_.‘THIS FILE COPY MUST BE RETURNEB

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Arctic and Alpine Research, Vol. 10, NCopyrighted 1978. All rights reserved.

INFORMATION SECTIONNORTHERN FORESTRY CENTRE5320-122 STREETEDMONTON7 ALBERTA

T6H 3S '‘'.5

EARTH HUMMOCKS OF THE CANADIAN ARCTIC AND SUBARCTIC*

C. TARNOCAIT AND S. C. ZOLTAI$

ABSTRACT

Studies in the western Arctic and Subarcticand in the central Arctic indicate that earthhummocks have an average diameter of 80 to160 cm and an average height of 40 to 60 cm.They have developed on materials having 58to 99% total clay and silt content and eitherhigh ice content or pure ice layers in the near-

surface permafrost. Earth hummocks arecryogenic in origin with their developmentbeing controlled by three major factors: soiltexture, soil moisture, and soil temperature.Most of the earth hummocks have developedduring the last 5000 yr, when the climate be-came colder.

INTRODUCTION

The term earth hummock was first used bySharp (1942) and describes a particular,dome-shaped type of nonsorted net or circle(Washburn, 1956, 1973; Mackay andMacKay, 1976). They are widely distributedin the subarctic forest of the MackenzieValley, the Yukon, and the western Arctic(Mackay, 1958, 1963; Mackay et al., 1961;Tarnocai, 1973; Zoltai and Pettapiece, 1973;Zoltai and Tarnocai, 1974) and are com-monly found in the central Arctic and on theArctic islands (Tarnocai and Boydell, 1974;Tarnocai et al., 1976; Tarnocai, 1976; Zoltaiand Woo, 1976). Earth hummocks are a cir-cumpolar phenomenon and are also found in

northern Europe (Lundqvist, 1969), Siberia(Naumov, 1969), Greenland (Raup, 1965),and Iceland (Thoroddsen, 1914).

The term "earth hummock" or "hummock"is commonly used in the Mackenzie Valleyliterature. Synonymous terms used elsewhereinclude mud hummock and thufur. In thispaper the terms "earth hummock" or "hum-mock" will be used.

The purpose of this paper is to describe theexternal and internal characteristics of earthhummocks and to suggest a theory for theirdevelopment. This work is based on studiescarried out in the western and central Arcticand western Subarctic of Canada.

DESCRIPTION OF THE STUDY AREA

The study area is composed of two basicunits (Figure 1): the Mackenzie Valley andnorthern Yukon areas (western area), and thenorth-central part of the District of Keewatin

*Contribution No. L.R.R.1. 713.(Land Resource Research Institute, K. W.

Neatby Building, Ottawa, KI A 006, Canada.$Northern Forest Research Centre, Edmonton,

Alberta T6H 3S5, Canada.

and some of the Arctic islands in the Districtof Franklin (eastern area).

Only the extreme northern part of the west-ern study area falls beyond the arctic treeline; the remainder of this area falls into thesubarctic forest region. Most of the westernstudy area is mountainous with elevations1000 m or higher. There are, however, largelowlands along the arctic coast while the Mac-kenzie Valley and some of the adjacent areas

C. TARNOCAI AND S . C. ZOLTAI / 581

KIN 'a

ILLIAMi

ISLAND

BEAd/FORT SEA

7o.Resolute

.1704

BY674578WISCI

4550ALES

I AMPBRODEUR

REN

Study Area

Northern limit of trees

--•— Southern limit of continuouspermafrost

Southern limit of discontinuous65° permafrost

568. Earth hummock site

tNtA

70

TERR.

0 50 100 200 Km

DIST

MAC

- f

-130°

140°

60°

y20°5° DISTRICT

100° 95^ 9

FIGURE 1. Map showing the study area and the location of sites included in this paper.

are characterized by gently undulating tohilly topography. The area east of the Mac-kenzie and Richardson mountains was at onetime covered by the Laurentide ice sheet. Therest of the area was either unglaciated or af-fected only by local mountain glaciation.Fine-textured materials (till, colluvial, andsedimentary in origin) are the most commonsurficial deposits in this area.

