Read this: The coastal plain transitions to the Piedmont region along the Atlantic Seaboard fall line, a line which marks the elevation at which waterfalls first appear on streams and rivers. The Piedmont region of central North Carolina is the state's most urbanized and densely populated section. It consists of gently rolling countryside frequently broken by hills or low mountain ridges. Small, isolated, and deeply eroded mountain ranges and peaks are located in the Piedmont, including the Sauratown Mountains, Pilot Mountain, the Uwharrie Mountains, Crowder's Mountain, King's Pinnacle, the Brushy Mountains, and the South Mountains. The Piedmont ranges from about 300 to 400 feet (91 to 122 m) in elevation in the east to over 1,000 feet (300 m) in the west. Because of the rapid population growth in the Piedmont, a significant part of the rural area in this region is being transformed into suburbs with shopping centers, housing, and corporate offices. Agriculture is steadily declining in importance. The major rivers of the Piedmont, such as the Yadkin and Catawba, tend to be fast-flowing, shallow, and narrow.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What do you call the line that marks the elevation that waterfalls first appear on rivers?
Atlantic Seaboard fall line