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Advances in Periglacial Geomorphology E<li,ed by :V1. J. Clark 191988 John Wiley & Sons Ltd

10 Earth Hummocks (Thufur)

E. SCHUNKE

Geographisches lnstitut, Universitiit G6ttingen

and

S. C. ZOLTAI

Canadian Forestry Service, Edmonton, Canada

ABSTRACT

Dome-shaped hummocks (thufur) occur in arctic, alpine and subarctic environments where climate allows characteristic snow distribution and seasonal ground frost patterns. They occur in level or IJ.ear-level areas that have imperfect drainage. Their size varies within narrow limits, averaging 50 cm in height and 100 cm in diameter. The soil material is fine textured, stone-free or sparingly stony. Their internal structure shows displaced or distorted layers. Mounds are formed by the permanent displacement of local surface material in frost-sensitive soil in the presence of plentiful moisture under climatic conditions that generate seasonal frost penetration. Such conditions are conducive of frost-generated soil movements-cryoturbation. Internal structure indicates that cryoturbation is playing an active role in generating and preserving the shape of the hummocks. The exact mechanism of the initiation of hummock formation is not known. Speculation and circumstantial evidence indicate that uneven surface (microrelief, texture) and related differences in vegetation may create thermal variations in the soil that can initiate hummock formation. Once formed, differences in moisture content, insulative vegetation cover and soil texture act to create cryoturbations that will maintain the hummocks.

RESUME

Les buttes gazonnees (thufurs) en forme de domes sont tres frequentes dans les milieux arctiques, subarctiques et alpins, caracterises par une forte differentiation de la couverture de neige et des structures de gel saisonnier.

231

232 Advances in Periglacial Geomorphology

Les buttes gazonnees se trouvent sur des surfaces horizon tales aussi bien que

sur des surfaces legerement indinees a drainage incomplet. Les dimensions des

buttes ne varient que dans des Iimites assez etroites: dIes atteignent en moyenne

50 cm de haut et 100 cm de diametre. Les thufurs se forment dans des materiaux

mineraux fins, dans des limons et des limons sablonneux tres pauvres en

pierrailles. La structure interne des thufurs montre souvent une perturbation de la stratification originelle et un deplacement des fines. Ce deplacement

superficiel permanent des fines est responsable de la morphogenese des thufurs.

Les facteurs les plus importants sont la gelivite des sols en condition d'humidite

et l'existence d'un gel saisonnier qui favourisent les cryoturbations. L'observation

des coupes montre clairement que ces cryoturbations jouent un role actif dans

la formation et dans la conservation de la forme de ces buttes. Les stades initiaux

de la formation des buttes restent encore problematiques: probablement ils

dependent du microrelief, des irregularites de la granulometrie et de la couverture

vegetale. Le developpement de la forme est plus evident: la tendence au

soulevement s'accentue grace aux differentiations dans l'humidite, la texture

du sol et l'isolation thermique par la vegetation, qui s'accentuent de plus

en plus.

ZLISAMMENFASSLING

Kuppelformige Erdbulten (Thufur) mit vollsUindiger Vegetationsbedeckung

haben im arktischen, subarktischen und alpinen Milieu, dessen klimatische

Ausstattung zu einer charakteristischen Differenzierung der Schneeverteilung

und des saisonalen Bodenftostes fuhrt, weite Verbreitung. Die Erdbulten treten

auf eben en wie auf schwach geneigten Arealen auf, denen es an einer guten Drainage mangelt. Die GroBe der Formen variiert in engen Grenzen: Die Hohe

betragt zumeist um 50 CIn und der Durehmesser um 100 em. Das Lockermaterial

der Erdbulten weist sehIuffige bis feinsandige Kornung auf und ist in aller Regel