All of the eastern study area is far beyondthe arctic tree line. The area is hilly with ele-vations generally below 300 m although a fewareas reach 600 m. All of this area was af-fected by glaciation and the coastal lowlands,which generally lie below the 200-m contour,were subjected to subsequent marine inunda-tion. This inundation resulted in the deposi-tion of considerable quantities of marine clayand silt and it is in these areas that earthhummocks are usually found today.

Although the climate of the study area iscold and all areas are affected by permafrost,

the western section of the study area is signifi-cantly warmer than areas at the same latitudein the eastern section. For example, in thewestern section of the study area the meanannual air temperature ranges from -4°C atFort Simpson to -11°C at Tuktoyaktuk withmean July air temperatures of 17 and 10°C,respectively (Burns, 1973). In the eastern sec-tion, however, the range is from -12°C atBaker Lake to -16°C at Resolute with meanJuly air temperatures of 11 and 4°C, respec-tively (Thompson, 1967). Although summertemperatures are low in the Keewatin area,they are especially low on the High Arcticislands where snow and frost can occur at anytime during the summer.

Most of the subarctic portion of the westernstudy area is within the discontinuous perma-frost zone. The northern part of the westernstudy area (northern Subarctic and Arctic)and all of the eastern study area are withinthe continuous permafrost zone (Figure 1).

MATERIALS AND METHODS

During the past six years, 173 hummocksites were examined (110 in the western studyarea and 63 in the eastern study area) in con-junction with systematic terrain studiescarried out in the Mackenzie Valley, northernYukon, District of Keewatin, and Arcticislands. Approximately 25% of the sites wereexamined and sampled in detail; the re-mainder were examined and sampled in lessdetail during brief helicopter stops.

FIELD METHODS

Aspect, slope, and soil drainage were deter-mined at each location. At detailed study sitesthe external dimensions of ten hummockswere measured. The height of the hummockwas measured from the mineral soil surface inthe trough to the corresponding surface onthe mound. The diameter was measured fromthe toe of the hummock in the interhummocktrough across the center of the moundparallel to the slope. Two diameter measure-ments were made for each hummock, the sec-ond being at right angles to the first measure-ment. The number of hummocks intersectedby a 50-m transect line was also counted ateach detailed site.

At each detailed site a soil pit was dug toproduce a cross section of a hummock extend-ing at least 50 cm into the permafrost; onshort stops made during helicopter traversesthe pit was dug only to the permafrost. Soilprofile characteristics were described andsamples were collected. Core samples werealso collected from the active and permafrostlayers and soil temperatures were measuredwith a thermistor on a predetermined grid10 cm behind the exposed soil face.

LABORATORY METHODS

Soil samples from representative horizonswere air dried and the portion less than 2 mmin diameter was used for analysis. The par-ticle-size distribution was determined by thepipette method (McKeague, 1976). Bulk den-sity, moisture, and ice content were deter-mined by drying the core samples at 105°Cand measuring the weight of water lost(McKeague, 1976). Radiocarbon dates weredetermined by the Radiocarbon Laboratoryof the Brock University Geological SciencesDepartment (BGS) and the Geological Surveyof Canada (GSC).

C. TARNOCAI AND S. C. ZOLTAI / 583

RESULTS

DISTRIBUTION OF EAR"' H HUMMOCKS

Earth hummocks occur commonly in thelowlands of the western (Mackenzie andYukon) portion of the study area. In thesouthern lowlands they cover very small areas(less than 5%) but in the central lowlands ap-proximately 50% of the mineral terrain is af-fected (excluding the mountainous terrain)and in the northern lowlands, to the arctictree line, 80% is affected. Beyond the treeline, in the tundra area, approximately 95%of the lowland mineral terrain in the westernportion of the study area is affected by earthhummocks.

Earth hummocks cover much smaller areasin the eastern portion of the study area andare not found in the high, north-centralplateau of the Keewatin (southern part of theeastern section of the study area). They do,however, cover extensive areas (approxi-mately 40% of the mineral terrain) in theMurchison River basin and the areas south-west of Pelly Bay, but they are very sporadicalong the Hayes River and on the BoothiaPeninsula. Earth hummocks cover only smallareas of the Arctic islands and are foundmainly in the coastal lowlands.

