crop-marks revealing large-scale patterned ground structures in cultivated areas, southwestern...

Crop-marks revealing large-scale patterned ground structures in cultivated areas, southwestern Jutland, Denmark LElF CHRISTENSEN

Christcnwn. L. 1974 12 01 : Cropmarl ts rcvci?ling I:irgc-acnlc patterned ground structures in cultivated arcas, southwestern Jutland, Dcnmark. Borrcis, Vol. 3, pp. 153-180. Oslo. lSSN 0300-9483. d$ Subsurface large-scale pattenicd ground sti-uctut-cs, obset-vcd as crop-mat-ks in cereal fields with spring-barley and oats have bccn detcclcd in southweslci-n Jutland. It is assumed that the structures rcvcaled ai-c fossil ice-wedge polygons. Mode of origin, time of foi-mation and palncoclimatic significance of icc-wedge casts arc discussed. Textui-c of poorly lay-crcd wedge-fill materials can be distinguished ft-om that of thc stratified materials transcctcd by thc wedges, causing stress conditions in crop, in diy summet-s, seen a s difl'ercntial growth and cereal ripcning. Thc watcr-holding capacity is highest in wedge-fill materials. Plants gt-owing above the ice-wcdge casts arc able to withstand a certain dcficiency in precipitation during spring atid early summer. Ccreals growing outside icc-wcdgc casts sufler owing to lack of available moisture, and gi-owth may be I-cstricted, even lo the cxtcnt that the plants can wither. Consequently, cropmarks of variablc growth appcar in the fields. A growth season with high pi-ecipitation makes it difficult to trace crop marks.

Leif Christriiseti, Dapitrfinrtit of Micropcilcrcotzlology, Acirlirrs lliiivclrsify, DK-8000 Aur- hrrs C , Deitimrk, 1st Jrriie, 1974.

B O W

The term 'patterned ground is used by Wash- burn (1956) for more or less symmetrical microrelicf forms on the surface of the carth. Forms appearing as nets, polygons, stripes, and steps are typical of polar and subpolar regions, but arc not necessarily limited to arctic regions. Thcy are particularly characteristic of the periglacial environment in arctic desert and tundra regions.

Patterned ground structurcs also existed along the marginal parts of the great ice sheets, which covered large regions in Europe and North America during the cold stages of the Plcistoccnc. Structures such as involutions, stone polygons, and ice-wedge casts can be found today in Denmark as fossil structures preserved in thc subsoil. However, extensive cultivation in Dcnmark restricts observations to profilcs in pits where glacial and glaciofluvial materials arc commercially utilized. Grcat difficulties in the continuous mapping of the lateral extension of such structures outside pits can therefore arise.

I n this study, some cxamples are given of fossil subsoil large-scalc patterned ground struc-

tures in areas outside commercial pits. Poly- gonal patterns are revealed as crop marks in some southwestern Jutland cereal fields, and can be secii at the surface or on aerial crop photographs taken during the growing season of different cereal crops. External conditions, such as soil composition and meteorological factors, related to the discovery of crop marks in temperate zones have been ascertained.

Previous work Discovery of archaeological sitcs by means of low-level aerial photography has shown that it is very important to consider the external conditions such as soil composition and climate prevailing in the area at the time of photography (Martin 1971). Crop marks are produced nearly every year in permeable soils, and crop marks are more frequent in a dry summer. Many sitcs, however, may become detectable only after heavy rain storms. Less permeable soils produce soil- or crop marks only at certain times of the year (Agachc 3062, 3964).

