Ch. 1] SWAMPS 27 



At the lower end the fan merges into a more or less broad valley 

 rilled with alluvium, or the fan may pass gradually into a playa which, 

 during times of flood, is converted into a lake. The playas are exceed- 

 ingly level and are dry most of the time. The lakes that occupy them 

 during and after a flood are broad and shallow (Russell, 1885). The 

 relative frequency of floods, the supply of debris, and the size of the 

 playa govern the types of deposits that are laid down. The sediments 

 vary in texture from layer to layer, but in general they are fine- 

 grained. A variety of organic deposits depending on food supply and 

 length of time required for the lakes to dry up is laid down. Some of 

 the playas, such as Searles Lake in California, give rise to economic 

 deposits of potash, salt, and borates (Foshag, 1926). 



Many of the streams in the semi-arid parts of western United States 

 occupy broad valleys filled with alluvium. This alluvium consists of 

 several different kinds of layers, some of which are discolored brown 

 or black on top, and they probably represent more or less long inter • 

 vals during which soil developed. Some of the layers are remarkably 

 uniform in thickness for a distance of a mile or more. The layers are 

 lenticular and presumably represent deposits of single floods or of 

 single periods of similar climate. The texture of the deposits ranges 

 from clay to sand. For further information see Chapter 23. 



This alluvium has been subjected to alternate periods of fill and 

 scour. During periods of scour, deep gullies form which in time swing 

 laterally to erode some of the fill previously deposited in the valley. 

 At the present time a period of scour characterizes most of the South- 

 west, particularly New Mexico, Arizona, Wyoming, and Utah, but 

 only to a relatively small extent central and eastern Nevada and cen- 

 tral California. The cause of the present scour has been attributed 

 to overgrazing, but it can also develop from natural causes, because 

 as Peterson shows in Chapter 23 of this symposium there have been 

 at least three periods of scour and subsequent fill since Late Pleistocene 

 time, caused by geologic agencies alone. 



Swamps 



The stagnant water in swamps favors the accumulation of woody 

 and other types of plant material that give rise to coal deposits (Cady, 

 1942). Such environments commonly are found near the outer edges 

 of coastal plains, particularly in areas bordered by barrier beaches or 

 bounded by deposits laid down in distributaries of deltas. The essen- 

 tial feature is the empoundment of water in a broad, low area in which 

 vegetation, particularly trees, can flourish. Depending upon the rela- 

 tive distribution of wooded and open areas, different types of organic 



