fall line
noun
1
the fall line
Skiing the route leading straight down any particular part of a slope【滑雪】直接下滑线。
2
- a narrow zone that marks the geological boundary between an upland region and a plain, distinguished by the occurrence of falls and rapids where rivers and streams cross it瀑布线。
2.1
the Fall Line
(in the US) the zone demarcating the Piedmont from the Atlantic coastal plain（美国）美东瀑布线。