weitgehend steinfrei. Die innere Struktur der Erdbulten laBt haufig eine

Deformation oder Umlagerung des Feinmaterials erkennen. Die Bildllng der

Erdbulten geht auf langsame und langfristige Umlagerllngen des

oberfIaehennahen Loekersubstrates zuruck. Voraussetzungen hierflir sind

frostempfindliehe Boden mit ausreichender Durehfeuehtung sowie die

Ausbildung von saisonalem Bodenfrost. Diese klimatisehen und edaphisehen

Bedingungen bewirken frostbedingte Bodenbewegungen von der Art der

Kryoturbation. Die innere Struktur der Erdbulten dokumentiert, daB fi.ir die

Genese und den weiteren Bestand der Erdbulten diese kryoturbaten Prozesse

eine aussehlaggebende Rolle spielen. Die Einzelheiten jener Mechanismen. die

zur Einleitung der Thufur-Bildung fiihren, sind nicht genau bekannt.

Wahrscheinlicb sind es die Differenzierungen des Oberbodens (Mikrorelief,

Bodentextur u.a.) und der Vegetation, die thermisehe Differenzierungen im

Earth Hummocks (Thujur) 233

Substrat bewirken, die schliesslich durch subkutane Feinmaterialumlagerung

die Biiltenbildung initiieren. Nach dem Initialstadium fiihren die mit ihm

gegebenen Differenzierungen der Bodenfeuchte, der Bodentextur und der

Isolationswirkung der Vegetation zu jenen kryoturbaten Prozessen im Boden, die die Bildung ausgereifter Erdbiilten hervorrufen.

10.1 INTRODUCTION

This paper summarizes recent cryopedological field studies on earth hummocks

(thufur) in the American and European arctic and subarctic. These periglacial

forms have for many years attracted the interest of scientists in many countries.

The earliest descriptions of Icelandic thufur (singular: thufa) appear in Gruner

(1912) and Thoroddsen (1913). In Sweden, earth hummocks (jordtuva) were

studied by Bergstrom (1912) and G. Lundqvist (1944), and are described by

J. Lundqvist (1962). Sod-covered hummocks, found under permafrost

conditions, have been reported from various parts of the USSR (Kachurin, 1959).

In North America, Sharp (1942) describes earth hummocks from the Yukon Territory, but he includes several unrelated forms in his discussion. 'Turf

hummocks' are described from Greenland by Raup (1965), but it is not clear

whether these are earth hummocks.

In fact, the casual and non-specific use of the term 'earth hummock' and

the eagerness of some authors to liken various periglacial forms to the Icelandic

thufur, have created a great deal of confusion. In many cases, forms that have entirely different origins have all been called earth hummocks. In most of these instances the internal structure dT'lhe mounds was not investigated, and they

were grouped as earth hummocks on the basis of their superficial appearance

alone. Recent studies, however, have shown that one group of non-sorted circles,

occurring in large numbers in similar environments under arctic and subarctic

conditions, share both internal and external characteristics and have a common

possible genesis. This paper therefore describes and defines earth hummocks

(thufur) on the basis of their external and internal characteristics, indicates factors important to their development, and establishes their significance in terms

of climatic and non-climatic factors.

The practical importance of earth hummocks lies in the fact that they are

formed by frost heaving. In non-permafrost areas, such as Iceland, thufur can

develop on previously tilled land within a few decades. Such fields are unsuitable

for mechanized agriculture unless the thufur are destroyed by levelling. In

permafrost areas, earth hummocks indicate actively heaving surfaces, as shown by the tilting of trees. In addition, they are associated with ice accumulations

just below' the permafrost table. Any disruption of the insulating surface

vegetation causes this ice to thaw, resulting in very unstable conditions where

erosion and mass movement will damage the land surface. An understanding

234 Advances in Periglacial Geomorphology

FIGURE 10.1 Closely spaced thufur, Iceland

FIGURE 10.2 Earth hummock development on gentle slopes in fine-grained soils with imperfect surface drainage

Earth Hummocks (Thujur) 235

of the mechanisms that lead to the establishment and maintenance of

earth hummocks may well lead to the avoidance of the problems associated with

them.