HOREAS 3 (1974)

-0 R WAY N

Differences i n soil composition as seen by differences in vegetation (Svensson 1964a, b, 1972, 3973; Christensen 1973a, b) can therefore reveal geological information. Crop photography and the vcgctational expression o f subsoil patterns have been used i n aerial archaeology i n Denmark by Stiesdal ( 1 959) and Newcomb (1970). Crop photography and the study of fossil patterned ground structures revealed as crop marks have been studied on aerial photographs mainly in England by Shotton (1960), Williams (1964), and Gruhn & Bryan (1969). In Sweden, investigations of aerial photographs by Svensson (1964a, b, 1972, 1973) have revealed many sites o f fossil polygonal patterns on the coastal plain of Laholm. After a dry spring, patterns can here be seen in cultivated fields as lines of higher plants. The fossil polygonal patterns compare well with recent ice- wedge polygonal patterns from arctic and sub- arctic regions, and are presumed to have formed during former pcriglacial conditions.

After the NGrvang investigations of 1942, 1943, and 1946, the study o f the fossil periglacial environment was largely neglected in Denmark. During the last ten years, however, there seems to have been a revival of interest, as indicatcd by the works of Svensson (1963, 1970, 1972), Christensen (1968, 1973a, b), Berthelsen (1970), and Bind (1971).

Fig. I . Sketch map of Denmark giving the po\ition of thc area shown in Fig. 2 (shadctl). Line C indicates the maximum cxtcnt of Wcich\.clian icc sheets in Jut- land. Linc D is a younger reccssion line.

Fossil periglacial phenomena in Denmark are mainly known from western Jutland, west of the Main Stationary Line, Line C (Fig. l), the outermost extension of Weichselian ice sheets. Cailleux (1942) described fossil eolian phenomena in western Jutland. Ngirvang (1942, 1943, 1946) studied ice-wedge casts, fossil stone polygons, fossil involutions, and fossil congeli- fluctionary deposits. Svensson (I 963, 1970, 1972) and Christensen (1973a, b) described oc- curreiices of fossil ice-wedge polygons from the study of aerial photographs, and Svensson (1970) interpreted closed, water-filled depressions in the Skjern region as possible pingo depressions. All phenomena arc presumed to have been formed under periglacial conditions when the Weichselian ice sheets were stagnating at the Main Stationary Line (Line C in Fig. I ) .

Findings of fossil stone polygons, ice-wedge casts, and fossil involutions in eastern Jutland (Ngirvang 1946; Christensen 1968) also indicate that periglacial processes were active during the retreat of the Weichsel ice from thc Main Stationary Line (Line C) to the Eastern Jut- land Stationary Line (Line D in Fig. 1). Gripp (1 964) maintains that frost fissures (ice-wedge casts?) are absent or rare north of the Pom- meranian recession line in Schleswig-Holstein, northern Germany.

BOREAS 3 (1974) Large-scale pcrtterrzed grorcitd structures 155

Glacial t i l l , mainly Fluviaglacial sand Outwash plains and clay. S a a l e and gravels, morain- -:::::::::::::A river valleys,mainly

ic sand w i t h some clay. S a a l e

sand and grave l . Weichsel

Feature less a r e a s Postglacial mar ine Eolian sand of La te - and Post- deposits g lac ia l non mar ine peat, sand and clay . Occurrences o f fossi l

patterned ground struc- * lawns

tures

h';g. 2. Di\tribution of Iai-gc-scale, fossil, patterned ground \lructures in southwestern Jutland as seen on low- level ohlique aerial photographs. Only localities with an at-en of polygonal patterns moi-e than I km? ;IIC shown on the map. Deposit map compiled l'i-om: Boriiehu\ch & Miltheis, K . (1935), Jcsscn (1522, 1925), and Milthcrs, V. (1925, 1948).

Ice-wedge casts are furthermore known from northern Jutland (NQrvang 1946; Andcrsen 1961) and Zealand (Andersen 1950; Hansen 1966). In northwestern Zealand ice-wedge casts and fossil involutions have been found and fossil patterns of ice-wedge polygons mapped by means of aerial photographs (Berthelsen 1970). Cailleux (1957) described fossil pingo remains from southeastcrn Zealand.

