Why Is the Mountain Falling Apart at the Bottom? Broken Rock Finding Its Angle

Rock exposed on a cliff face is broken up by several processes and the fragments fall. Water entering cracks and freezing expands by roughly nine per cent, which levers the rock apart, and repeated cycles of freezing and thawing are the dominant mechanism in cold and temperate mountains. Heating and cooling stresses the surface. Roots grow into joints and widen them. Chemical weathering weakens the rock along planes of weakness. The loosened fragments fall, bounce and roll down to accumulate at the base, sorted by the process, since larger blocks carry more momentum and travel further out from the cliff while smaller ones stop higher up, producing a slope that is finer at the top and coarser at the bottom.

The accumulation settles at a characteristic gradient determined by the material:

• •Loose angular fragments interlock and resist sliding up to a limiting angle
• •That angle is typically between thirty and forty degrees for rock debris
• •Angular material stands steeper than rounded material, since the pieces lock together
• •Adding more material at the top causes movement until the angle is restored
• •The same limiting angle governs a heap of sand, of grain and of any granular material
• •Water changes it, with a little moisture increasing stability and saturation destroying it

The difficulty of moving on such a slope follows from the same physics. Each step disturbs the local arrangement, and because the material sits at its limiting angle, disturbance causes movement, so the ground slides under the foot and a portion of every upward step is lost. The sorting means the fragments underfoot change size unpredictably. Loose rock dislodged by anyone above travels a long way and gathers company, which is the main hazard and is why parties on such ground either spread out or keep close together directly below one another. Descending is faster and is done by allowing the surface to move, which works on fine material and is dangerous on coarse. And the whole surface is unstable enough that established paths do not persist.

Several features are produced by the same downslope movement of material and distinguishing them is useful in the field. A talus cone is a fan built below a single gully that funnels material to one point, and a row of them merges into a continuous apron along a cliff base. A rock avalanche deposit is the result of a single catastrophic collapse rather than gradual accumulation, and it typically travels much further than the angle of rest would predict, which remains incompletely explained. Solifluction lobes form where saturated soil creeps slowly downhill over frozen ground, producing tongue-shaped bulges. Blockfields cover plateau surfaces with shattered rock and are not slopes at all. And screes developed under colder conditions than the present are common in temperate mountains, where they are now stabilised and vegetated.

These accumulations are not permanent features and their fate depends on the setting. Where they are stable long enough, plants colonise from the edges, which binds the surface and eventually converts it to soil-covered slope, and the presence of lichen and vegetation is the field indicator that a slope is no longer actively accumulating. Where a glacier or a stream removes material from the base, the slope is undercut and continues to be fed. In permafrost regions the accumulation may creep downhill as a mass, developing into a rock glacier that moves metres per year and has ice within it. And the rate of supply from the cliff is itself changing in many mountain regions, with warming permafrost releasing rock faces that had been held together by ice.

Freezing water levering cracks apart is the main supplier, and falling fragments sort themselves with the largest travelling furthest from the cliff. The accumulation settles at the angle at which interlocking fragments resist sliding, typically thirty to forty degrees. Crossing it is difficult because the material sits at that limit, so every step disturbs it and slides.