10.2 THE CHARACTERISTICS OF EARTH HUMMOCKS

10.2.1 External morphology

The terms 'thufur' (Thoroddsen, 1913) and 'earth hummocks' ( Sharp, 1942)

describe a type of ground patterning which can be categorized as hemispherical

and domed non-sorted circles or nets (Washburn, 1956, 1980). Unless otherwise

qualified, the term earth hummock as used in this paper can be taken as the equivalent of thufur. The paper thus implies that these two terms are

interchangeable, but that they are both restricted to forms which meet the strict

morphological and genetic criteria discussed below.

In ground view the hummocks are generally circular or oval, but elongated

hummocks occur on gentle slopes if their angle is less than 6°. Their height

varies from 20 cm to 100 cm, and averages around 50 cm. Their basal diameter

is between 50 cm and 150 cm, with the majority close to 100 cm. Earth

hummocks usually occur closely spaced in distinct fields (Figure 10.1), although

they may be found on occasion as scattered individuals. When closely spaced,

they are separated from each other by narrow grooves or somewhat wider troughs. In general, the distance between hummocks is less than the diameter

of the mounds (Tarnocai and Zoltai, 1978).

Earth hummocks occur on flat or gently sloping areas of fine-grained soils

where the internal drainage is imperfect but there is no excess surface water

(Figure 10.2). They invariably develop in fine-grained, stone-free or sparingly

stony soils of volcanic-aeolian, lacustrine or glacial origin. Their distribution

is widespread in the arctic and subarctic regions of the northern hemisphere,

though infrequently they are also found in alpine areas outside the polar regions.

The associated vegetation is tundra or meadow, or open subarctic woodland

in North America.

The individual mounds are usually covered with vegetation, but in the

high arctic their apices may be bare. In permafrost areas, the insulating

vegetation is usually thicker in the interhummock troughs, hence the permafrost

table is higher under the trough than under the mounds. This results in the

development of small 'basins' in the permafrost table under the mounds-a

mirror image of the ground surface (Figure 10.3). In non-permafrost areas, different vegetation develops on the earth hummocks compared \vith that in

the intermound troughs due to different drainage conditions caused by

microrelief (Lotschert, 1974). Such vegetation differences undoubtedly cause

contrasts in the thermal regIme of various parts of the hummock, as is

discussed later.

236 Advances in Periglacial GeomorphologJ'

FIGURE 10,3 Section through earth hummock showing the development of a 'basin' in the underlying permafrost table

FIGURE 10.4 Section through earth hummock showing disrupted and displaced sedimentary layers of volcanic ash, Iceland. (Scale in centimetres and decimetres)

Earth Hummocks (Thujifr) 237

10.2.2 Internal morphology

The internal morphology of earth hummocks is characterized by disrupted and

displaced horizons and strata. The intruded material may be parts of soil

horizons, organic layers or sedimented layers such as volcanic ash (Figure 10.4).

Tongues of such materials may extend downwards, especially at the perimeter of the mound, thence tending to\vards its centre and then often turning upwards.

The material displaced by these intrusions often shows flow patterns that

normally develop in a viscous liquid.

Earth hummocks consist of fine-grained mineral soil. The combined clay­

and silt-sized particles dominate in the soil (58-99 per cent in Canada; average

of 60 per cent in Iceland), and the remainder is mostly fine sand (Tarnocai and

Zoltai, 1978; Schunke, 1977a). The texture of the unfrozen hummock material is usually homogeneous, but there may be a slight increase in coarser-textured

particles on the sides and tops of the mounds. Mackay (1980) found that the

sand content was higher at the top of the permafrost table and at the centre

than on the sides, indicating a cell-like circular pattern. Finely disseminated

organic carbon is common within the unfrozen earth hummock material; the concentration may reach 2-5 per cent ( Zoltai and Tarnocai, 1974)

Hollow spaces are not observed inside the hummocks, although the soil is

usually loosely structured within the rooting zone of plants. The bulk densities of

the mineral soil in hummocks have a fairly wide range (1.00-2.18 g/ cm3), such

as is characteristic of soil materials affected by cryoturbation. In areas of high

rainfall the earth hummocks show homogeneous soil moisture content. In areas

of continental climate, however. the soil moisture content is low at the hummock

apex, increases towards the centre, ��d reaches a maximum above the permafrost

table. The ice content of the permafrost in hummocky areas is high, being well

in excess of 100 per cent by weight. The ice-rich layer, however, is only about 1 m thick (Zoltai and Tarnocai, 1974). The presence of such concentration of

segregated ice is not necessarily related to earth hummock formation, but it

does reflect the availability of plentiful water in the imperfectly drained

hummocky areas.