EXTERNAL MORPHOLOGYIn the western Arctic and Subarctic the ex-

ternal dimensions of 484 hummocks weremeasured at 61 different locations to obtainthe average sizes of hummocks at detailed in-vestigation sites. In addition, single hum-mocks were measured at 250 different loca-tions. In the central Arctic the dimensions of82 hummocks were measured at 25 differentlocations.

There is a great deal of variation in the sizeof individual hummocks even at the samelocation. It is not unusual to find hummocksthat are twice the size (both in height anddiameter) of others. In the Mackenzie Valleythe average height of hummocks at the inten-sively studied sites was between 40 and 60 cm.The highest individual hummock was 78 cmand the lowest identifiable hummock was14 cm high. In the central Arctic the hum-mocks tend to be lower. Here, the averageheights varied between 22 and 40 cm, with anoverall average of 32 cm.

The diameter of hummocks is equally vari-able on the intensively studied sites in the

584 / ARCTIC AND ALPINE RESEARCH

Mackenzie Valley where they ranged between110 and 160 cm, with an overall averagediameter of 132 cm. In one area in the west-ern Subarctic, near Willowlake River, hum-mocks were found which were up to threetimes larger in diameter. Here the averagediameter was 320 cm, ranging between 210and 440 cm. In the central Arctic the diam-eter of the hummocks averaged 117 cm, withmost hummocks in the 80- to 140-cm range.

The distance between hummocks (i.e., thewidth of the interhummock depression) isusually less than the diameter of the hum-mocks. On average, a 50-m transect wouldcross 23 hummocks, but in most areas thisnumber varied from 18 to 30. There is someindication that hummocks tend to be fartherapart in poorly drained locations.

On flat areas the hummocks are generallycircular (Figure 2B) in outline, but onslopes greater than 1% they tend to be elon-gated downslope. There appears to be adirect relationship between the steepness ofslope and the elongation of hummocks (Fig-ure 3). On steep slopes, generally over 12%,these structures are greatly elongated and ver-tical development (height) rarely takes place.They are either nonsorted nets or stripes,rather than hummocks.

In horizontal cross section the outline of ahummock resembles a section of a circle or anoval. In vertical cross section the sides risesteeply and flatten out gradually, resulting ina distinct apex. Although a portion of theapex may be flat, it is generally rounded (Fig-ure 2).

Differential accumulation of organicmatter on the surface tends to level the hum-mocky terrain, especially in forested areas. Inundisturbed areas an organic layer (up to15 cm thick) develops on the tops of hum-mocks, but it may be 20 to 25 cm thick in thetroughs (Zoltai and Tarnocai, 1974). Forestfires usually burn the organic mat on top ofthe hummocks, but seldom in the wettertroughs. In the tundra of the arctic regionsthe organic layer on top of the hummocks isthinner, averaging 7 cm. In some areas thetops are bare of organic matter and livingplants, although up to 20 cm of organicmaterial may be present in the troughs andEriophorum tussocks may grow on the lowerpart of the sides.

A

FIGURE 2. (A) Earth hummocks from the air near Murchison River; (B) cross section of an earth hum-mock burned by the 1968 Inuvik forest fire; (C) cross section of an earth hummock from the subarcticforest region; and (D) earth hummocks on Bathurst Island.

u C9 0

C)Organic Layer Granular Mineral Horizon

Apex

/ 5 m

INTERNAL MORPHOLOGYThe permafrost table, which is the mirror

image of the surface topography, separatesthe active-layer portion from the perenniallyfrozen portion of the hummock. The mostcommon internal features of these two dis-tinct portions are shown in Figure 4. Thegranular, relatively dry, mineral horizon nearthe apex is very commonly found on hum-mocks, except for those associated withpoorly drained (wet) conditions. A large por-tion of the central core of the hummock iscomposed of a structureless, massive mineralmaterial containing intrusions of bothorganic and mineral soil materials andorganic smears or very well dispersed organicmaterials mixed with mineral materials.

