150
10o
50
Fig. 12.6 Runrf;f regimes of selec.ted rivers in the periglac.ial
:one of North America, (Modifiedrt-om H. M. Frenc.h (]97ó)The
Periglacial Environment. Longman, London, Fig.8.1 , p. ló9,
hased on M. Churc,h (l974)Permafrost Hydrology; Proceedings
of Workshop Seminar, Environment Canada, Ottawa 7-20.)
curTent rates of weathering and mass movement Supplying
more debris than can be transported by existing rivers. Some
of this material may, however, be a relic of higher rates of
rock breakdown and sediment supply during the Pleistocene.
I2.2.6 Aeolian activity
Present-day periglacial areas characterized by extreme aridity
are favourable environments for aeolian activity, and
there is abundant evidence for much more widespread and
intense wind action in areas marginal to the great northern
hemisphere ice sheets during the Pleistocene. In addition to
strong winds and the freeze-drying of sediments, low precipitation
and low temperatures are associated with minimal
vegetation cover. Aeolian erosion is evident in faceted and
grooved bedrock surfaces, deflation forms in unconsolidated
fluvial sediments, and in the formation of ventifacts
(see Section 10.2.2.I). Wind is also impor-tant in the movement
and deposition of snow which, together with topographic
features, determines areas of snow accumulation
and removal and thereby the distribution of an insulating
Snow cover,
Of great geomorphic significance are various aeolian
deposits, especially the loess which blankets much of those
areas in North America and Eurasia which were located on
Periglacial processes and landforms 301
the southern margin of the Pleistocene ice sheets. Loess is
composed of angular to sub-angular particles ancl cohesion
between grains allows the development of steep, high cliffs
where thick blankets have been dissected by fluvial action.
Although loess can form in warm deserts, most is believed
to have originated in past or present periglacial environments.
Glacial grinding, frost cracking and salt weatheťing
appear capable of producing sufficiently fine particles which
can be picked up from sites such as glacial outwash plains
and carried considerable distances by the wind before being
deposited as extensive loess blankets, exceptionally up to
100m thick and covering tens of thousands of square
kilometres (see Section I0.3.'/).
During the glacial phases of the Pleistocene, the steep
pressure gradient between the high pressure cells over the
Laurentide and Fenno-Scandian ice sheets and low pressure
to the south would have produced high mean wind velocities
and conditions conducive to significant aeolian transport
and deposition. Dating of discrete vertically stacked
layers of loess interbedded with soils indicative of more
temperate and humid conditions in China and Europe has
indeed indicated a close correlation between loess deposition
and phases of ice sheet extension (see Section r43.2).
12.3. Periglacial landforms
l2.3.1 Patternedground
First described more than a century ago, patterned ground
is one of the most conspicuous features of periglacial envřonments.
It can vary widely in scale from patterns composed
of elements only a few centimetres across to those with
dimensions of 100 m or more. Although some forms of patterned
ground are found in other environments, especially
deserts, the extent of its development is much greater in the
periglacial zone than elsewhere since it is most characteristic
of surfaces subject to intense frost action.
The most widely used descriptive classification of patterned
ground has been developed by A. L. Washbum. This
ernphasizes both shape and the degree of sorting of the
constituent materials. Five basic pattems are recogrrized circles,
nets, polygons, steps and stripes - although one
form may grade into another. Each of these types is subdivided
on the basis of whether the pattem results from the
sorting of suďace materials into fractions of large and small
size or whether it has developed without sorting. Slope
angle is an additional factor. Circles, nets and polygons are
most common on flat surfaces, whereas steps and stripes
tend to form on gradients of between 5 and 30., with transitional
forms developing on gentler slopes of between 2 and
5o. Mass movement is generally too active on slopes steeper
than 30o to allow pattems to form. A summary of the characteristics
and environmental associations of the various types
of pattemed ground is presented in Table 12.5.
