Alabama Landscapes

The Fall Line

According to the Glossary of Geology (American Geological Institute) a Fall Line is:

" an imaginary line or narrow zone connecting the waterfalls on several adjacent near parallel rivers, marking the points where these rivers make a sudden descent from an upland to a lowland ..."

This term was originally used to describe the east-facing cliff (or scarp) from New Jersey south to the Carolinas that separates hard Piedmont rocks of the Appalachian Highlands from the softer Atlantic Plain rocks.   Although waterfalls and rapids long the Fall Line  effectively stopped water transport inlands they provided power for  water wheels and flumes.  hence, many of the early cities during colonial times developed along the Fall Line.  They included Trenton, Baltimore, Richmond, Raleigh and Columbia. 
 

The term is more generally used as the boundary between the Atlantic Plain and the  eastern and southern edge of the Appalachian  Highlands (Figure FL1).  In Alabama, the term "Fall Line" is also used to describe the  locations upstream that major rivers (Black Warrior, Cahaba, Coosa and Tallapoosa) were no longer navigable before the construction of locks and dams.  This "Fall Line" approximates the boundary between the Atlantic Plain and the Appalachian Highlands. 

Figure FL1.  Location of the Fall Line in Alabama.  B: Birmingham, C: Clanton, H: Huntsville, M: Montgomery, O: Opelika, T: Tuscumbia, Tu: Tuscaloosa.  1-4 locations of topographic profiles (see Figure FL2).

Along the Fall Line, the Coastal Plain consists of sand and gravel of  the Tuscaloosa Group that support the Fall Line Hills physiographic district.   The sand and gravel are in contact with not only metamorphic rocks of the Piedmont Upland to the south and east of Clanton--the "true" Fall Line in its original definition--but also Paleozoic sedimentary rocks of the Valley and Ridge, Cumberland Plateau and Highland Rim (Figure FL1).  

The physiographic expression of the Fall Line varies along its length in Alabama.

Where the Coastal Plain is in contact with the Piedmont in the eastern part of the state a definite slope change occurs, with the Piedmont rocks occupying the high ground (Figure FL2).   Elevation changes in the order of 150-200 feet occur.  In the 1800s several small towns sprung up here to utilize water power (e.g., Griffin Mill, Whatley Mill).

Figure FL2 Topographic profile across the Fall Line. Black arrow: boundary between the Coastal Plain and the Piedmont Upland (Profile 1 on Figure FL1)

 Between Montgomery and Clanton, the Fall Line between Piedmont and the Coastal Plain is a lowland about 10-12 miles wide (Figure FL3), caused by the erosive power of the Coosa River system.  Coastal Plain rocks generally sit at a maximum elevation of 600 feet whereas the Piedmont Upland is at about 790 feet.   One major exception is in the vicinity of Union Grove (Chilton County, Exit 219 I-65) where  a thin cap of Coastal Plain rocks covers Piedmont rocks at an elevation of about 780 feet.

Figure FL3  Topographic profile across the Fall Line. Black arrow: boundary between the Coastal Plain and the Piedmont Upland (Profile 2 on Figure FL1)

In the central part of the state, the Coastal Plain is in contact with the Valley and Ridge.  Here the Fall Line is a diffuse zone (between the broken arrows on Figure FL4) where Coastal Plain sediments occupy the higher elevations and Valley and Ridge rocks occupy the valleys. 

Figure FL4 Topographic profile across the Fall Line. Black arrows: transition between the Coastal Plain and the Valley and Ridge (Profile 3 on Figure FL1)
 

The complex relationship between geology and physiography along the Fall Line in the central part of the state can be seen along the Cahaba River  in  Bibb County.  

In most of the northwestern part of the state, the Fall Line occurs along the boundary of the Coastal Plain and the Cumberland Plateau (Figure FL5).  Here an elevation drop of about 120 feet occurs

Figure FL5 Topographic profile across the Fall Line. Black arrow: boundary between the Coastal Plain and the Cumberland Plateau (Profile 4 on Figure FL1)
 

An abbreviated version of this article occus in the Encyclopedia of Alabama (http://eoa.auburn.edu/face/Article.jsp?id=h-1618).

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