The subsurface organic layer most oftenoccurs in the form of a tongue from theorganic layer in the interhummock depres-sion. Very often, however, this layer is con-tinuous under the hummock, both above andbelow the permafrost table. Tongues extend-ing upward into the hummock from this sub-surface organic layer are characteristic. The

F IGURE 3. Ratio of downslope diameter (Dd) andcross-slope diameter (Dc) of hummocks on variousslopes.subsurface organic layer sometimes joins thesurface peaty layer at the interhummock de-pression. In this situation, the mineral coresare truly separated and enveloped by organic

Interhummock depression

Structureless, Massive MineralHorizon

FIGURE 4. Schematic diagram showing the external and internal morphology of earth hummocks.

586 / ARCTIC AND ALPINE RESEARCH

Soil Parent Material

layers. The subsurface organic layer and theunderlying mineral layer are very high in iceand often contain pure massive ice.

TEXTUREAll hummocks examined were associated

with fine- to medium-textured (clay to siltloam) glaciolacustrine, glacial till, marine, orcolluvial materials. The particle-size distribu-tion of these materials (Figure 5) indicatesthat the total silt and clay content varies be-tween 58 and 99%, with the silt content rang-ing between 17 and 64% and the clay contentbetween 17 and 57%. The sand fraction ofthe material shows a variation between 1 and32%. All of these above figures are related tothe perennially frozen parent material whichis probably least, if at all, affected bycryoturbation.

On examination of the active layer of an

earth hummock it was found (Figure 6A) thatthe clay content was less than 40% on the sideand the surface but 40% or greater in themid-central portion of the hummock. The siltcontent, however, is just the reverse with thesurface and side of the hummock generallybeing 44% or higher and mid-central portionbeing less than 44%. This mid-central por-tion of the hummock shows the highest totalsilt and clay content, being 84% or higher.Other parts of the hummock have total siltand clay content less than 84%. The totalsand content, in general, varies little. Theside of the hummock, however, tends to haveslightly more sand than does the middle andthe mid-central portion has less sand contentthan does any other part of the hummock.

MOISTURE AND ICE CONTENTThe surface of the active layer of all hum-

100

90

80

70

60

50

40

30

20

I0

0

FIGURE 5. Distribution of silt and clay content in the perennially frozen portion of hummocks throughoutthe study area. The Y samples are from the Mackenzie Valley and the North Slope of the Yukon; the DC,C, and ZJ samples are from northern Keewatin and Boothia Peninsula; the WZ samples are fromSomerset Island; and the B and DB samples are from Bathurst Island.

C. TARNOCAI ANDS. C. ZOLTAI / 587

41 1-- J • 41 0 BCy

42..2

Cz

— _H—. ____ -- — —--. • • 24 5 ----7•7Bm • 33.8 ---- ...._\

39.1 •34.0 ----n -- — -------------- ---

(. 35 .5 •38 . 5 3870 iii-- is? OmI • 39 4 38 7

Om --z

34 4

B

Om

1 48 • 34

Cz

8 494: - H

:93 :Bm •BO35 7

9 • 452 \39 3421 • 42 7 89.5--- 40.3 --

mt• 47 82 4

32.43545.3 42 6 • •• 41.7 904 I/ 82 2 41.237.6 40.1 40.7829 B4782 - 7 BCy 82.4

73 2 430 •41 2B 6 84.2

Cz

— • 44.4 44.640 384 9

Scale10 crn

c rn

D.

FIGURE 6. Hummock Y94, showing the distribution of (A) silt and clay content; (B) moisture content (%)on a volume basis; (C) bulk density (g cm -3 ); and (D) isotherms throughout the thawed layer. The brokenline ( ) represents the soil horizon boundaries, where Om and L-H are organic horizons, Bm is agranular brownish horizon and BCy a strongly cryoturbated massive transitional mineral horizon. Thesymbol — • — • — represents the frost table with the perennially frozen Cz horizon below this.