]5
],o
o5
al
cD
s
o
o
a
E
O
ó
Jasons Creek,Devon lsland 1970
302 Exogenic processes and landforms
Table 12.5 Characteristics of different forms of pattemed ground
Circles Occur singly or in groups. Typical dimensions 0.5-3 m.
Nonsorted type characteristically rimmed by vegetation,
sorted type bordered by stones which tend to increase in
size with size ofcircle. Found in both polar and alpine
environments but are not restricted to areas of permafrost.
Unsorted circles also recorded from non-periglacial
environments
Polygons Occur in groups. Nonsorted polygons range from small
features (<1 m across) to much larger forms up to 100 m
or more in diameter. Sorted polygons attain maximum
dimensions of only 10 m. Stones delimit polygon border
and surround f,ner material. Nonsorted forms are
délineated by furrows or cracks. Some types of polygon
occur in hot desert environments but most are best
developed in areas subject to frost. Ice-wedge polygons
only form in the presence ofpermafrost
Nets A transitional form between circles and polygons.
Usually fairly small (<2 m across). Earth hummocks
comprising a core of mineral soil surmounted by
vegetation are a common form of unsorted net.
Steps Found on relatively steep slopes. They develop either
parallel to slope contours or become elongated downslope
into lobate forms. In unsorted forms the rise of the step is
well vegetated and the tread is bare. In the unsorted type
the step is bordered by larger stones. Lobate forms are
known as stone garlands. Neither type confined to
permafrost environments
Stripes Tend to form on steeper slopes than Steps. sorted stripes
composed of altemating stripes of coarse and fine
material elongated downslope. Nonsorted variety
delineated by vegetation in slight troughs. Not confined to
periglacial environments
Source,. Based on discussion in A. L. Washburn (19'79) Geocryology.
Edward Amold, London, pp.122-56.
Numerous factors contribute to the form of the great
variety of patterned ground including the interactions between
sorting, patteming and slope processes (Fís. I2.1).
An important distinction is that between those forms of
pattemed ground in which initial cracking of the suíface is
essential and those in which it is not (Tabte 12,6). Causes
ofcracking include contraction on drying (desiccation cracking),
stretching of the surface as a result of heaving (dilation
cracking), and thermal contraction of seasonally or
perennially frozen ground (frost cracking). Only the latter
is restricted solely to periglacial envirónments. Desiccation
and dilation cracking are important in the formation of small
non-Sorted polygons, whereas frost cracking is instrumental
in the development of larger forms. The most important patteming
processes which do not depend on initial cracking
include differential frost heaving and mass displacement.
Frost heaving is apparently a primary cause of a number
of sorted forms of pattemed ground; it helps to segregate
larger stones, which tend to move upward and outward,
from the finer-grained matrix which forms a central core.
Continued development at a number of discrete sites eventually
leads to,coalescence and the formation of polygonal
rims formed primarily of stones. Sedimentation and mass
Fig. 12.7 Relationship between patterning, sorting and slope
processes in the formation of patterned ground. (After A. L.
Washburn (l979) Geocryo\ogy. Edward Arnold, London, Fig.
5 .4 l , p. l60.)
displacement contribute to the development of non-sorted
circles while surface wash is probably a factor in the formation
of sorted stripes.
The regularity of many forms of patterned ground has
suggested to Some researchers that convection occurring
when frozen ground thaws may be an important mechanism
in its development. The idea is that water-saturated soil
close to the suďace waríns up while the temperature of that
adjacent to the still frozen subsoil remains close to freezing
point. Convection in this situation is possible because water
is densest at 4oC, so water at the thawing front is less dense
than the slightly waímer water close to the surface. When
the active layer thaws the cooler water at the thawing front
rises while descending plumes of warmer water lead to
localized melting of the underlying ice. This produces a
regular undulating interface between frozen and unfrozen
soil which, it is thought, can be reflected in the surface
topography. The exact mechanism by which this occurs is
not clear, but it probably involves a range of processes
including frost heave. Striped forms of patterned ground
according to this model are explained by the downslope
elongation of convection cells within the soil.