10.2.3 Dynamic features

The dynamic nature of earth hummocks is indicated by their external and internal

characteristics. Externally, some hummocks may be split across the apex, or

the organic mat may be ruptured near the base, indicating internal pressures.

Trees grO\ving on hummocks are invariably tilted by the heaving of hummocks

(Figure 10.5) (Zoltai, 1975). Internally, the presence of disrupted and displaced

materials points to cryoturbation activity.

The age of earth hummocks is difficult to determine. In North America, radiocarbon dates indicate that most earth hummocks originated 2500-5000 BP

238 Advances in Periglacial Geomorphology

FIGURE 10.5 Tilting of trees caused by heaving of the earth hummocks upon which they are growing

FIGURE 10.6 Relationship between thufur and snow retention, Iceland. Deep snow has an insulating effect and therefore decreases frost penetration

Earth Hummocks (Thufur) 239

( Zoltai et al., 1978), although they have remained active since then. In Iceland,

thufur formation is taking place at the present time, as is shown by the

development of thufur on cultivated land (Schunke, 1977a). Other thufur,

however, are of greater antiquity, as shown by the deposition of volcanic ash

layers of kno\vn age that conform to the shape of the mounds.

10.3 CLIMATIC CONDITIONS FOR

EARTH HUMMOCK FORMATION

Virtually all earth hummocks in North America are underlain by permafrost

where the seasonally-thawed layer freezes every winter. The exceptions are those

occurring in alpine areas (Scotter and Zoltai, 1982) or those that are inactive

fossil forms (Tarnocai and Zoltai, 1978). In the USSR earth hummock-like

structures were identified from permafrost regions (Kachurin, 1959), but

hummocks in non-permafrost areas were reported from eastern Siberia

(Naumov, 1963). In Europe, earth hummocks are again not restricted to

permafrost terrain. The climate is severe enough to allow relatively deep seasonal

frost penetration, although in the lowlands of Iceland the seasonally frozen layer

is only 30-40 em thick (Schunke, 1977a). Local climatic conditions, especially

the distribution of snow, become important (Figure 10.6). Deep and late-thawing

snow has an insulating effect on the soil and therefore tends to decrease frost

penetration. Thorarinsson (1951) notes that thufur are absent from regions where

snow cover is particularly thick and melts late. The occurrence of earth

hummocks in Scandinavia is virtually restricted to areas above the tree line

(J. Lundqvist, 1962). This may be J;�lated to snow redistribution, which is more

effective in treeless areas than in those with trees. In Greenland, too, earth

hummocks are not limited to permafrost terrain (Schunke, 1977b).

10.4 THE GENESIS OF EARTH HUMMOCKS

The mechanism of earth hummock formation is not known with certainty. It

is known that the mounds are produced by the permanent displacement of local

surface soil materials. The process that can produce such displacement under

the given circumstances is cryoturbation. All the necessary ingredients are

present: heave-sensitive soil, plentiful soil moisture and seasonal frost

penetration. Should any of these conditions be lacking, hummock formation

does not take place.

