6 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
deposits which, in the South Atlantic and the Gulf Coast regions, 
have an average width of over 100 miles and which are not residual 
soils, but there should be added numerous small areas of residual 
soils scattered throughout areas of other kinds of soil. 
Cumulose or swamp soils (Ref. No. 3, pp. 35-38). — This type of 
soil is related to residual soils in that it has been formed largely from 
materials not transported. When plants grow where water fills the 
soil most of the time, the lack of air in the land surface hinders the 
decay of organic matter to the extent that large deposits of this 
material finally collect. Such accumulations going on for ages 
result in what are commonly known as peat bogs or muck swamps. 
They contain, as a rule, only such mineral matter as has been washed 
in from adjoining areas. Cumulose soils are widely distributed and 
vary greatly in area. In this country they are most numerous in 
the northern United States, while larger areas of slightly different 
type, known as seacoast swamps, are common along the Atlantic 
and Gulf coasts. Such soils are generally useless for agricultural 
purposes until drained. The management of marsh soils, however, 
is considered in Lesson X. 
While soil in many cases has been derived as above explained from 
the rock directly under, or from plant remains in place, there are many 
kinds of soil which were formed in other sections of the country and 
have been brought to their present location by some natural agency. 
The three most important agencies transporting soil materials are 
water, ice, and wind. 
Alluvial soils (Ref. No. 2, pp. 43-50). — The action of water as a 
soil-forming agent is a matter of common observation. Whenever 
streams flood and overflow their banks they deposit some of the sedi- 
ment brought down from higher up in their valleys. In this way 
they frequently form layers of sand or fine gravel when the stream is 
rapid, and of silt when it is moving very slowly, and in the broad 3 
lakelike floods which occupy the larger valleys of the more important 
rivers the finest sediment, or clay, is frequently deposited in deep 
layers. Soils thus transported by water are called alluvial soils. 
They are always stratified, and the strata frequently vary a great 
deal in the size of grains, so that a layer of gravel is often found under 
one of coarse sand, and a layer of coarse sand under one or more of 
fine silt. For this reason alluvial soils differ greatly in character, and 
one must examine the subsoil of any alluvial field if he desires to 
know its condition and value. Alluvial soils include large agricul- 
tural areas, and when well drained are among the most productive 
soils of the earth's surface. The high percentage of organic matter 
which they commonly contain and the frequent renewing of fertility 
by repeated overflows (in case of the low-lying alluvial soils) are 
reasons why they keep productive. The Nile Valley in Egypt is a 
