas River Valley is a low-lying province surrounding the valley of the Arkansas River and its major tributaries. The highest point in Arkansas (Mount Magazine – 2753 ft.) is found among several mountain ridges that rise above the lowlands in the southern portion of the river valley. The mountain ridges are separated by broad valleys. Surface rocks in this province consist of a sequence of coal-bearing sandstones and shales.


Geology of the Arkansas River Valley

The Arkansas River Valley represents the northern extent of the Ouachita orogenic (mountain building) system in Arkansas. Once flat-lying, these Pennsylvanian sedimentary rocks have been compressed into well developed east-west trending open folds (anticlines and synclines) and faults, which gradually diminish northward into the Ozark Plateau Province.

Shaded relief map of the Arkansas River Valley

Shaded relief map of the Arkansas River Valley. Canoe and cigar-shaped structures represent anticlines and synclines.

Diagram showing the structures in the river valley

Diagram showing structures in the river valley. Most of the higher elevations exposed in the river valley are actually what is left of the center if the syncline. Nearly all of the rocks from the surrounding anticlinal limbs have been eroded away.


The Arkansas River and surrounding low-lying land as seen from Petit Jean Mountain

The Arkansas River and surrounding low-lying land as seen from Petit Jean Mountain

Sandstone and shale on Magazine Mountain

Sandstone and shale on Magazine Mountain

Ouachita Mountain Valley
Ouachita Mountains Province

The Ouachita Mountains consists of a series of sharp ridges, mostly east-west trending and often buckled and distorted, separated by narrow to broad valleys. Surface rocks from this province are mostly shales, sandstone, novaculite, chert and minor limestone.

Ouachita shaded relief map

Shaded relief map of the Arkansas River Valley. Canoe and cigar-shaped structures represent anticlines and synclines.


Geology of the Ouachita Mountains

The Ouachita Mountain contains thick sequences of sedimentary rocks deposited in a deep ocean basin that have been uplifted and compressed northward into east-west trending complex folds (anticlines and synclines) and thrust faults due to major orogenic (mountain building) processes. The most intensely deformed area is in the central portion of the fold belt.

Syniclinal fold in rocks near Hot Springs Arkansas

Synlinal fold in rocks near Hot Springs Arkansas

Vertical bedding at Y-City

Vertical bedding at Y-City


Stanley Shale in Hot Springs Arkansas

Inclined bedding in the Stanley Shale in Hot Springs Arkansas

Folded strata in he Big Fork Chert near Hot Springs

Folded strata in the Big Fork Chert near Hot Springs

Gulf Coastal Plain
West Gulf Coastal Plain Province

The West Gulf Coastal Plain is a south sloping plain of gently rolling hills and can be divided into two areas based on the age of the rocks. The area in orange consists of Tertiary clays, sands and silts with lignite deposits and Quaternary gravels, sands and clays.

The area in green consists of Cretaceous sand, clay, gravel, marl, limestone, chalk and Quaternary sand and gravel.

Cretaceous, Tertiary and Qaternary image
Geology of the Tertiary Area

This part of the state contains mostly early Tertiary aged rocks, mostly sands, silts and clays, deposited in streams, swamps and shallow marine water. Lignite beds occur throughout the sequence. Quaternary alluvium and terrace deposits are present from rivers in the area. Underneath the Tertiary deposits are the sands, gravels, limestones, chalks and marls seen in the Cretaceous area.

Wilcox sand and clays with lignite beds.

Wilcox sand and clays with lignite beds.

Tertiary outcrop near Little Rock

Tertiary outcrop near Little Rock


Geology of the Cretaceous Area

The geology of the Cretaceous area within the West Gulf Coastal Plain is characterized by gently southward-dipping sedimentary rocks deposited mostly in shallow marine water of the Gulf of Mexico that once extended into Arkansas. Alluvium deposits from older and present day streams are common

The Annona Formation at Whitecliffs near Millwood Lake

The Annona Formation at Whitecliffs near Millwood Lake

Gypsum beds in the DeQueen Limestone

Gypsum beds in the DeQueen Limestone

Tokio Gravel

Tokio Gravel

Mississippi River Alluvial Plain
Mississippi River Alluvial Plain Province

The Mississippi River Alluvial Plain is a relatively level plain with elevations varying between 100-300 feet except for Crowley’s Ridge with some of its highest points reaching 400-560 feet. This province contains mostly unconsolidated sediments such as sand, clay, silt, gravel and loess.


Geology of the Mississippi River Alluvial Plain

The geology of the Mississippi River Alluvial Plain is characterized by unconsolidated flat-lying alluvium from present day and older streams, overlying poorly consolidated Tertiary formations that are dipping slightly to the south.

Quaternary terrace sand and gravels on top of Crowleys Ridge

Quaternary terrace sand and gravels on top of Crowley’s Ridge

Tertiary deposits

Tertiary deposits


View of flat-lying alluvial plain

View of flat-lying alluvial plain

Loess canyon on Crowleys Ridge

Loess canyon on Crowley’s Ridge