|The Fall Line and major cities of the Eastern U.S.
(see text below image)
| On the left is an unmodified image from the Multi-angle Imaging SpectroRadiometer (MISR) taken from NASA's Terra satellite on March 24, 2000 and shown on the Earth Science Picture of the Day on September 8, 2001. It shows the western Atlantic Ocean and eastern U.S. from southernmost New York through New Jersey, Pennsylvania, Delaware, Maryland and Virginia to northernmost North Carolina. Chesapeake Bay is at center right.
| The image shows the geologic texture of the region, and the various geologic provinces are labelled on the middle image. The most evident is in the northwest: the Valley and Ridge province of folded and faulted Paleozoic sedimentary rocks, where the rock layers make landforms that in this image look like the grain of old wood. Next to the southeast is the Blue Ridge, which consists mostly of Precambrian and Paleozoic metamorphic rocks. Southeast of it is the Piedmont, a hilly region of Precambrian and Paleozoic igneous and high-grade metamorphic rocks. Farthest southeast is the Coastal Plain, a region of undeformed and thus nearly flat-lying Cretaceous and Cenozoic sedimentary rocks and sediments.
(The geologic boundaries on the middle image were only qualitatively "eyeballed" from the USGS Geologic Map of the U.S. and should not be taken as a precise authoritative statement of professionally mapped geologic contacts.)
| The Fall Line is the boundary between the hilly Piedmont and the flat Coastal Plain. It's called the Fall Line because the first falls or rapids in rivers that one encounters as one comes inland from the ocean are usally found at this boundary, as the streams drop off the Piedmont and onto the Coastal Plain.
Note that, as you compare the left two images, the Fall Line seems to have a white dusting that almost looks like snow. However, it's not snow, as the image at the right shows.
|The white that marks the Fall Line in this image is the string of major cities that lie on the Fall Line (see the image on the right). Where rivers cross the Fall Line, cities commonly develop because ships reach the inland limit of navigation and must surrender their loads to land-based carriers. Philadelphia, Baltimore, and Richmond developed this way without any planning. Washington was cited on the Potomac River deliberately at the Fall Line for the same navigational reasons.
Farther south, Columbia SC is a Fall Line city. on the Congaree River. In Georgia, Augusta, Milledgeville, Macon, and Columbus are Fall Line cities on the Savannah, Oconee, Ocmulgee, and Chattahoochee Rivers. In Alabama, Tuscaloosa is a Fall Line city on the Black Warrior River. All these cities were the economically inevitable result of the intersection of rivers with a geologic boundary between the hilly Piedmont and low flat Coastal Plain.