Thus banked off from the stream, the outer portions of a flood plain are often ill-drained and swampy, and here vegetal deposits, such as peat, may be interbedded with river silts. A map of a wide flood plain, such as that of the Mississippi or the Missouri (Fig. 77), shows that the courses of the tributaries on entering it are deflected downstream. Why? The aggrading streams by which flood plains are constructed gradually build their immediate banks and beds to higher and higher levels, and therefore find it easy at times of great floods to break their natural embankments and take new courses over the plain. In this way they aggrade each portion of it in turn by means of their shifting channels. =Braided channels.= A river actively engaged in aggrading its valley with coarse waste builds a flood plain of comparatively steep gradient and often flows down it in a fairly direct course and through a network of braided channels. From time to time a channel becomes choked with waste, and the water no longer finding room in it breaks out and cuts and builds itself a new way which reunites down valley with the other channels. Thus there becomes established a network of ever-changing channels inclosing low islands of sand and gravel. [Illustration: Fig. 73. Terraced Valley of River in Central Asia] [Illustration: Fig. 74. Terraces carved in Alluvial Deposits] Which is older, the rock floor of the valley or the river deposits which fill it? What are the relative ages of terraces _a_, _b_, _c_, and _e_? It will be noted that the remnants of the higher flood plains have not been swept away by the meandering river, as it swung from side to side of the valley at lower levels, because they have been defended by ledges of hard rock in the projecting spurs of the initial valley. The stream has encountered such defending ledges at the point marked _d_] [Illustration: Fig. 75. River Terraces of Rock covered with Alluvium _c_, recent flood plain of the river. To what processes is it due? Account for the alluvium at _a_ and _b_ and on the opposite side of the valley at the same levels. Which is the older? Account for the flat rock floors on which these deposits of alluvium rest. Give the entire history which may be read in the section]