The point of emergence for a stream that has been ﬂowing underground—or that once disappeared underground to ﬂow through a cavern, perhaps—and that regains the surface is called a resurgence. Often the resurgence occurs where an underground stream ﬂowing through permeable rock strata encounters an impermeable rock layer. If the latter intersects the surface, it can guide the stream back above ground. One notable example is the spot where the Echo River in Kentucky emerges from the Mammoth Cave system. Another is on the Metolius, which begins abruptly northwest of Black Butte, Oregon.