26o STUDIES FOR STUDENTS 



flocculated clay, giving rise to conditions unfavorable for either under- 

 ground or surface drainage. In the case of piedmont slopes it was 

 seen that in desert climates the greatest seasonal contrasts exist between 

 too much and too little water, but that all parts fare much alike and 

 become dried out through the greater portion of the year. Over 

 the lower flood plains, on the contrary, such striking contrasts of 

 swamp and desert are permanent features through series of years. 

 Using the delta of the Colorado River as an "example of one developed 

 under highly arid conditions, Macdougal describes in a recent work 

 how: 



At places where the river is cutting into gravelly and sandy bluffs, within 

 the compass of one hundred feet may be found the most vivid contrasts of rank 

 swamp vegetation and water -loving plants having broad leaves and delicate 

 tissues with the. toughened spinose, and hairy xerophytic forms of the desert. 



The quantity of food furnished by the swampy jungles is sufi&cient to support 

 a vast amount of native animal life, and furnishes inviting feeding-grounds for 

 migrating birds. The countless millions of young willow and poplar shoots 

 supply food for the beaver, which bids well to hold out long in the impassable 

 bayous and swamps against its trapper foe. 



Nearer the gulf are found great sloughs, in which are extensive fields of the 

 "wild rice," while the land subject to the action of the overflow of the tides sup- 

 ports a carpet of salt-grass.^ 



Similar areas of more or less permanent swamp, increasing toward 

 the seaward margin, may be noted as characteristic of other large 

 deltas, such as those of the Nile and the Indus, developed as with the 

 Colorado in truly desert regions. In ancient river deposits, therefore, 

 an appreciable proportion of paludal deposits must be expected to 

 occur over the terminal portions of the delta under all climatic con- 

 ditions, and such must be allowed for in making inferences in regard 

 to the ancient climate. As means for separating these geographic 

 and climatic factors, however, may be noted the close association over 

 the desert delta of paludal and desert conditions and the much smaller 

 proportion of swamp which is permanent than in the case of more 

 pluvial climates. In the long seasons of dessication all but the lowest 

 bottoms become dried out and mud-cracked.^ 



1 "The Delta of the Rio Colorado," Bulletin of the American Geographical Society, 

 January, 1906, pp. 4, 10, 11. 



2 D. T. Macdougal, op. cit.; see Fig. i. 



