Talus.  At the foot of cliffs there is usually to be found a slope of rock fragments which clearly have fallen from above.  Such a heap of waste is known as talus.  The amount of talus in any place depends both on the rate of its formation and the rate of its removal.  Talus forms rapidly in climates where mechanical disintegration is most effective, where rocks are readily broken into blocks because closely jointed and thinly bedded rather than massive, and where they are firm enough to be detached in fragments of some size instead of in fine grains.  Talus is removed slowly where it decays slowly, either because of the climate or the resistance of the rock.  It may be rapidly removed by a stream flowing along its base.

In a moist climate a soluble rock, such as massive limestone, may form talus little if any faster than the talus weathers away.  A loose-textured sandstone breaks down into incoherent sand grains, which in dry climates, where unprotected by vegetation, may be blown away as fast as they fall, leaving the cliff bare to the base.  Cliffs of such slow-decaying rocks as quartzite and granite when closely jointed accumulate talus in large amounts.

Talus slopes may be so steep as to reach the angle of repose, i.e. 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 long rains and after winter frosts the cohesion between the waste and the sound rock beneath is loosened by seeping water underground.  The waste slips on the rock surface thus lubricated and plunges down the mountain side in a swift roaring torrent of mud and stones.