48 SOILS 



be seen in alluvial soils. The largest area of 

 alluvial soil in the country is the flood plain or 

 delta of the lower Mississippi. It reaches from 

 the mouth of the Ohio southward for 1,100 miles. 

 The whole area is flooded periodically and receives 

 each time a deposit of the mud tnat gives the 

 Missouri its Indian name, meaning "Big Muddy." 

 It is exactly such conditions as this that have en- 

 abled the valley of the Nile to produce bountiful 

 crops for 4,000 years without artificial fertilisation. 

 The same process is responsible for thousands of 

 meadows, swales, and swamps in northern United 

 States, and it may be seen in action on the banks 

 and at the mouth of every stream. Alluvial soils 

 are made mostly of very fine sand, and silt and clay. 

 They vary greatly in chemical composition, but 

 are usually very rich. 



Drift Soils. Of even greater agricultural im- 



Eortance are "drift" soils, those that were formed 

 y the action of the great ice sheet of the geologic 

 past. They are distinguished from all others by 

 having many rounded rocks or boulders, which 

 were worn smooth and rounded by glacial action. 

 Some drift soils are assorted or in layers, having 

 been laid down by successive streams of water 

 issuing from the ice; others are not in layers, having 

 been deposited directly by the ice. The deposits 

 of drift soil are not always spread evenly over 

 the land. Sometimes the underlying rock comes to 

 the surface, making patches of sedentary soil; 

 sometimes drift soil is neaped into broad rounded 

 knolls, from several feet to 300 feet high. These 

 "morains" or "drumlins" are a distinctive feature 

 in the farm landscape from eastern Massachusetts 

 to North Dakota and north into British Columbia. 

 The average depth of drift soils is about 30 to 50 