As is the case with several periglacial features, the initiation of the formative

process is more difficult to explain than its subsequent maintenance or

development. The initiation of mound development may be connected \'lith the

movement of moisture toward a freezing front. The moving moisture may carry some fine-grained soil particles. Cellular centres often develop in relatively

homogeneous fine-textured soil, and clay-sized soil particles may be concentrated

240 Advances in Periglacial GeomOlphology

20-150 em

A

I 120-80 I em

L

FIGURE 10.7 A model of cellular moisture and fine sediment movement in an earth hummock (after Schunke, 1981)

in such centres by water movement associated with freezing (Figure 10.7)

(Schunke, 1981). Another mechanism for the initiation of earth hummocks may

be the random development of frost-heaved spots (Mackay, 1980), which may

remain without insulating vegetation for several years. Such spots have different thermal regimes from the surrounding vegetated areas and may serve as the focus

for mound development. Fields of hummocks can develop by the infilling of

spaces between hummocks.

Once an embryonic earJh hummock has been initiated, the vegetation will

develop differentially on the mounds compared with the intervening areas. The

small mounds are somewhat better drained, and an insulating, more mesic

vegetation develops on them than in the moister troughs. This results in differential thermal regimes: the drier mound will lose heat more slowly, than

the moist surroundings. In the more moist spots the freezing front penetrates

faster and deeper than under the mound. This sets up lateral pressures toward

the centre of the mound, displacing more materials and eventually forming the

earth hummocks (Schunke, 1977b). An interesting difference between earth hummocks with and without a

permafrost substrate is the depth of cryoturbation. In non-permafrost soils the maximum deformation takes place at 30-60 cm (Schunke, 1977b), and layers

at a greater depth are completely undisturbed (Figure 10.8). In permafrost areas

the entire hummock above the permafrost table shows severe mixing. These

differences show that in non-permafrost areas the seasonal frost (depth and

rate of penetration) is responsible for the maintenance of the earth hummocks.

In permafrost areas the greater depth of mixing may be accomplished by a

cellular motion, maintained by basin-shaped depressions in the permafrost table under the hummocks (Mackay, 1980).

Earth Hummocks (Thufur) 241

FIGURE 10.8 Section through an earth hummock in an area without permafrost, Iceland. l\1aximum deformation occurs at depths of 30-60 cm, while deeper layers remain

undisturbed

The presence of a permafrost taGle constitutes another difference between the

permafrost and non-permafrost environments for hummock formation. The

permafrost table acts as a layer virtually impervious to water and as a base that

can firmly withstand pressures associated with cryoturbation. In non-permafrost areas where such a firm impervious base is absent, earth hummocks develop

in layered deposits, such as volcanie ash, that have a uniformly fine texture but are underlain by different materials. If underlain by coarser material, moisture tends to accumulate at the textural boundary. If underlain by finer materials,

internal drainage may be impeded, thus giving moister conditions. If underlain

by bedrock or dense till, these materials will (like permafrost) resist pressures and direct the resulting forces upward.

to.5 DEFINITION AND TERMINOLOGY

The equivalent terms 'earth hummocks' and 'thufur' should be restricted to

those mounds that have the internal (texture, structure, moisture, cryoturbation)

and external (size, shape) characteristics described in this paper, and that occur within arctic, alpine or subarctic regions of the northern or southern

hemispheres. To conform to traditional usage, those forms which were produced

242 Advances in Periglacial Geomorphology

FIGURE 10.9 Small, very closely spaced hummock-like mounds in arctic Canada, possibly formed by water deepening of desiccation cracks rather than by true earth

hummock processes

FIGURE 10.10 Classic thufur development in an agricultural field in Iceland

Earth Hummocks (Thufurj 243

under permafrost conditions may be called earth hummocks, and those occurring in non-permafrost soils should continue to be called thufur. According to the

current state of knowledge, these forms are produced by the same process, and they differ only in the degree of cryoturbation: earth hummocks show the effects

of frost churning throughout the mound above the permafrost table, while in

thufur the disruption occurs only in the upper 60 cm. When in doubt, a non­

genetic term such an non-sorted circle (Washburn, 1956) can be used.

Superficially similar, but basically different, forms should not be confused

with earth hummocks or thufur. To clarify these distinctions, it is worth

considering some of the major categories of features which cannot be equated

with true earth hummocks:

(1) Small hummock-like mounds that occur in the high arctic of Canada on

moderate to steep (5-20°) slopes may be mistaken for earth hummocks.

