The places where water has cut a deep pass through a cuesta or high ridge are called water (or wet) gaps. They have persistent ﬂow, even if it is ephemeral. Dry gaps occur where water has ceased to communicate across such a cutting. Water gaps channel streams through low-lying valleys or narrow gorges across ranges of hills, contrary to the common impression that watercourses ﬂow out of mountains but do not cross them. When a once-blocked river regains its former course and keeps it by eroding the land as fast as uplift raises it, a water gap is said to arise through antecedent drainage, and its agent is called an antecedent stream. The Delaware Water Gap, a tall and verdant slot where the Delaware River cuts through Kittatinny Mountain near Stroudsburg, Pennsylvania, is the deﬁnitive example of this landform. Gap towns are settlements built near, and commanding, strategic water gaps.