The Appalachian Piedmont and Atlantic Coastal Plain are physiographic provinces that are separated by the fall zone. The fall zone (also called the Fall Line) is the contact where the hard crystalline rocks of the Piedmont dip under and disappear beneath the sediments of the Coastal Plain. The landscape and rock types shown in northern Delaware are classical examples of the larger geologic features that dominate the geology of eastern North America.
There are reasons why the major cities line up parallel to the coast and in accordance with the trend of the mountains. The fall zone was the limit of navigation to the European explorers. This and the availability of fresh water suggested the location of the initial settlements. Earth materials in the region—stone, sand and gravel, brick and china clay, mica, and feldspar—sustained economic development for several hundred years. Waterpower provided energy essential to the Industrial Revolution. Commerce and communication by ship benefited by access to tidewater. Northeast-southwest land travel and communication benefited from the relatively flat topography of the inner Coastal Plain. No wonder that our initial population concentrated along the fall zone. Delaware's Piedmont provides outstanding illustrations of the influence of geology on our history and society. Our forebears lived close to the land and understood its basic dictates. Pressures of land use and environmental protection now prompt us to rediscover the essentials of geology and apply modern science to advance applications appropriate to today’s needs.
Image of the Fall Line from the USGS National Atlas: http://nationalatlas.gov/articles/geology/features/fallline.html