12.3.2 Ground-ice phenomena
l2.3 .2.1 Ice wedges
Ice wedges are downward-tapering bodies of ice (Fig.
12.8). They can be over 1 m across at the top and reach a
depth of 10 m or more. They have a polygonal form in plan,
SLOPE PROCESS
Sorted steps
and stripes
Sor te d
and poly
Nonsorted
steps and
Nonsorted
circles, nets and polygons
PATTERNlNG PROCESS
I
Lr-.
Periglacial processes and landforms 303
Table 12.6 Processes contributing to various types ofpattemed ground
PATTERNED GROUND MORPHOLOCY
Circles Polygons Nets Steps Stripes
FORMATIVE PROCESSĚS
Cracking essential
Desiccation cracking
Dilation cracking
Salt cracking
Seasonal frost cracking
Permafrost cracking
Frost action along joints
Cracking not essential
Primary frost sorting
Mass displacement *
Differential frost heaving *
Salt heaving *
Differential thawing
and eluviation
Differential mass movem9nt
Rillwork
*,|
*.)
*?
*, *
*9 *?
*, *
*?
*9
*?
x9
***
* * *?
* * *?
*,|
*,
N and S indicate, respectively, nonsorted and sorted types of pattemed ground.
Source: Modified from A. L. Washburn (1979) Geocryology. Edward Arnold, London, Table 5.1, p. 158.
Fig. 12,8 Ice wedge in the right bank of the Aldan River, 170 km
upstream of its junctíon h,ith the Lena River, Yakutia, USSR.
(Photo courtesy A. L. Washburn.)
and ice-wedge polygons are associated with the most
impressive form of pattemed ground in periglacial environments.
Frost cracking appears to be the key mechanism
necessary for the development of ice wedges. Extreme cooling
of ice-rich permafrost in winter leads to thermal contraction
and cracking of the surface (Fig. l2.9). As the
wedge grows it compresses the surrounding sediments
which tend to bulge up afound the perimeter of each polygon
forming a low ridge. In other cases thawing and erosion of
the ice wedge produces a perim€ter trough. In dry permafrost,
where meltwater is scarce, the cracks may be filled
witir loess or other coarser sediments to form sand wedges.
Recent field studies have revealed that some ice-wedge
cracks originate near the top of the permafrost and then
propagate both upwards to the surface and downwards into
the ice wedge. Only a proportion of wedges tend to crack in
any one year, and the frequency of cracking Seems to Vary
inversely with the depth of snow cover. The average
amount of ice accreted each year varies enormously so the
total volume of ice present is not necessarily a good indicator
of how long an ice wedge has been growing. Observations
made of the development of ice wedges after the
artificial draining of a lake in northern Canada have demonstrated
that under ideal conditions they can grow rapidly
and reach a significant size afterjust a few years.
Ice-wedge casts form when an ice wedge is replaced by
sediments as a result of the long-term thawing of the permafrost.
The form of the ice wedge is preserved by sediment
and may survive long after the permafrdst in which
the original ice wedge developed has completely thawed.
Ice-wedge casts are consequently an important indicator of
the previous existence of permafrost. Their palaeoenvironmental
significance is also enhanced by the fact that ice
wedges only seem to form in present-day permafrost environments
where the mean annual temperature is below -6
to -8
oC.