10 •

Y33A

EU

20a_

30

WATER CONTENT % ON A VOLUME BASIS

10 20 30 400

50

Y33B

0

40

mocks examined had a very homogeneousmoisture content. The moisture content ofthese hummocks varied from 17 to 39% mois-ture on a volume basis. Hummocks examinedin the southern part of the Mackenzie Valleywere generally drier than elsewhere. Hum-mocks in the central and northern MackenzieValley were generally moister (35% orgreater) than the central Arctic ones wherethe moisture content was usually less than35%.

On examination, the moisture distributionwithin a hummock was found to vary greatlywithin a distance of 1 m. Moisture profiles areshown in Figure 7. Moisture profile Y33A is asection from the apex of the hummock andY33B is a section from the interhummock de-pression. The surface on the apex of thishummock is fairly dry, having a moisture con-tent less than 20%. The moisture content in-creases rapidly with depth and reaches a valueof 38% at a depth of 30 cm. From this depthto the frost table (55 cm) the moisture contentshows little variation. The moisture profile inthe interhummock depression, 100 cm dis-tance from the above moisture profile, showsa higher surface moisture content but herethe moisture content initially increases more

slowly with depth and reaches its highestvalue of 48% at a depth of 20 cm, just abovethe frost table which occurs at a depth of25 cm.

Thus, the earth hummock can be charac-terized as having a fairly low moisture contentin the surface layer. The moisture content in-creases with depth and reaches its highestvalue just above the frost table (Figure 6B).

The ice content of permafrost (the upper50 cm) beneath hummocks was low in thesouthern part of the Mackenzie Valley (FortSimpson area) with an average ice content of19% on a weight basis. No ice layer or lenseswere found. The ice was in the form of verysmall, segregated ice crystals and, sometimes,very thin vein ice. In these hummocks thenear-surface permafrost layer had a lowermoisture (ice) content than the overlyingactive layer. This is a complete reversal of theice distribution pattern found farther north,as the near-surface permafrost of hummocksfrom all other regions contained very highamounts of ice. The ice content, on a weightbasis, of the perennially frozen layer of hum-mocks varied from 44 to 179% in the centralMackenzie Valley; 50 to 138% in the deltaand North Slope area; 39 to 320% (74 to 98%

FIGURE 7. Moisture content of the active layer of a perennially frozen soil taken from a hummock.Moisture profile Y33A is taken from the top of the hummock and Y33B from the interhummock depres-sion. These two profiles are 100 cm apart. (Lat. 65°12 ' N, Long. 124°47' W, 16 June 1973.)

C. TARNOCAI ANDS. C. ZOLTAI / 589

-5 0

+10

0/'

o. 0I /

DB2B

DB2A

YI04A

Y1048

SURFACE 0

40

50

30

20

10

80

60

70

+20

on a volume basis) in northern Keewatin; and32 to 130% (61 to 96% on a volume basis) onthe Arctic islands. The ice in these hummocksoccurred in the form of well-developed veinice, ice lenses, ice crystals, and massive ice,especially at the permafrost table and in theinterhummock areas.

BULK DENSITYThe mineral materials from the hummocks

examined had a fairly wide range of bulk den-sities (1.00 to 2.18 g cm -3). This wide range ofvalues is characteristic of soil materials af-fected by cryoturbation. Where soil materialsare mixed with various amounts of organicmatter and compacted by this process, thebulk density values can be very low due toorganic matter content or very high due tocompaction. The average figure for bulk den-sity, using all of the data collected in the studyarea, is 1.60 g cm'. There was no indicationthat the values are different between arcticand subarctic regions.

The bulk density values from different por-tions of a single hummock (Figure 6C) indi-cate that the surface of the hummock has thelowest value. This coincides with the granularstructured soil horizon (Bm). Below this, thebulk density values are higher, reaching thehighest value in the central portion of thehummock.

THERMAL. CHARACTERISTICSSoil temperatures of hummocks throughout

the study area were measured. These datawere taken only once for each site, at the timethe site was visited. Thus, no relationship canbe worked out for the study area because nolong term data are available.

