ALLUYIAL DEPOSITS Sll 



heat and frost; L e.^ through, mechanical agencies rather than 

 through the processes of rock decomposition.^ 



But, as has already been noted, only a part of the sediment 

 carried by any stream reaches its mouth. A comparatively 

 small, but, from the present standpoint, very important portion 

 is carried during seasons of high water beyond the usual chan- 

 nels and spread out over the flood plains, as described on p. 276. 



Fig. 32. — Section across an alluvial plain. 



Such deposits are plainly stratified, and consist of mineral mat- 

 ter in a finely comminuted condition derived, it may be, from 

 the breaking down of a great variety of rocks. Their physical 

 and chemical properties, as well as the periodic character of 

 their deposition, are favorable to the formation of soils possess- 

 ing great strength and fertility. Both fertility and rate of depo- 

 sition in such cases are augmented through plant growth, which 

 takes place with great rapidity wherever climatic conditions 

 are favorable. So soon as the water leaves the flood plain, 

 a host of moisture-loving plants, as reeds and rushes, spring up 

 in countless numbers to die down again in the fall, and yield their 

 carbon and nitrogenous constituents to serve as fertilizers, 

 and augment the crop of the following year. Moreover, the 

 remaining stems and fallen leaves serve to retard the running 

 waters of each succeeding flood, catching in their meshes the 

 floating sediments which might otherwise be carried seaward. 

 The Anaeostia, which empties into the Potomac River east of 

 Washington, serves as a good illustration of the working of these 

 agencies. A century ago the stream was navigable by coasting 

 crafts as far as Bladensburg. Now, owing to shallow waters, 

 only rowboats can navigate beyond the Navy Yard at Washing- 

 ton. Each season the stream, murky with suspended silt from 

 cultivated fields along its shores, comes down, till, ponded back 

 by tides, it begins to deposit its load. As year by year its bed 

 is thus raised, water plants, encroaching more and more from 



^Proc. Eoyal Soc. of London, Yol, XXXIX, 1885, p. 213. 