12.3 .2.2 Pingos
Pingos, named after the Eskimo word for a hill, are large
perennial ice-cored mounds, They,are roughly circular to
elliptical in plan and range in size from 3 to 70 m high and
from 30 to 600m in diameter (Fig, I2.I0). Large dilation
cracks generated by the progressive growth of the ice core
3a4 Exogenic processes and landforms
A 1st Winter
B
02m
Fig. 12.9 Hypothesized development of an ice wedge initiated
by frost cracking due to thermal contraction. The initial crack,
which may be only a few millimetres wide, collects snow and
meltwater from thawing of the active layer (A). This water freezes
below the permafrost table to form a thin ice vein (B). The initial
fracture represents a zone of weakness so will be a favoured site
for frost cracking in subsequent years. However, cracking does
not necessarily occur each year and expansion during summer
heating will tend to close cracks developed during the preceding
winter. Further ingress of snow and melnvakr adds new ice until
an ice wedge is eventually formed (C and D). (After A. H.
Lachenbruch (1962) Geological Society ofAmerica Special Paper
70, Fig. ], p. 5,)
commonly mark the summit. These may widen sufficiently
to expose the ice core and initiate thawing, thereby giving
rise to a collapsed pingo. Various materials can envelop
the ice core, including clays, silts, gravels and even bedrock.
Drilling has shown that in some cases the core of
clear ice extends to depths greater that the pingo height,
Vertical growth rates of pingos may reach 0.2 m a-t but
they tend to decline rapidly as the pingo grows. Large
pingos certainly take thousands ofyears to develop, as indicated
by the radiocarbon dating of two pingos in the Canadian
Arctic which gave approximate ages of 4500 and 7000 a.
The ice core forming a pingo is thought to grow through
the freezing of water forced upwards at pressure. Two
mechanisms have been proposed to explain the generation
ofthis pressure. In closed system pingos cryostatic pressure is
involved. This may be brought about in two related ways. If
Fig. 12.10 Pingo, Wollaston Peninsula, Northwest Territories,
Canada. (Photo courtesy A. L. Washburn.)
a lake in a permafrost zone is infilled by sediment and
vegetation, insulation of the underlying ground will be
reduced (Fig. 12.1l) and freezing will encroach from the
base, sides and top, This will trap a body of water which on
freezing expands and domes the overlying sediments. Alternatively,
diversion of a river or draining of a lake may have
a similar effect by reducing ground insulation. Support for
this model of cryostatic pressure comes from the distribution
of pingos in the Mackenzie Delta area of the Canadian
Arctic where 98 per cent of the 1380 pingos recorded are
located in, or closely adjacent to, lake basins.
The open system pingo model involves the freezing of
ground water flowing under hydrostatic pressure as it forces
its way towards the surface from below a thin permafrost
layer (Fig. 12.12). It has been calculated that very high
pressures are required to dome pingos and that these exceed
those commonly attained within unconfined ground water.
Consequently the open system type may in fact form under
temporary closed-system conditions as open taliks are
frozen in winter. Open system pingos extend into the zone
of discontinuous permafrost where mean annual temperature
may be only a little below freezing since here water
circulation is freer. They tend to occur in clusters in favourable
areas of drainage where the hydrostatic pressure is
more constant, In contrast, the closed system type usually
occur as isolated features on flat surfaces related to lakes
and are confined to the zone of continuous permafrost where
minimum mean annual ground temperatures are around
-5,C.
12.3 .2.3 Palsas
Palsas are mounds or more elongated forms occurring in
bogs and containing perennial ice lenses. They differ from
pingos in containing peat as a major constituent and a core
composed of discrete superimposed ice lenses rather than a
'lst Autumn
c 5OOth Winter
D SOOth Autumn
Periglacial processes and landforms 305
permaf rost permafrost
r*. nJI Schematic representation of the development of a closed system pingo following the infilling Óf a lake. (Modified from A
Washburn, (1979) Geocryology. EdwardArnold, London, Fig.5.49, p. 183.)
terrain. In addition to the collapse of material into the space
previously occupied by ice, thermokarst features may be
significantly affected by flowing water released by thawing.
High, short-term rates of erosion can occur, especially
where ice masses are exposed along cliffs or stream banks.