According to Hanscn (l96.5), fossil periglacial phenomena are of no importance in

interpreted crop polygons in the Lahnlm plain of southern Halland as fossil ice-wedge polygons, whilc Hillefors (1966) observed ice-wedge casts in northern Halland. Tak-Sneider (1968) reports some observations on ice-wedge casts from the central Swedish end moraines in the Jonkoping arca. The distribution suggests formation in Younger Dryas time.

General characteristics of large- eastern and southeastern Denmark. In the ncighbouring regions of Sweden, however, jutland Johnsson (1956, 1957, 1958, 1962, 1963),

scale patterned ground structures in

Svensson (1964a, b), and Lundquist (1962) de- Large-scale aerial photo coveragc makcs it scribed fossil periglacial phenomcna from clear that fossil large-scale patterned ground Scania. The fact that Johnsson (1959) suggests structures are much more common and widcly other interpretations indicates that not all ob- scattered in W Denmark than indicated in the servations necessarily refer to fossil periglacial literature. The structures frequently do not phenomena. Svensson (1964a, b, 1972, 1973) appear on conventional vertical acrial photo-

156 Leif Christc~rzscri BOREAS 3 (1974)

graphs, but can be seen from low-level flights and registered on oblique aerial photographs. The distribution of fossil large-scale patterned ground structures in southwestern Jutland has been compiled (Fig. 2) from a discontinuous low-lcvel aerial photo coverage. Occurrences of fossil, large-scale, patterned ground structures are mainly found in soils composed of sands and gravels, such as the Weichselian outwash plains and river valleys around the Varde and Sneum rivers. Fossil patterns have also been found in the Saale landscape composed of glacial and fluvioglacial sands and gravels.

The tracing of patterns of differential growth, visiblc in the field from ground level and on low-level, oblique aerial photographs, has made it possible to present six examples selected from three regions in southwestern Jutland.

Different types of film have been used: pan- chromatic black and white; colour; infra red scnsitivc black and white (TR-films); and colour infra red (CIR-films).

Fig. 3A. Low-levcl oblique aerial photo of crop mat-ks revealing a light lai-goscale polygonal pattern as seen on a darkcr background in several cultivated fields. A gi-ey mottling is seen between polygonal sides. Photographed on 26th July, 1966, by J . K. St. Joseph. Copyright.

B r aiii mi n g e region Crop nuirk7

Some field? yituated in the Saale landscape east of Brammingc clcarly reveal a large-scale polygonal pattern as light crop marks in barley on a darker background in late July 1966 (Figs. 3A, B). Intersection between polygonal sides is mainly at right angles, forming an oriented orthogonal pattern, according to the Lachenbruch (1962, 1966) classification of polygonal shapes. The orthogonal patterns have shapes approaching quadrangular and rec- tangular patterns. The patterns in the cereal are usually continuous with a non-uniform width. The maximum mesh for polygons usually ranges in size from 10 to 30 m (Fig. 4 and Table 3 ) .

In the fields covered with polygonal patterns a grey mottling can also be seen between polygonal patterns (Fig. 3A). Crop marks are most clearly rcvealcd in fields with spring barley,

UORF:AS 3 (1974) LurgP-scnle pcirtrrned ground structures 157

I' ig . 3 H . Sketch of Iat-gc-scale polygonal patterns ft-om A. Continuation of polygonal pattern i n beet ficld separating the two barley fields is not visihlc for photointerpretntio~i. Di\tnncc bctwcen arrows is 32 m. The dilfuse crop marks seen in the tmidcy ficld at thc top of the photo A ale not shown on the sketch.

while crops such as grasses and beets obscure patterns on the aerial photographs (Figs. 3A, B).

