the steepest angle at which the material will lie. This angle
varies with different materials, being greater with coarse and
angular fragments than with fine rounded grains. Sooner or later a
talus reaches that equilibrium where the amount removed from its
surface just equals that supplied from the cliff above. As the
talus is removed and weathers away its slope retreats together
with the retreat of the cliff, as seen in Figure 9.

GRADED SLOPES. Where rocks weather faster than their waste is
carried away, the waste comes at last to cover all rocky ledges.
On the steeper slopes it is coarser and in more rapid movement
than on slopes more gentle, but mountain sides and hills and
plains alike come to be mantled with sheets of waste which
everywhere is creeping toward the streams. Such unbroken slopes,
worn or built to the least inclination at which the waste supplied
by weathering can be urged onward, are known as GRADED SLOPES.

Of far less importance than the silent, gradual creep of waste,
which is going on at all times everywhere about us, are the
startling local and spasmodic movements which we are now to
describe.

AVALANCHES. On steep mountain sides the accumulated snows of
winter often slip and slide in avalanches to the valleys below.
These rushing torrents of snow sweep their tracks clean of waste
and are one of Nature's normal methods of moving it along the
downhill path.

LANDSLIDES. Another common and abrupt method of delivering waste
to streams is by slips of the waste mantle in large masses. After