However, when examined and measured at twelve locations throughout

the Canadian Arctic Archipelago ( Zoltai, unpublished data), they were

found to be considerably smaller than true earth hummocks: their average

height is 20 cm (range lO-52 cm), and their average diameter is 31 cm

(range 12-68 cm). The mounds are closely spaced (Figure lO.9); a 30 m

long straight line touches an average of 76 mounds. The ball-like mounds

are completely covered with dwarf shrubs (1-2 cm high) of Dryas or

Cassiope. Internally, they are composed mainly of fine sand

(0.08-0.4 mm), and show no evidence of cryoturbation. The Bernard soil

on Banks Island (Tedrow and Douglas, 1964) has developed on areas of

such small mounds. The PaLent material is stone-free or very sparsely stony

colluvial, aeolian, lacustrine or fluvial deposits. At six locations in the

Canadian study, desiccation polygons (average diameter 28 cm) were

found on level ground above the mound-covered slopes. It is, therefore,

suggested here that the small mounds are formed by water eroding and

overdeepening the desiccation cracks that originated in a uniform readily

erodible fine sand material. These desiccation mounds are not earth

hummocks as here defined.

(2) Other mounds, not associated with periglacial environments, are the

'gilgai' that are formed by the s\velling of clay ( Hallsworth et al., ]955).

(3) 'Pimple mounds' are believed to be erosional pedestals that were protected

by the roots of single trees (Cain, ]974). (4) 'Mirna mounds' appear to originate by the excavations and earth-moving

activities of pocket gophers (Geomys spp., and Thomomys spp.) (Cox, 1984).

(5) The hummocky microrelief affecting over 90 per cent of the forested land

in the Appalachian region of North America is caused by the uprooting of trees by wind (Beke and McKeague, 1984). There is a characteristic

mound and pit relief, and the soil horizons are disrupted (arbotur bated).

244 Advances in Periglacial Geomorphology

None of these numbered forms are earth hummocks. On the other hand, mounds

that lack cryoturbation features but are otherwise comparable to earth hummocks have been noted in northern Canada (Tarnocai and Zoltai, 1978),

and these are believed to be old inactive forms of earth hummocks. The

dimensions of the mounds are similar to those of the active forms, but soil

horizons are well developed under both the hummocks and the troughs, and the soils are not disrupted. As there are no detailed descriptions of fossil earth

hummocks, it is not known in what form they might be preserved. Studies of

earth hummocks in terms of human influences are equally lacking; however,

the development of particularly regular and striking forms on man-made

hayfields in Iceland (Figure 1 2.10) introduced the term 'thufur' to the

international periglacial terminology.

10.6 PERSPECTIVES

When dealing with a little-known periglacial phenomenon, it is prudent to apply

a generalized, non-specific nomenclature and classification. Thus the term 'non­

sorted circle' (Washburn, 1956) served for many years to describe various forms

that were somewhat similar. How'ever, as knowledge increased through careful

observations, analyses and measurements, it became apparent that certain groups of non-sorted circles were distinctly different from others. Earth hummocks

and thufur are different from most other non-sorted circles in their internal

and external morphology and perceived genesis, hence these terms are proposed

for the specific forms to which they have been applied in this paper. An

agreement on common terminology would greatly facilitate communications

between various workers.

In the last few years there has been an explosion of information on soil conditions under freezing conditions. However, little of this has been applied to the study of periglacial phenomena. Knowing the conditions under which

earth hummocks and thufur are formed, it should be possible experimentally

to initiate and monitor the development of such forms, with full instrumentation.

This vl'Ould supply substantive confirmation of models hitherto based on

speculative indirect evidence.

10.7 REl-'ERENCES

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BEKE, G. J. and McKEAGUE, J. A. (1984). Influence of tree windthrO\v on the properties and classification of selected forest soils from Nova Scotia, Canadian Journal of Soil Science, 64, 195-207,

C\JN, R. H. (1974). Pimple mounds: a new viewpoint, Ecology, 55, 178-82. Cox, G. \:y'. (1984). Mounds of mystery, IVatural History, 93, 36-45. GRUNER, M. (1912). Die Bodenkultur Islands, Arch, Biantol., vol. 3, Berlin, 213pp.

