16 BULLETIN 1419, U. S. DEPARTMENT OF AGRICULTURE 
dense stand of growing timber, the sedimentary material below yields 
Gradually by plastic flow. In some cases this may lead to rapid sub- 
sidence, shallowing of ditches, damage to roads, or disalignment of 
tiles, and so increase the cost of maintenance. Subsidence of the sur- 
face layers, or their overburden, is certain to follow in areas which con- 
tain water pockets in or between layers of peat. The swamp forest 
shown in Plate 7, A, was submerged for a time, because of the sink- 
ing of the surface layer of peat. The underlying saturated layer of 
sedimentar}^ material could not support the pressure of the increas- 
ing density in the stand of timber. The surface layer emerged after 
the trees had died. 
The roadbed in Plate 7, B, sank into water pockets which occur 
between the surface layers of moss and sedge peat and the underlying 
water-logged layer of sedimentary peat. It has been further found 
that the displacement and upheaval of beds of peat along the sides 
of a sinking highway is not an uncommon experience. An instance 
of this kind is shown in Plate 7, C. In areas with the profiles exem- 
plified by 1-2, 1-3, or 1-2-3 in Plate 1 a saturated basal layer of 
sedimentary peat, resting frequently on a water-logged mineral sub- 
stratum, has no sustaining or cohesive power to support additional 
loads. But when the layer is dense and closely compressed and the 
water content reaches a critical stage at which the material becomes 
plastic, the weight of any overburden causes lateral expansion and 
consequently a more or less gradual lifting up of adjacent parts of 
the overlying layers. On the other hand, a surface layer of sedimen- 
tary peat, such as is shown in profiles 2-1, 1-2-1, 2-3-1, and 3-2-1, 
hardens under conditions of excessive drainage; it may become so firm 
and unyielding as to be practically useless for agricultural and other 
purposes. In addition, the downward percolation of surface waters 
is prevented, and the level of the ground water may fall to a depth 
sufficient to cause extensive damage from drought. 
It is unnecessary to point out the great importance which attaches 
to irrigation for peat land having profiles of these types. Irrigated 
cultivation should be practiced not only during times of the growing 
season but upon all well-decayed surface peat soils. In fact, irriga- 
tion may be looked upon increasingly as the principal mode of water 
supply in the future adaptation of peat lands to crops. Maintaining 
an even, constant moisture condition would result in a slower shrink- 
age and more favorable decomposition; it would probably decrease 
to a minimum the rate of settling of an overburden, such as a cover 
of sand or road material. However, where the outlets are inadequate, 
it would be advisable to dike off the area and install a pumping 
system. But peat lands of the profile series beginning with 1 in Plate 
1, which are of a relatively great depth and usually lack a natural out- 
let or fall, are difficult and expensive to drain. Investigations by the 
United States Department of Agriculture, indicate that subsidence 
immediately begins and continues at a fairly rapid rate until equilib- 
rium is reached at or near the water level in the drained and cul- 
tivated area of peat. If cultivation is to be continued it becomes 
necessary to provide deeper drainage and the same cycle of subsid- 
ence is repeated. It deserves strong emphasis that areas of this 
type of peat land are better suited as reserves. 
Fibrous peat lands of the profile series beginning with 2 in Plate 1 
do not require to be drained to any great depth. They can be made 
