The projection of the direction of the lower end of the stream may be said to be the axis of the extra-debouchure current. From this axis, where the flow is strongest, the movement diverges more or less to right and left. Since the velocity of the diverging water is reduced more rapidly than that of the water which follows the axis of flow, deposition is likely to take place faster on either side of the axis than along the axis itself. The result is that the extra-debouchure current tends to build up levee-like ridges on either side, making a sort of sluice-way for itself. This sluice-way is gradually extended seaward, and at the same time gradually filled. As its capacity is reduced, more and more of the water flows over its sides. Presently the escape of the water over the little side-levees will develop a break at some point, and a line of distinct flow then diverges from the main current. This distributary repeats the history of its main. Thus the processes of levee-building, channel-filling, and levee-breaking follow one another, until some such system of currents as shown in Fig. 190 is developed. The result is that a delta’s growth is not simply in the line of extension of the main stream, but in a more or less semicircular area, the center of the circle being a point slightly below the position of the debouchure of the stream when the delta began. At any stage in its development the margin of the delta is more or less crenate (Fig. 191), or characterized by delta fingers (Fig. 190), the projections corresponding to the positions of the debouchures of the latest streams flowing across it. The extreme ends of the delta lobes (Fig. 190), and of groups of the delta fingers, often have something of the shape of the Greek letter from which the name originated, but the resemblance in form between a well-developed delta and the Greek letter is not striking. Deltas are sometimes built in bays, and in such cases their forms are predetermined on all sides but one. The head of a delta is sometimes arbitrarily located at the point where the first distributaries are given off. Since this point shifts widely with time, the definition can hardly be accepted. On this basis the head of the Mississippi delta is about 200 miles above its lower end. In reality it is much farther north. [Illustration: +Fig.+ 190.—The terminus of the delta of the Mississippi. (C. and G. Survey.)] [Illustration: +Fig.+ 191.—A miniature delta.]