What Does Fall Line Mean?
The fall line is the geological demarcation between the highlands and the plains. It's that narrow stretch of land where rivers and streams are marked by waterfalls and rapids.
Within the fall line (or fall zone, as some call it) the transition between the uplands and lowlands is so sudden and so fierce, a stream's water may discover itself in a losing battle with gravity as it suddenly and unexpectedly falls an indeterminate distance, temporarily unsupported by its familiar streambed, only to continue its journey to the sea at a lower, flatter elevation as it lazily crosses the plains.
Trenchlesspedia Explains Fall Line
The fall line is a byproduct of the same geological forces that build mountains, epeirogeny and orogeny.
Orogeny is folding and faulting, a process not unlike kneading dough -- minor upheavals in unstable areas of the Earth's crust rise and fold onto themselves over many millennia as they're subjected to compressive, lateral forces from the surrounding more stable crust. Dropoffs forming along the crustal boundaries are the upland boundary of the fall line. Later (or geologically more recent) upheavals, not as yet absorbed by orogeny's mountain-building work are epeirogeny and mark the lowland side of the fall line
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