TYPES OF SOILS 



29 



in the banks of talus at the base of cliffs almost anywhere. 

 The soil brought down by avalanches also belongs to this 

 class. Since the materials that compose it are coarse, rough, 

 and irregular, a colluvial soil is of little value for cultivation, 

 though it may support a luxuriant growth of lichens, mosses, 

 ferns, and small shrubs. The soil on hillsides may also be 

 regarded as partly colluvial. 



Glacial soils. Glacial soils have been derived from many 

 kinds of bed rock by the glacial ice that once covered a great 

 part of the northeast- 

 ern states and various 

 other parts of the earth. 

 They consist of sand, 

 clay, and gravels either 

 separate or intermin- 

 gled. Such soils are 

 the rule in the states 

 north of the Ohio River 

 and east of the Great 

 Plains, but south and 

 west they gradually 

 thin out and disappear. 



Alluvial soils. These 



FlO. 14. A ridge of glacial debris that has 

 have been transported b een sor ted by running water 



by streams that during 



periods of flood pick up much material that is dropped as soon 

 as the current slackens. The soil in our ordinary bottom lands 

 has been formed in this way, and the soil in the delta region 

 along the lower Mississippi is of the same nature. Alluvial 

 soils are extremely fertile, since they consist in great measure 

 of the richest soil washed down from other fields. 



Soil constituents. Ordinary arable land is a mixture of 

 various ingredients. When these are separated, we know 

 them as sand, clay, silt, humus, and the like. Peat is a black 