These soil temperature measurements pro-vide information concerning the thermalcharacteristics of the hummock at the timethe data were obtained. Soil temperature pro-files taken of the interhummock depressionand the top of the hummock from site DB2 onBathurst Island and site Y104 in the Mac-kenzie Delta are shown on Figure 8. One ofthe main characteristics of these data (Fig-ure 8) is that the soil temperatures of theHigh Arctic hummock (DB2) on BathurstIsland are much colder than those of thehummock in the Mackenzie Delta. Anothermajor characteristic is that the soil tempera-ture profiles of the interhummock depressionindicate a variation in thermal characteristics

within a relatively small distance (less than100 cm). The interhummock depression is thecoldest part of the earth hummock and thesoil temperature profile generally shows asteeper initial decline when compared to soiltemperatures in the central portion of thehummock.

Isotherms drawn on a cross section of hum-mock Y94 (Figure 6D) show that the warmestpart of the hummock is that part which is ex-

SOIL TEMPERATURE °C

FIGURE 8 Soil temperature profile of the inter-hummock trough (DB2B and Y104B) and the topof the hummock (DB2A and Y104A). Soil tem-peratures were measured at 3:00 PM on 22 July1975 on Bathurst Island (DB2) and at 2:00 PM on28 July 1973 in the Mackenzie Delta area (Y104).

590 / ARCTIC AND ALPINE RESEARCH

posed to the morning sun. Figure 6D alsoshows that the temperature gradient isgreatest in the interhummock depression andleast in the central portion.

EFFECT OF CRYOTURBATIONSince earth hummocks develop on fine- and

medium-textured materials, the evidence ofsorting is minimal. There is evidence of sort-ing or movement of coarse material by the in-crease of coarse and very coarse sands at theside of the hummock. In addition to this,elongated stones and pebbles generally pointupward or slightly to the side of the hummockindicating possible movement by frost action.

Under stable conditions, soil horizons de-velop as a result of pedogenic processes andfollow the normal sequence from the soil sur-face to the unaltered parent material. As a re-sult of cryoturbation, this normal sequence ofsoil horizons is greatly disrupted, as evidencedby dislocated soil horizons and intrusions oforganic matter and parent material.

DYNAMICS OF EARTH HUMMOCKSThe internal and external characteristics of

the earth hummocks indicate that, during thelifetime of an earth hummock, a great deal ofactivity takes place. Externally, the hum-mocks are not only domed, but some of themare also ruptured at the apex, indicating in-ternal pressure development (Crampton,1977). Some hummocks, especially in theforested region, are heaved to such an extentthat the organic mat is split at the base. Thisheaving can be several centimeters high andthe split at the base can be 5 to 10 cm wide.

The internal morphology reveals evengreater activity. Probably the most strikingfeature is the presence of organic or mineralintrusions throughout the active layer of thehummock. Organic smears and subsurfaceorganic layers near the permafrost table arealso very characteristic features. Verticallyaligned elongated stones also indicate that in-ternal pressure development and internal soilmovement take place. Some investigators(Mackay, 1958; Mackay et al., 1961) also

found relatively recent plant fragments andEskimo artifacts buried inside the earth hum-mock. Soils associated with earth hummockshave very strongly disrupted and dislocatedsoil horizons, indicating strong cryoturbation.In addition, earth hummocks developed ongentle slopes are subject to downslope move-ment and have an elongated form. All ofthese features indicate that earth hummockshave a very dynamic character.

AGEIt is very difficult to determine the exact

age of a hummock. Radiocarbon dates ofcryoturbated organic matter give some indi-cation of the age, but this age does not neces-sarily indicate either the time at which cryo-turbation took place or the time at whichhummock development began. Fifty radio-carbon dates of organic matter from earthhummocks (Zoltai et al., in press) indicatethat the average age is approximately 2700 yrBP and the oldest is 11,200 ± 100 yr BP(GSC-2018). The occurrence of recent datessuch as 490 ± 80 yr BP (BGS-403) is an indi-cation of recent burial of organic matter.

