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
sedimentary 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, {. 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 pay 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
L, which are of a relatively great depth and usually i 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
ype of peat land are better suited as reserves.
ibrous peat lands of the profile series beginning with 2 in Plate 1
do not require to be drained to"anv ereat depth. Thev can be made