=The cut-off of the meander.=—As the meander swings toward its extreme position it becomes more and more closely looped. Adjacent loops thus approach nearer and nearer to each other, but in the successive positions a nearly stationary point is established near where the river makes its sharpest turn (Fig. 170, _G_, and Fig. 454, p. 417). At length the neck of land which separates meanders is so narrow that in the next freshet a temporary jamming of logs within the channel may direct the waters across the neck, and once started in the new direction a channel is scoured out in the soft silt. Thus by a breaking through of the bank of the stream, a so-called “crevasse”, the river suddenly straightens its course, though up to this time it has steadily become more and more sharply serpentine. After the cut-off has occurred, the old channel may for a time continue to be used by the stream in common with the new one, but the advantage in velocity of current being with the cut-off, the old channel contains slacker water and so begins to fill with silt both at the beginning and the end of the loop. Eventually closed up at both ends, this loop or “ox-bow” is entirely separated from the new channel, and once abandoned of the stream is transformed into an ox-bow lake (Fig. 171 and p. 415). [Illustration: FIG. 171.—An ox-bow lake in the flood plain of a river.] =Meander scars.=—Swinging as it occasionally does in its meanderings quite across the flood plain and against the bank of the earlier degrading river in this section, the meander at times scours the high bank which bounds the flood plain, and undermining it in the same manner, it excavates a recess of amphitheatral form which is known as a meander _scar_ (Fig. 172). At length the entire bank is scarred in this manner so as to present to the stream a series of concave scallops separated by sharp intermediate salients of cuspate form. [Illustration: FIG. 172.—Schematic representation of a series of river terraces. _a_, _b_, _c_, _e_, successive terraces in order of age. _d_, _d_, _d_, _d_, terrace slopes formed of meander scars.] =River terraces.=—Whenever the river’s history is interrupted by a small uplift, or the base level is for any reason lowered, the stream at once begins to sink its channel into the flood plain. Once more flowing upon a low grade, it again meanders, and so produces new walls at a lower level, but formed, like the first, of intersecting meander scars. Thus there is produced a new flood plain with cliff and terrace above, which is known as a _river terrace_. A succession of uplifts or of depressions of the base level yields terraces in series, as they appear schematically represented in Fig. 172. Such terraces are to be found well developed upon most of our larger rivers to the northward of the Ohio and Missouri. The highest terrace is obviously the remnant of the earliest flood plain, as the lowest represents the latest.