Text Box: Coastal Plain Introduction | Topography and Geology | Sections | Embayed Section | Cape Fear Arch | Sea Islands Downwarp | Peninsular Arch 1 | Peninsular Arch 2 | East Gulf Coastal Plain | Mississippi River Alluvial 1 | Mississippi River Alluvial 2 | West Gulf Coastal Plain | Resources
             The Atlantic-Gulf Coastal Plain province in the United States extends from Cape Cod in Massachusetts southwestward to the Texas-Mexico border.  The total straight-line length of the Atlantic and Gulf coastlines in the province is 3,410 miles, but the tidal shoreline (which includes the indentations along the coast) is more than 13 times as long–40,920 miles.  The province includes all or part of 19 states; no other province includes as many.  The geomorphic characteristics of the province continue underwater as the Georges Bank for many miles north and east of Cape Cod, and extend approximately 1,000 miles into Mexico. 
Continental Shelf
Not all of the province is presently above sea level.  The submerged continental shelf is considered to be a part of the province, primarily because many coastal features on the land continue seaward onto the shelf, and a portion of the shelf was dry land until rising sea level reached its present position about 5,500 years ago.  (Some maps show this feature as a separate province.)  The continental shelf extends more than 200 miles from the Gulf coastline of southern Florida, although the average width along the Atlantic and Gulf coasts is less than 100 miles, and is only 5 miles wide off the coast of southeastern Florida.  Much of the seaward edge of the shelf, where it merges with the much more steeply dipping continental slope, is at a depth of about 600 feet.  The inclination of the continental shelf is mostly very slight, averaging about 0.1°.  The area of the continental shelf extending from Cape Cod to the Texas-Mexico border is 264,478 square miles, which is slightly more than one-third of the total area of the province. 
General Characteristics
Generally, and as used here, the term Coastal Plain refers only to the non-oceanic portion of the province.  When only this portion is considered (450,193 square miles), the province is the second largest in the conterminous United States, second to the Central Lowlands province, and is very slightly larger than the Great Plains province.  Much of the Coastal Plain is about 100-200 miles wide, although it is much narrower in the northern portion.  It is widest where it extends approximately 550 miles up the Mississippi River Valley into southern Illinois.  The inner boundary extending from near New York City to about central Alabama is the Fall Line.  This is the line, more often zone, of contact between the higher, older, more resistant metamorphic rocks of the Piedmont Plateau to the west with the lower, younger, less resistant sedimentary rocks of the Coastal Plain.  Falls and rapids in many eastward-flowing rivers occur at this contact.  The steepest and most spectacular Fall Line rapids occur at Great Falls of the Potomac, about 14 miles northwest (upriver) of Washington, D.C.  This feature, now included in Great Falls Park, consists of cascading rapids and waterfalls up to 20 feet in height; here the river drops a total of 76 feet within a 0.66-mile section of the Potomac.  The Potomac River has been effective in eroding these rocks, although the Fall Line here consists mostly of fairly resistant 750-million-year-old schists and gneisses.  Mather Gorge, just below the main area of rapids and falls, has been cut mostly within the last 35,000 years.  For a period from about 35,000 until 13,000 years ago, the Potomac River cut downward in the region of the gorge at the rate of about 2.6 feet per 1,000 years. (For discussion of the technique used and comparison of results with the cutting rate in Holtwood Gorge near Harrisburg, Pennsylvania, see the Ridge and Valley province.)
There is a distinct change of rock types along the entire Fall Line, and there is often a noticeable elevation change, even when there are no rapids or waterfalls, notably between the Potomac and Delaware rivers.  Other features include a significant escarpment north of the Rappahannock River, the Sandhills of North and South Carolina and Georgia, and cliffs at Raven State Park in North Carolina.  However, there is not always a marked topographic feature to demarcate the position of the Fall Line:  in places along the northern extent of the line there is a transition zone 5-10 miles wide, rather than an abrupt change. 
The Fall Line has significantly influenced river flow and navigation on the rivers that cross it.  Many rivers bend at the Fall Line and flow parallel to it for many miles before taking a more direct route to the Atlantic Ocean.  Most of the rivers from the James River in Richmond, Virginia and northward bend toward the south or southwest, but two rivers just south of the James–the Appomattox and Nottoway rivers–are offset toward the north.  Also, the Fall Line is the inland limit of navigation along rivers inward from the Atlantic Ocean.  Because the Fall Line was a natural site for ports, as well as for the development of manufacturing due to the availability of water and hydroelectric power, many cities developed along this feature, including New York (although some do not consider it to be a Fall Line city), Trenton, Philadelphia, Baltimore, Richmond, Raleigh, Columbia (South Carolina), as well as the Georgia cities of Augusta, Macon and Columbus. 
Coastal Plain Introduction | Topography and Geology | Sections | Embayed Section | Cape Fear Arch | Sea Islands Downwarp | Peninsular Arch 1 | Peninsular Arch 2 | East Gulf Coastal Plain | Mississippi River Alluvial 1 | Mississippi River Alluvial 2 | West Gulf Coastal Plain | Resources