At site Y93, where hummocks have de-veloped on an old, stabilized flowslide, logsapproximately 10 cm in diameter buried to adepth of 220 cm by the flowslide have beendated at 5500 ± 100 yr BP (BGS-214). Cryo-turbated organic matter from two hummocksat this site give dates of 4670 ± 90 yr BP(BGS-213) and 2400 ± 80 yr BP (BGS-202).These dates indicate that the flowslide surfacebecame stabilized and earth hummocksformed 1000 to 3000 yr after the flowslideburied these logs.

When the relatively young hummocks(2400 ± 80 yr BP) at site Y93 were comparedto nearby hummocks dated much older (8780± 130 yr BP, BGS-201) at site Y84, no mor-phological or pedogenic differences werefound. The younger hummocks were at thesame stage of morphological and pedologicaldevelopment as the older hummocks at siteY84.

DISCUSSION

DEVELOPMENT OF EARTH HUMMOCKSEarth hummocks are formed as a result of

displacement of soil materials due to factors

associated with frost action (Washburn,1956). Earth hummocks examined in thisstudy indicate that three major factors—soil

C. TARNOCAI AND S. C. ZOLTAI / 591

texture, soil moisture, and soil temperature—can be identified in their development. Theinteraction of these three factors produces theearth hummock.

Earth hummocks examined in this studyhave developed in materials where the totalsilt and clay content is 68% or higher. Theamount of silt and clay has some effect on theexternal morphology of the earth hummock.Earth hummocks developed on high clay con-tent materials tend to have steeper sides; onthe other hand, hummocks with lower claycontent have much more gently sloping sides.

Permafrost generally occurs within 1 m ofthe surface in all but a few of the mostsoutherly hummock locations in the studyarea. The hummocks on nonpermafrost ter-rain are believed to be fossil forms on the basisof their morphological and pedological char-acteristics. In Sweden the earth hummocksare not related to permafrost (Lundqvist,1969), although most occur above the treeline (Lundqvist, 1963). It is not indicatedwhether these are active or fossil features.

Soil moisture and soil temperature aremainly responsible for the development ofcryostatic pressure. Progressive freezing fromboth the surface and the permafrost tableduring the fall freeze-up produces cryostaticpressure (Washburn, 1956). This pressure de-velops when these two freezing fronts meeteach other in some places sooner than inothers, resulting in unfrozen pockets. In thisclosed system, the moving freezing front re-sults in the development of cryostatic pressurewhich may displace these still unfrozen, highmoisture content materials. During this pro-cess the materials are squeezed to positions ofless pressure. This process can also be pro-longed, as was suggested by Washburn(1956): first, since cryostatic pressure lowersthe freezing point, materials remain plastic inspite of the fact that their temperature isbelow 0°C; second, variation of cryostaticpressure might lead to regelation that wouldfacilitate the movement of soil materials.

Mackay and MacKay (1976) measured thecryostatic pressure in some earth hummocksat Inuvik and found that, at present, little orno cryostatic pressure develops during thefreeze-back period. They found that themiddle of the hummock had a low moisturecontent at freeze-back because of water lossupwards and downwards to the freezingplanes.

Zoltai (1975) found that ground move-ments, as registered by the ring structure oftrees growing on earth hummocks, have beenvery rare during the last two decades. On theother hand, above average levels of activitywere indicated for the period between 1847and 1943. These movements, however,usually affected single trees only, rather thanthe entire hummock. This study also indicatesthat higher levels of activity are related tohigher than normal late summer tempera-tures and rainfall. These climatic conditionsresult in a higher soil moisture content andthicker active layer (greater storage capacity),hence a greater amount of moisture availablefor ice lens formation and less chance thatdessication could occur between the two mov-ing freezing fronts. In this situation the verywet and plastic (unfrozen) soil material is ableto move.