Although some thermokarst features can be attributed to
climatic change, many others form as a result of the
'normal' variability of the periglacial environment. Thermokarst
most commonly originates from a modification of
surface conditions and this can be brought about in a number
of possible ways including disturbance of vegetation,
cliffretreat and changes in stream courses.
Thaw lakes are perhaps the most ubiquitous thermokarst
Fig. 12.12 Schematic representation of the development of an
open system pingo. (Modifiedfrom F. Muller (I9ó8) in: R.W.
Fairbridge (ed.) Encyclopedia of Geomorphology. Reinhold, New
York. Fig.2, p.846.)
single ice mass, The mode of ice formation is uncertain but
probably originates from differential frost heaving with
peat playing a crucial insulating role. Palsas are commonly
found in areas of discontinuous permafrost.
12.3.2.4 Thermokarst
Thermokarst is a term used to enconipass a variety of
topographic depressions formed by the thawing of ground
ice. It is so called because ofthe superficial resemblance of
the landforms produced to'those characteristic of řue kaíst
Fig. 12.13 Small thaw lake s, Báthirst Island, Northwest
Territories, Canada. (Photo courtesy Geological Survey of
Canada.)
ater expulsioi
Dilation crack
306 Exogenic processes and landforms
form, and arguably one of the most characteristic landforms
of periglacial environments 1Fig. 12.13). They are bodies of
water which fill small, shallow depressions, The majority
are less than 5 m deep, and they are rarely more than 2km
across. The origin of thaw lakes seems to be related to the
melting of permafrost which contains a volume of ice that
exceeds the normal pore space of the sediment. Subsidence
occurs and shallow depressions are formed which fill with
water. They are particularly common in poorly drained
lowland regions where ground ice is abundant (Fig, 12.14).
They ate usua\\1 f,l\ed, quite tapid\1 by sediments andpeat,
and have a limited life span of only a few thousand years.
In some.localities, such as in the northem coastal plain of
Alaska, thaw lakes are somewhat elongated with lengthwidth
ratios of 2: 1 or 3 : 1 and tend to be orientated in a
specific direction. Their origin is uncertain, but they are
probably related to winds prevailing from a particular
direction which preferentially deposit sediments along lake
'banks normal to the wind direction, These deposits insulate
the banks from further thawing and protect them from wave
erosion.
Thaw slump§ are aícuate embayments facing downslope
and formed by the exposure and thawing of ground ice.
Basal unděrcutting of river banks or mass movement are
sufficient to expose ground ice and initiate the process.
Debtis satutated by the rne\twatet ptoduced í§o\es downslope
as a mud flow or by gelifluction. The headwalls of
thaw slumps may reach 8 m in height and retreat at rates of
over 7 m a-t, making this a significant denudational process
in some periglacial environments. Thermocirques are
Fig. 12.14 Thaw lakes in the Tuktoyaktuk Peninsula which lies within the continuous permafrost zone of the Northwest Territories,
Canada.The-area shown is about I80 km across. (Landsat image courtesy R. S.Williams Jr)
]rge-scale variants of thaw slumps formed when retreating
.,opes intersect ice wedges. The surface thawing of ice,,,
edge polygons can produce linear and polygonal troughs
:unounding a central mound.
Alases are major therrnokarst depressions from 3 to 40 m
].ep and l00 m to 15 km across. They are compound fea:lres
resulting from climatic change or a widespread distur:lnce
of the surface such as that arising from a forest fire.
_ hese environmental pefiurbations can lead to permafrost
::sradation which initiates a sequence of surface collapse,
..e formation and pingo development (Fig. 12.15). Eventual
, ,riescence of individual alases gives rise to alas valleys
. :rch may be tens of kilometres long. Alas formation has
,-,:ected large areas in central Yakutia in eastem Siberia
.,. :ere conditions are parlicularly favourable. It is estimated
1rt in this area 40-50 per cent of the Pleistocene surface
:! been subjected to alas development, which apparently
lce wedges
A r._x.._+J* I,tL* .
