THE WORK OF RUNNING WATER. 1^9 



is not confined to a definite channel, and the deposits made are there- 

 fore spread more or less on either side of the line which represents the 

 continuation of the stream's course. 



As the depth of the water into which the stream flows increases, 

 the current diminishes. Out to the point where the depth of the stand- 

 ing water is less than the depth of the current, the latter affects the 

 bottom, and the surface of the deposits made slopes gently seaward; 

 but where the depth of the standing water is such that the projected 

 stream current is ineffective at the bottom, all the load rolled along the 

 bottom is dropped, and a depositional slope is established (Fig. 188), 

 its upper edge being below sea-level by an amount corresponding roughly 

 to the depth of the current which brings the detritus. The outer slope 

 is relatively steep and well-defined where the detritus is coarse, and 

 relatively gentle and ill-defined where it is fine. Thus the stream 

 tends to construct a sort of platform in the water just beyond its debou- 

 chure. The successive deposits on the outer abrupt slope will dip con- 

 formably with its surface (Fig. 189). The finest sediment will be carried 

 beyond the steep slope, and conform to the topography of the bottom 

 beyond (c, Fig. 189). 



At the beginning, the top of the delta platform is at the level of 

 the bottom of the stream's channel at the point of debouchm-e, but 

 it is gradually aggraded as water continues to flow over it. Its land- 

 ward margin is presently built up to sea-level and then above it, and 



Fig. 189. — Longitudinal section of the delta at a later stage of development. 



as the delta grows the delta-land is extended seaward (compare Figs. 188 

 and 189). At the same time the channel of the stream above the 

 original head of the delta is aggraded, for the current there is checked 

 by the aggradation of the delta. Thus alluvial deposits continuous 

 with the delta are extended landward. 



The projection of the direction of the lower end of the stream may 

 be said to be the axis of the extra-debouchiu-e current. From this 



