size of those which it could move before, and must drop all those
of larger size.

Will a river deposit more at low water or at flood? when rising or
when falling?

STRATIFICATION. River deposits are stratified, as may be seen in
any fresh cut in banks or bars. The waste of which they are built
has been sorted and deposited in layers, one above another; some
of finer and some of coarser material. The sorting action of
running water depends on the fact that its transporting power
varies with the velocity. A current whose diminishing velocity
compels it to drop coarse gravel, for example, is still able to
move all the finer waste of its load, and separating it from the
gravel, carries it on downstream; while at a later time slower
currents may deposit on the gravel bed layers of sand, and, still
later, slack water may leave on these a layer of mud. In case of
materials lighter than water the transporting power does not
depend on the velocity, and logs of wood, for instance, are
floated on to the sea on the slowest as well as on the most rapid
currents.

CROSS BEDDING. A section of a bar exposed at low water may show
that it is formed of layers of sand, or coarser stuff, inclined
downstream as steeply often as the angle of repose of the
material. From a boat anchored over the lower end of a submerged
sand bar we may observe the way in which this structure, called
cross bedding, is produced. Sand is continually pushed over the
edge of the bar at b (Fig. 42) and comes to rest in successive
layers on the sloping surface. At the same time the bar may be