IIIIII
H r Collapsed pingo L
W
Fig. 12.15 Schematic,representation ofthe sequence of
,lel,elopment and decay of alases: (A) original lo*-land surface
,l,ith ic,e wedges; (B) eJfec,t rlf int,rease in depth of actile layer
promoted by climatic warming oi ground disturbant,e; (C) initial
rhermrlkarst stage; (D) young alas stage; (E) mature alas stage;
t F) old alas stage ; (G) phase of possible pingo formation: (H )
development of collapsed pingos. (After P. J. Williams (l979)
Pipelines and Permafrosí. Longman, London, Fig.2,2, p.2l ,
ntodffied.f1,om T. Czudek and J. Demek (I970) Quaternary
Research 1, Fig. 9, p.11].)
Periglacial processes and landforms 307
occurred predominantly during a warm phase of the Holocene
between 9000 and 2500 a np.
12.3.3 Depositiona| forms related to mass moyernent
The downslope movement of debris in association with
periglacial mass movement processes gives rise to a range
of locally significant landforms. Deposits associated with
gelifluction assume a variety of forms, including sheets,
benches and lobes. Geliíluction sheets have a smooth.
gently sloping surfaces and a bench-like lower margin.
whereas gelifluction benches are terrace forms. Particularly
characteristic are gelifluction lobes which consist of
tongue-shaped forms, 30-50 m across, which are elongated
downslope (Fig. 12,16), They tend to occur on steeper
slopes (10-20') than benches. Where the downslope elongation
is very marked the term gelifluction str€am is userl.
Both benches and lobes have steep fronts, 1-6 m high,
which may be subject to erosion, They appear to form as
stones, which have been pushed downslope, accumulate
and dam the movement of material above,
Some gelifluction deposits are crudely stratified and most
exhibit a preferred downslope orientation of the long axis
of theř angular constituent particles, which may range in
size from boulders down to silt-sized material. Relict gelifluction
deposits are difficult to distinguish from other solifluction
debris, although they should be less weathered than
similar deposits from non-periglacial environments.
Block slopes are slopes mantled with angular boulders,
mostly between 1 and 3 m across, covering 50 per cent or
more of the ground surface. In the case of tllock streams
the boulders are concentrated in valley bottoms or occur as
Fig. I2.16 Gelífluction lobe, Heste skoen, Mesters Vig, northeast
Greenland. (Phottl (,zlulesy A. L. Washburn.)
1
Ý 4 + l - , >/
308 Exogenic processes and landforms
narow downslope linear deposits on hillsides. Downslope
movement is indicated for both forms by the orientation of
the long axes of the boulders which tends to be roughly
normal to the slope contours. Gelifluction and frost creep
are the favoured transporting mechanisms for most occurrences.
Block fields, consisting of relatively flat bouldercovered
suďaces, are also common. They probably form
largely by in situ frost shattering and the heaving ofjointed
bedrock and involve no significant downslope movement.
Rock glaciers are tongue-shaped masses of angular
boulders resembling in form a small glacier. They have a
steep front, exceptionally exceeding 100m in height, which
stands at the angle of repose of the constituent material.
Rock glaciers usually descend from cirques or cliff-faces
and may reach a length of l km or more. Large boulders
and smaller stones typically form the surface which commonly
exhibits arcuate ridges and lobes. Active rock glaciers
contain ice at depth, either filling voids (ice-cemented type)
or forming a core (ice-cored type), Some of the ice-cored
variety are probably derived from debris-covered glaciers,
whereas others may be transitional to ice-cored moraines.
In localities where rock glaciers and ice glaciers are both
currently active the distinction between them is essentially
one of the quantity of debris carried. Where debris-rich glacier
ice becomes stagnant sufficient ice may remain mixed with
fine sediment at depth to sustain slow movement, largely by
intemal deformation and basal shear. Frost creep and gelifluction
may also contribute to downslope movement. Irrespective
of the actual mechanism of movement, rock glaciers
are important transportational agents in highland periglacial
environments.
