138 GEOLOGY 



While aridity and altitude are conditions which favor the devel- 

 opment of canyons, as shown by the fact that most canyons are 

 in high and dry regions, they are not indispensable. Niagara 

 River has a canyon below its falls (PL IX), and the surrounding 

 region is neither high nor arid. The narrow- part of the valley is 

 so young that side erosion has not yet widened the valley or lowered 

 its angle of slope to such an extent as to destroy its canyon char- 

 acter. This canyon is often called a gorge, a term frequently 

 applied to small valleys of the canyon type. 



Rate of Degradation 



The amount of mechanical sediment which the Mississippi 

 River carries to the Gulf of Mexico is estimated to represent a rate 

 of degradation for the Mississippi basin of about one foot in 5,000 

 years. But the mechanical sediment carried to the Gulf does not 

 really represent the total degradation of the basin, for the water 

 which sinks beneath the surface is dissolving more or less rock sub- 

 stance, especially lime carbonate. This material is carried to the 

 sea in solution, and does not appear in the sediment on which the 

 above estimate is based. Taking into account the matter dissolved 

 by the water and carried to the sea in solution, the average rate 

 of degradation for the Mississippi basin has been estimated at one 

 foot in 3,000 to 4,000 years. 1 



The sediment carried to the Gulf by the Mississippi River is 

 gathered from nearly all parts of the basin of this stream, but 

 much more of it comes from some parts of the basin than from 

 others. On the whole, the rate of erosion is probably greatest 

 toward the margins of the basins, where the land is in its topo- 

 graphic youth or early maturity. It is notably less in the middle 

 courses of the valleys, and is exceeded by deposition in some places 

 along the lower courses of the Mississippi and some of its main 

 tributaries. 



The average elevation of North America is probably not far 

 from 2,000 feet. If the present rate of degradation, say one foot 

 in 3,500 years, were to continue, it would take something like 



recent unpublished data seems to indicate that this rate is, 

 haps, too high. 



