Water gap
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A water gap is an opening or notch which flowing water has carved through a mountain range. Water gaps often offer a practical route for roads and railroads to cross a mountain ridge.
[edit] Geological implications
A water gap is usually an indication of a river that is older than the current topography. The river likely established its course when the landform was at a low elevation, with a very low stream gradient and a thick layer of unconsolidated sediment. The river therefore established its channel without regard for the deeper layers of rock.
A renewed period of uplift caused increased erosion, removal of the overlying sediments and exposure of the underlying rock layers. Rejuvenated drainage caused streams to follow weaker layers of rock, but larger watercourses, as long as the uplift did not exceed the rate of erosion, were able to cut through the harder rocks which generally became ridges. Water gaps are common in the Ridge-and-valley Appalachians of eastern North America.
Alternatively, a water gap can be formed through headward erosion of two streams on opposite sides of a ridge, ultimately resulting in the capture of one stream by the other.
