THE CHANGING FACE OF NATURE 17 



flow is lessened, in consequence of which a bar is often formed at 

 its mouth or in larger streams, a delta. Deposits are laid down 

 below an obstruction like a bowlder in midstream. While the 

 stream cuts on the outside of a wide curve where its flow is rapid, 

 it deposits on the inside where it is slow. Often the spring 

 freshets raise the height of a river so it overflows its usual channel. 

 The current beyond this is relatively sluggish so that over the 

 flood plain there is formed a deposit of mud washed from the 

 hills upstream (Fig. 16). This flood plain is frequently enriched 

 by this annual addition of humus, so that river bottoms proverbi- 

 ally have good soil. 



The accumulated soil and rock debris swept from a continent 

 by the various agencies of erosion and transportation deposited 

 in quiet bays or ocean deeps gradually transforms to rock again, 

 so new lands are formed as old ones are destroyed. When exten- 

 sive deposits accumulate offshore or in the deltas of great rivers, 

 the very weight of material seems to cause the earth's crust to 

 gradually sink. Thus the deposits may continue to accumulate, 

 as is apparently the case in the Gulf of Mexico, until they become 

 very thick. Then the lower mud layers, pressed upon by the 

 thousands of feet of overlying layers, baked by the heat of the 

 interior, since they are ever sinking farther from the surface, 

 are transformed into rock in a way analogous to the manner in 

 which we make brick or artificial stone, by compressing, then 

 heating the mud or clay. So sedimentary rocks have formed, 

 probably are forming in our Great Lakes now. Certainly they 

 have so formed here in the Chicago region, for our dominant 

 bedrock, the limestone, is of sedimentary origin. 



Movements of the earth's crust often upheave these beds of 

 mud transformed to rock, so adding to the surface of the land. 

 The outer crust of the earth is relatively cool, the inner portion 

 of the earth still exceedingly hot. Like any hot body it gradually 

 cools, and cooling shrinks; so it tends to shrink away from the 

 already cooled crust. This, of course, can never actually happen, 

 for the heavy crust crumples down on to the shrinking central 