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H/VLLSWORTH, E. G., ROBERTSON, G. K. and GIBBONS, F. R. (1955). Studies in pedogenesis in New South Wales. VII The 'gilgai' soils, Journal of Soil Science, 6, 1-34.

KACHURIN, S. P. (1959). Principles of GeoClyology. Part 1: General Geocryology, Academy of Sciences of the USSR, V.A.Obruchev Institute of Permafrost Studies, Moscow. Chapter Xl: Cryogenic physico-geological phenomena in permafrost regions, pp. 365-98. National Research Council of Canada Translation TT-1157 (1964).

U)TSCHERT, W. (1974). Uber die Vegetation frostgeformter Boden auf Island, Ber. a. d. Forschungsstelle 'Nedri As', 16, Hveragerdi, Iceland, 15pp.

LUNDQVIST, G. (1944). De svenska fjiillens natur, STF's Handbdcker Olll det svenska fJdllet, vol. 2, Stockholm, 440pp.

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tVfACKAY, J. R. (1980). The origin of hummocks, western Arctic coast, Canada, Canadian Journal of Earth Sciences, 17, 996-1006.

NAUMOV, E. M. (1963). Soils of the southern part of the Magadan region along the coast of the Sea of Okhost, in IVANOVA, E. N. (ed.), Soils of eastern Siberia, Izdatel'stvo Akademii Nauk SSSR, Moscow. Israel Program for Scientific Translations, Jerusalem (1969), pp. 175-223.

RAUP, H. M. (1965). The structure and development of turf hummocks in the Mesters Vig district, northeast Greenland, Medd. 0111 Gronland, pp. 166, Copenhagen, 112pp.

SCHUNKE, E. (I 977a). Zur C)kologie der Thufur Islands. Ber.a. d. Forschungsstelle 'Nedri As', 26, Hveragerdi, Iceland, 69pp.

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SCOTTER, G. \V. and ZOLTA1, S. C. (1982). Earth hummocks in the Sunshine area of the Rocky Mountains, Alberta and British Columbia, Arctic, 35, 411-16.

SHARP, R. P. (1942). Soil structures in the St. Elias Range, Yukon Territory, Journal of Geomorphology, S, 274-301. __

TARNOCA1, C. and ZOLTAI, S. C. (1978). Earth hummocks of the Canadian Arctic and Subarctic, Arctic and Alpine Research, 10, 581-94.

TEDROW, J. C. F. and DOUGLAS, L. A. (1964). Soil investigations on Banks Island, Soil Science, 98, 53-65.

THORARINSSON, S. (1951). Notes on patterned ground in Iceland, with particular reference to the Icelandic '!las', Geografiska Annaler, 33, 144-56.

THORODDSEN, Th. (1913). Polygonboden und 'thufm' auf Island, Pet. Geogr. Mitt., 59, 253-5.

WASHBURN, A. L. (1956). Classification of patterned ground and review of suggested origins, Bulletin Geological Society of America, 67, 823-56.

WASHBURN, A. L. (1980). Geocryology, Wiley, London, 406pp. ZOLTA1, S. C. (1975). Tree ring record of soil movements on permafrost, Arctic and

Alpine Research, 7, 331-40. ZOLTAI, S. C. and TARNOCAI, C. (1974). Soils and vegetarion of hummocky terrain,

Environmental-Social Committee, Task Force on Northern Oil Development, Report 74-5, Inf. Can. Cat. R72-13374, Ottawa, 86pp.

ZOLTAI, S.c., TARNOCAl, C. and PETTAPIECE, W. \V. (1978). Age of cryoturbated organic materials in earth hummocks from the Canadian Arctic, Proceedings 3rd International Conference on Permafrost, vol. 1, National Research Council of Canada, Ottawa, pp. 325-31.