INTRODUCTION. 15 



the continental shelf at the margin of which they have cut deep gorges. The 

 Hudson Eiver channel is particularly well marked and has been shown to 

 extend almost uninterruptedly to the edge of the shelf over 100 miles to the 

 east of its present mouth. The same is true of Chesapeake Bay. This sheet 

 of water so broad and deep to-day and affording the great highway of com- 

 merce for Virginia's export trade as well as the unparalleled local transpor- 

 tation facilities for the tidewater country, is the submerged lower valley of 

 the old Susquehanna Eiver which flowed across Maryland and Virginia and 

 found its way seaward past the Capes, its channel continuing across the sea 

 floor of the submarine portion of the Coastal Plain. 



The combined width of the submarine and subaerial portions of the 

 Coastal Plain province is quite uniform along the entire eastern border of 

 the continent, being approximately 250 miles. In Florida and Georgia the 

 subaerial portion is over 150 miles wide, the eastern submerged portion being 

 much narrower, and along the southeastern shore of the peninsula of Florida 

 it is almost wanting. Northward the submerged portion gradually increases 

 in width, while the subaerial portion becomes narrower. Except in the 

 region of Cape Hatteras, where the submarine belt becomes narrower with a 

 corresponding increase in width of the subaerial belt, this gradual change 

 continues as far as the southern part of Massachusetts, beyond which the 

 subaerial portion disappears altogether through the submergence of the 

 entire Coastal Plain province. Off Newfoundland the continual shelf is 

 about oOO miles in width. 



The surface of the Coastal Plain has a gentle slope from the "fall-line" 

 to the east, gradually declining eastward from five feet to the mile to one 

 foot or even less, except in the vicinity of the Piedmont Plateau, where the 

 slope is occasionally as great as 10 to 15 feet to the mile and in a few 

 instances even more rapid. The submerged portion which slopes even less 

 rapidly eastward is monotonously flat as desposition has destroyed most of 

 the irregularities produced by erosion when it formed a part of the land 

 rarea. The slight elevation of the subaerial portion which seldom reaches 

 400 feet, and is for the most part less than half that amount, has prevented 

 the streams from cutting valleys of great depth. The country, however, 

 in the vicinity of the larger streams shows considerable relief, although the 

 variations in altitude are only a few hundred feet. Throughout a great 

 portion of the area this relief is veiy inconsiderable, the streams flowing in 

 open valleys at only a slightly lower level than the broad flat divides. 



The land portion, or the subaerial division, of the Coastal Plain province 

 is marked by the presence of many bays and estuaries, representing sub- 



