At the beginning of its development the material of the alluvial cone is deposited much as in a talus cone (compare Fig. 170 with Figs. 67 and 68). Its deposition chokes the channel of the stream, and some of the water then seeks new courses to right and left of the apex of the deposit. This expands the area of deposition to right and left, while the water which flows over it lengthens it in the direction of flow. The course and behavior of the water after reaching an alluvial cone is instructive. As its velocity is checked, deposition often takes place in the channel, diminishing its capacity. As the channel is filled up, the water tends to overflow on either side. The overflowing water, being shallow, has so little velocity that much of its load is dropped on either margin of the channel, building up levees. The water ever and anon breaks through the levees, giving rise to distributary streams, each of which aggrades its channel and builds its own miniature levees (Fig. 171). Not rarely this process of channel-filling and levee-building goes on until the channels of the little rivulets are above the general level of the cone on which they rest. The rivulet then runs in a groove on the crest of a little ridge. The channels on the surfaces of fans and cones are fewest and deepest at their heads, and more numerous and shallower below. In some cases the surface-water disappears altogether before the outer border of the fan is reached, by sinking into the débris. [Illustration: +Fig.+ 170.—A talus cone. North Greenland Coast. The talus cone reaches the sea-level. Drawn from photograph.] Alluvial fans and cones have various forms, and often attain considerable dimensions. Their angles of slope depend on the amount of reduction of velocity which the depositing water suffers, and the amount and kind of load which it carries. The maximum slope of the cone is the angle at which the loose material involved will lie. The minimum slope of the fan, on the other hand, approaches horizontality. If many alluvial fans develop in proximity to one another, as at the base of a mountain range, they may expand laterally until they merge. A long succession of them may thus give rise to an extensive _alluvial piedmont plain_, or a _compound alluvial fan_. The lower edge of such a fan is often somewhat lobate. Such plains exist along the bases of many mountain ranges (Pl. VI), and may be seen in miniature even along low ridges. [Illustration: +Fig.+ 171.—Miniature levees on an alluvial cone. Slope of Gray Peak, Colo. (R. T. Chamberlin.)]