12.3.4 Asymmetric valleys
Asymmetric valleys are valleys with one slope significantly
steeper than the other. Virtually all valleys are asymmetric
to a certain extent, but some display a degree of asymmetry
sufficiently marked to warrant some special explanation.
Asymmetric valleys are known from a wide range of envřonments
and can clearly be caused by a number of factors,
but geomorphologists working in the periglacial zone have
long recognized the prevalence of asymmetric valleys
there. Collection of field data has indicated no uniformity in
the preferred orientation of the steeper slope in periglacial
asymmetric valleys (Table I2.1), although it has been suggested
that steeper north-facing slopes predominate in the
northern hemisphere below a latitude of about 70 oN.
Various conditions existing in periglacial environments
may contribute to valley asymmetry. In the northern hemisphere
longer exposure to the Sun on south-facing slopes
will result in a more rapid and prolonged thawing and a
greater abundance of meltwater, promoting gelifluction and
other mass movement processes. Consequently, southfacing
slopes will experience a more active reduction of
Table l2.7 Nature of slope asymmetry in the periglacial zone of the
northem hemisphere
oRIENTATIoN (ASPECT)
OF STEEPER SLOPE
LocALITY VALLEY
AI-IGNMENT
central Alaska
North-west Alaska
Southampton Island, Canada
Banks Island. Canada
Caribou Hills. Canada
Yakutia, Siberia (3 studies)
Andreeland, west Spitsbergen
Conwayland, west Spitsbergen
Kaffioya-Ebene, west Spitsbergen
Wollaston-Vorland. east Greenland
E_W
E_W
E_w
NW_sE
E-W
E_W
E*W
N*S
E_W
E_w
Sorrrce: Modified from H. M. French (19'76),The Periglacial
Environment, Longman, London, Table 8.3 p. 179.
slope angle, while theř colluvium will tend to divert streams
towards opposing north-facing slopes which will be subject
to undercutting and slope steepening. Moreover, as northfacing
slopes will be in shade longer during the summer
thaw season, permafrost, where present, will be preferentially
developed and will help to stabilize unconsolidated
sediments at a steeper angle. Prevailing winds may play a
role since snow will tend to accumulate on lee slopes and
the greater quantity of meltwater produced on thawing will
augment mass movement on such slopes. Differences in
vegetation cover may also be important. In a study in Alaska,
for instance, vegetation was found to be least developed on
the colder north-facing slopes which consequently experienced
less active mass movement.
Valley asymmetry has also been identified in areas subject
to periglacial conditions during the Pleistocene. Although
these have commonly been attributed to some of the
mechanisms just discussed relevant to periglacial environments,
it is difficult to separate any inherited periglacial
effects from other processes capable of producing asymmetry.
One important factor is the higher angle of inclination of
the Sun in mid-latitude Pleistocene periglacial environments
in contrast with the present-day high latitude periglacial
zone.
I2.3.5 Cryoplanation terraces and cryopediments
Cryoplanation terraces (also known as altiplanation
terraces) are level or gently sloping surfaces found in the
periglacial zone which are cut into bedrock on hill summits
or upper hillslopes. Cryopediments are a similar form
developed at the foot of valley sides or on marginal slopes.
Unless they transect structure it may be difficult to distinguish
them from structurally controlled benches, and in
any case lithological and structural factors are important in
their formation. Cryoplanation terraces range from 10m to
2km across and up to 10km in length, The risers between
terraces may reach a height in excess of 70 m and stand at
N
N
N
sW
N,S
N
S
W
S
N
an angle of 30o or more where debris-covered, and up to
90o where bedrock is exposed. The terraces are mantled by
selifluction debris and may contain pattemed ground.
