Geoscience Reference
In-Depth Information
down. Debris forms
scree
or
talus
deposits on slopes at
equilibrium angles in the range 35-45
, depending on
lithology and block shape.
Colluvium
is a general term for
reworked rock debris derived from slope or other
(especially glacial) sediments. It has undergone further
breakdown and chemical weathering, with a wider range
of particle sizes, and acquired a less blocky, more
grain/matrix form. Colluvium is unlikely to be well sorted
or stratified, reflecting the episodic downslope movement
of separate pulses of debris. Pedogenesis will begin but
subsequent burial testifies to the influx of new material.
Equilibrium slope angles range from 32
to 37
, rising
above 40
towards an upslope boundary with scree.
Soil
refers to material showing substantial pedogenesis and
pedological character, normally supporting vegetation.
Debris is generally characterized by high void ratios,
weak and irregular potential shear surfaces and zero
cohesion, except at low water content. Dry movements
occur when tangential forces exceed shear resistance,
assisted by
ground heave
through hydration or ice
formation, or rain splash. At very low velocities and in the
absence of clearly defined failure surfaces, this amounts
to the relentless
creep
of particles downslope either as
single blocks or grains or
en masse
by
solifluction
as lobes
or sheets. The latter term is used both generally and also
in the restricted sense of the permafrost environment.
Debris/soil slumps develop on well defined failure surfaces
by mass rotation. Cohesive debris, with suction forces or
cementation, may behave initially like rock mass and slide,
fall or topple (
Plate 13.20
)
before disintegrating. Wet
movements are promoted by mass fluidization, and the
resultant slurry velocity is usually moderate to fast as
mud flows
or very fast as debris flows, where grain
collisions provide buoyancy (see box, p. 308).
Devonian sandstone, in Kerry, Ireland.
Photo: Ken Addison
subsequently flow, carrying away
competent
overburden in
celebrated cases such as the Black Ven cliffs in Dorset.
Cambering
occurs as the rigid overburden creeps forward
before shearing over a steepening slope. Debris may distort
underlying strata during rock creep (
Plate 13.18
).
Slides release small rock volumes to spectacular
failures. 36,000 m
3
of rock failed in a single wedge slide
during glaciation at the site in
Plate 13.6
but quantities
two to three orders of magnitude larger were moved in
catastrophic events such as the Franks slide, Alberta, in
1903, and Blackhawk slide, southern California (
Plate
frost weathering involve smaller volumes. All rock failures
contribute blocky debris either to downslope scree or
avalanches, which continue as collision-buoyed flows. The
special case of rock glaciers is covered in
Chapter 15.
Slope landforms and slope development
Slope landforms are either residual scars from which
material has moved, or debris deposits downslope. These
components relate to each other and constitute features
of a slope landsystem of recognizable, recurring elements
(
Figure 13.19
).
Both are transient, since scars represent
local oversteepening beyond an unsustainable shear stress
(
F
s
=
1) and deposits are probably connected with the
fluvial landsystem. As with mass movements, slope
landform and development models abound, and one of
the most enduring and widely used is the
hypothetical
nine-unit land surface model
(
Figure 13.20
).
By no means
all elements are found on every slope but it is instantly
recognizable, for example, in the alpine landsystem
described in
Chapter 25.
Failure modes on debris slopes
Various terms are used for the forms of denudation
'debris' or 'detritus'. Several stages in the progressive
breakdown of rock are recognized.
Rock debris
or regolith
is blocky material derived from rock walls and retains
its angular character with little or no further break-
