THE PHYSICAL PROCESSES OF SOIL FORMATION. 7 



former cut to a depth of over 2000 feet into hard basalt rock, 

 the latter to over 5,000 feet, partly into softer materials, partly 

 into granite, are perhaps the most striking examples of this 

 power; the manifestations of which can, however, be as con- 

 vincingly seen in thousands of minor rivers and streams. 



All the materials so carried off from the higher slopes are 

 finally deposited on a lower level ; whether only a short distance 

 away on a lower slope (colluvial soils), or farther away in the 

 flood plain of streams, rivers, or lakes (alluvial soils). Other 

 things being equal, the finest materials are of course, carried 

 farthest, and often into the sea ; in which, however, they can- 

 noj long remain suspended, but are quickly thrown down, 

 forming river bars, flood plains, and deltas. The fineness of 

 the material of delta soils, like that of those made from glacier 

 flour, insures them the same advantage, viz. great fertility and 

 durability. 



It is calculated that the Mississippi River carries into the 

 Gulf of Mexico annually some 7469 millions of cubic feet of 

 earthy deposits, which would fill one square mile of surface 

 to the height of 268 feet, or would cover that number of square 

 miles to the depth of one foot. 



FIG. 3. Cliffs and caves on sea beach at l.a Jolla, Calif, slum ing effects of \Vave action. 



Wave-Action. The powerful effects of the beating of wave? 

 upon abrupt shores of seas or lakes are in evidence all over 

 the world, and these effects are so characteristic that they can be 

 recognized even where no sea or lake exists at present. Gravel 

 and sand are carried in the surf and serve as grinding ma- 

 terials, wearing even the hardest rocks into grooves, rills, chan- 



