=Terraces.= While aggrading streams thus tend to shift their channels, degrading streams, on the contrary, become more and more deeply intrenched in their valleys. It often occurs that a stream, after having built a flood plain, ceases to aggrade its bed because of a lessened load or for other reasons, such as an uplift of the region, and begins instead to degrade it. It leaves the original flood plain out of reach of even the highest floods. When again it reaches grade at a lower level it produces a new flood plain by lateral erosion in the older deposits, remnants of which stand as terraces on one or both sides of the valley. In this way a valley may be lined with a succession of terraces at different levels, each level representing an abandoned flood plain. [Illustration: Fig. 76. Development of a Meander The dotted line in _a_, _b_, and _c_ shows the stage preceding that indicate by the unbroken line] =Meanders.= Valleys aggraded with fine waste form well-nigh level plains over which streams wind from side to side of a direct course in symmetric bends known as meanders, from the name of a winding river of Asia Minor. The giant Mississippi has developed meanders with a radius of one and one half miles, but a little creek may display on its meadow as perfect curves only a rod or so in radius. On the flood plain of either river or creek we may find examples of the successive stages in the development of the meander, from its beginning in the slight initial bend sufficient to deflect the current against the outer side. Eroding here and depositing on the inner side of the bend, it gradually reaches first the open bend (Fig. 76, _a_) whose width and length are not far from equal, and later that of the horseshoe meander (Fig. 76, _b_) whose diameter transverse to the course of the stream is much greater than that parallel with it. Little by little the neck of land projecting into the bend is narrowed, until at last it is cut through and a "cut-off" is established. The old channel is now silted up at both ends and becomes a crescentic lagoon (Fig. 76, _c_), or oxbow lake, which fills gradually to an arc-shaped shallow depression. [Illustration: Fig. 77. Map of a portion of the Flood Plain of the Missouri River Each small square represents one square mile. How wide is the flood plain of the Missouri? How wide is the flood plain of the Big Sioux? Why is the latter river deflected down valley on entering the flood plain of the master stream? How do the meanders of the two rivers compare in size? How does the width of each flood plain compare with the width of the belt occupied by the meanders of the river? Do you find traces of any former channels?]