In both the cases, Figs. 200 and 201, not only must there be a number of weeping holes left in the lower part of the wall, but there must be sufficient well-built drains and culverts under the filling and through 162 the wall to carry away all ordinary or flood water coming down from the cliffs and hills above. Where a retaining wall is built along the margin of a river, the lower portion, which will be in contact with the water when the river is full, should be constructed of selected large heavy stones to withstand the scouring action of the water, and any brushwood or floating timber which may be brought down by flood water. Where retaining walls are built to support wet clay, or in embanked places on wet side-lying ground, the efficiency of the work will be much increased by constructing a layer, two or three feet in thickness, of dry, flat, bedded stones carefully hand-laid, from the foundation to the top of the wall, as shown in Fig. 199. These dry stones form a continuous vertical drain to take away water from any part of the earthwork down to the outlets left in the lower portion of the wall. The building of retaining walls entirely of dry stone is very questionable economy, and entails a constant expenditure in maintenance and renewal. The working out of one stone loosens the surrounding portion of the wall, and if not at once repaired, a length of the wall will fall down, bringing with it a large quantity of the earthwork. If readily obtained, large heavy stones should be selected for the coping of retaining walls, so as to minimize as much as possible the chance of their disturbance or displacement. Where lighter stones have to be used, or bricks laid on edge, they should be bedded and pointed in cement. In many places it is necessary to form wide and massive foundations of concrete on which to build the retaining wall; and in some cases of soft, treacherous ground, timber piling may be necessary.