STABILITY OF EARTH HUMMOCKSRecent studies (Zoltai, 1975; Mackay and

MacKay, 1976) indicate that earth hummocksin the western Arctic and Subarctic have beenrelatively stable features for at least the pasttwo decades. During the 1968 forest fire atInuvik extensive hummocky areas were bull-dozed to form a fire-break line. These bull-dozed hummocks have shown little indicationof regeneration in the past 8 yr. On the otherhand, Thoroddsen (1914) indicates that earthhummocks developed on wet clayey and siltysoils in Iceland and cleared and leveled forfarm fields became hummocky again within afew years if the soil was not thoroughlydrained. In the long term, however, internalcryoturbation features, relatively recentradiocarbon dates (500 yr BP, Zoltai et al., inpress), and tree ring studies (Zoltai, 1975) in-dicate that the hummocks are periodically, ifnot constantly, active.

Earth hummocks found in the southernpart of the Mackenzie Valley (e.g., sites R166and B89) indicate that cryoturbation ceased along time ago and, in addition, there are nodisrupted soil horizons (Tarnocai, 1973), suchas are characteristic of hummocks found inthe more northern parts of the study area.Examination of these southerly hummocksshowed that the external hummocky micro-topography was still present, in spite of thefact that no evidence of cryoturbation wasfound and no evidence of ground movementwas registered in the ring structures of trees.

592 / ARCTIC AND ALPINE RESEARCH

This indicates that these hummocks have laindormant for a long period of time.

The age of the terrain surface does not ap-pear to influence the stability of hummocks.Hummocks examined in the unglaciated por-tion of the northern Yukon, where cryotur-bated organic materials yielded a date of11,200 ± 100 yr BP (GSC-2018), are similarin development and stability to those found inthe glaciated part of the study area. Similarly,when earth hummocks on a stabilized flow-slide near Eskimo Lakes (site Y93) were com-pared to nearby hummocks (site Y84) wherethe cryoturbated organic material was twiceas old, no difference was found in either de-velopment or stability.

These observations suggest that earth hum-mocks probably develop quickly when all fac-tors are favorable and then they become rela-tively stable. There may be internal activity(cryoturbation) after this development or theymay become dormant for a long period oftime.

CLIMATIC IMPLICATIONSThe majority of dates for buried organic

materials are less than 5000 yr BP and a rela-

tively high number of dates are clustered be-tween 3000 and 3500 yr BP (Zoltai et al., inpress) both in the western and eastern parts ofthe study area. The climatic conditions be-came colder in northern Canada around 5000yr BP (Nichols, 1969; Ritchie and Hare,1971; Terasmae, 1972). Hummocks may haveexisted in the Arctic before this time sinceolder dates are found, but the majority of theearth hummocks probably developed between3000 and 5000 yr BP. The older dates couldalso be the result of burial of old organicmaterial by hummock development when thecooling period began. When all radiocarbondates in the Subarctic were compared (Zoltaiet al., in press), no dates greater than 5000 yrBP were found. This would suggest that hum-mocks in this area developed only during thecool climatic period since 5000 yr BP. Earthhummocks are found as far south as FortSimpson and Horn Plateau in the MackenzieRiver area and South Indian Lake, Manitoba(Beke et al., 1973). These most southerlyhummocks developed during the coldestperiod of the last 5000 yr and became inactivewhen climatic conditions moderated.

SUMMARY

Earth hummocks are very common inthe western Arctic and Subarctic and lesscommon in the central Arctic.

Their average diameter and height are132 and 50 cm, respectively, in the westernportion of the study area and 106 and 29 cm,respectively, in the eastern portion.

(3) Earth hummocks have developed onmaterials having total silt and clay contentbetween 68 and 99%. They are associatedwith a moist, cryoturbated active layer and ahigh ice content or pure ice layer near the

permafrost table.Internal morphology indicates that

earth hummocks are all affected by cryo-turbation, as is indicated by subsurfaceorganic layers, organic and mineral intru-sions, organic smears, and broken and discon-tinuous soil horizons.

Earth hummocks have developed as aresult of cryogenic processes above the perma-frost table; the majority of them have prob-ably developed since 5000 yr BP, when cli-matic conditions became severe.

ACKNOWLEDGMENTS

The data for this paper were obtained as aresult of investigations carried out under theEnvironmental-Social Program, NorthernPipelines, of the Task Force on Northern Oil

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