Cryoplanation tenaces are thought to be formed by
several processes working in conjunction. In essence they
appear to form by the combined effects of the break-up of
bedrock by frost action and scatp recession. Various stages
are involved (Fig. 12.i7). According to one model, development
begins with the formation of a nivation hollow or
bench associated with snow patches. Nivation then proceeds
to erode a cliff which recedes as debris is transported
away from the cliff base, largely by gelifluction and slope
rvash. continued cliff recession on either side of an intertluve
finally forms a summit tenace with residual rock masses.
Some researchers, however, believe that most descriptions
of cryoplanation terraces are of relict forms, and that in
many cases the features described may be no more than
benches related to the differential erosion of contrasting
lithologies.
Cryopediments probably develop in a similar way
except that slope wash may be more active than gelifluction
in carrying debris away. Although many cryoplanation terFig.
I2.17 Stages in the development oJ' ctloplanatiun tet,rat es
in resistant rock: (A) original sulface; (B) .formation of nivaíion
hollows; (C) initial development of c,ryoplanation terrat:e; (D)
mature staqe of ctloplanation terrace developmení,. (E) initial
formation of cryoplanation swfac,e ; (F ) development of
clloplanation summit sulíaces. Arrows indicate the diret:tion of
surface modffication. (Based on model of J , Demek ( 1969)
Biuletyn Peryglacjalny 18, ] 15-25. after H. M. French (t976)
The Periglacial Environment. Longman, Londtln, Fi1.7.1l ,
p l60.)
Periglacial processes and landforms 309
races and cryopediments are associated With present-day
permafrost, frozen ground does not seem to be a prerequisite
for their formation.
Further reading
Students of periglacial geomotphology have no shortage of
excellent texts which they can consult. Washbum (1919)
provides a fully illustrated and referenced survey of processes
and landforms; a considerable strength of this text is
the extensive reference made to the work of Russian and
E,ast European scientists who have made very important
contributions to perigiacial studies. Other general sources
include Embleton and King (19'/5), French (1976) and
Williams and Smith (l989). The volumes edited by Church
and Siaymaker (1985) and Clark (1988) provide detailed
up-to-date treatments of many aspects of periglacial geomorphology,
while a number of useful paper§ are contained
in Zeitschrift fúrGeomorphologie Supplementband, 7I.
Various regional studies are included in Boardman (1987),
although here the emphasis is on relict periglacial forms in
the British Isles,
The problems involved in the human utilization of high
latitude environments have received much attention. Although
these are not specifically considered here, discussions of
various aspects of applied periglacial geomorphology are to
be found in Brown (1970), Hanis (1986), Sugden (1982)
and Williams (1919). Journals containing articles on periglacial
geomorphology include Biuletyn Peryglacjalny (a
Polish publication but with many arlicles in English),
Canadian Journal of Earth Sciences, Quaternary Research
and Arctic and Alpine Research, in addition to other joumals
which publish papers on geomorphology generally. Useful
annual reviews of periglacial geomorphology are to be found
in Progress in Physical Geography.
Harris (1986) provides a detailed discussion of the nature
and formation of permafrost, while the pioneer work by
Taber (1929, 1930, 1943) on frost action is still worth consulting.
Thom (1979) presents some intriguing data on the
occurTence of freeze-thaw cycles'and the efíicacy of frost
shattering in the alpine environment of the Colorado
R.ockies, and White (1916) compares the role of frost shattering
and hydration shattering. Both field and laboratory
investigations of frost shattering are reviewed by McGreevy
(l981), and Pavlik (1980) provides a brief discussion of the
factors controlling ground ice formation. Salt weathering in
relation to tor formation is considered by Selby (1912).
Mass movement pfocesses are covered by Harris (1987)
and Rapp (1986), and also in the more general texts,
especially Washbum (1979). Benedict (1916) gives a useful
review of frost creep and gelifluction and also provides a
detailed discussion on mass movement mechanisms in alpine
environments (Benedict, 1970). Fluvial processes in specific
periglacial environments are discussed in Church (1972)