Ground surface observations of cereal fields in this particular area during the 1970 growth season clearly revealed polygonal patterns as lines in spring barley fields (Fig. 5 ) . Cereal plants are more densely developed in the lines, and the individual plants are taller with broader leaves compared with surrounding cereal plants outside lines. The barley plants in the middle of the lines attain a maximum height in July. There is a continuous decrease in height away from the centre of the lines, so it is difficult to measure the exact width of lines in cereal fields, especially prior to ripening. The maximum difference in height observed between plants in the middle of the line and those outside the line has been measured as 42 cm in late June, 1970. Generally a difference in height of 10 to 20 cm is common in a year with a low precipitation in May and June.

Furthermore, the width of lines change con- siderably along the strike from a minimum of 0.4 m to maximum of 3 m. Patterns with a width of less than 0.4 m are normally un- detectable from ground level, but may be visible on low-level, oblique aerial photos. Aerial photos taken with IR and CIR films seem to be especially helpful in revealing narrow lines in cultivated areas.

I n July, ripening processes change the colour of spring barley to yellow in the polygonal patterns, while the cereal crop outside the polygonal patterns remains green due to tillering, which appears much later than the normal period of tillering (Fig. 6). These late tillers ripen late, and turn from green to yellow at a later stage than the main shoots and normal tillers.

Observations in the fields after harvesting did not display any topographic differences or soil contrasts between areas where higher barley plants were observed during growth season and the surrounding areas.

Treble 1. Summary statistics of maximum mcsh in rcgulnrly oricntcd oi-thogoiial large-scalc patterned ground structures at B~-amminge mid Varde as measui-cd 011 vertical acrinl crop photogi-aphs.

Locali ty crop' Slatistical parameter rc\c31- ing tiumhcr m e d i a n mode mean oh- variance stan- skew- kui: coefficient large- o c scrvcd d:u-d I lCSS

scale mciiwrc- rangc dcvia- variation pat- ments tion tern';

t o y i s of

2.5 km ywing cast 0 1 harlcy 31 17.05 17.8 18.158 16.8 22.673 4.762 -0.310 2.035 26.224 Urnrn- oiid rniiiw o a t s

4 kni south- s p i - i i i g w c \ , t o l harlcy 41 30.5 32.4 29.259 24.7 41.199 6.419 0.228 2.210 21.938 VLI rdc

4.5 k111 spring south- harlcy 85 30.5 32.4 32.676 33.4 45.363 6.735 -0.197 2.978 20.612 west o f and Vatde o a t s

c I

0

N = 3 1

Class intervals i n metres

/.'ig. 4. 1 l istogrnm showing mensurcments of numhcr of maximum mcsh i n regularly oricntcd orthogonal 1x1 ye-scale pattci-t ied ground structures east of Bram- mingc :IS WCII on a vertical aerial crop photo. See Tahlc 1 f o r wmmnt-y statistics.

Clitiwite

Considering the meteorological conditions in the period preceding the date of photography, 1966 was a year with a higher precipitation than normal from February to April, and less than normal in May (Fig. 7A). Detailed daily records of air tempcraturcs and precipitation from May to August at the meteorological station at Spangsbjerg, just north of Esbjerg, are shown in Fig. 7B. The records show dry, sunny periods in the middle of May and from the end of May to the middle of June. This was followed by cooler days with showers from latc June to mid-July, and another dry, sunny period precedes the date of photography, 26th July.

1970 was a year with an April precipitation of more than three times the mean for April, while May and June were slightly above the normal May and June levels (Fig. 8A). Detailed daily records of air temperature and precipitation from May to August (Fig. 8B) indicate several dry sunny days in the middle of May and for most of June. The precipitation in June was mainly due to thunderstorms at the station.

Structures in the soi l

Some trenches excavated in late July 3970 in the central parts of a field (Fig. 3A) revealed

BOREAS 3 (1974)

Fig. 5. Sevcral lines fot-ming orthogonal crop patterns in ii spring 1x11-Icy field :it Hrammingc. Thc individual plants ai-e taller in the lincs. It is not posihlc to sec a n y difference i n mntui i ty in the photographs. I'hotogl-aphed 26th June, 1970.

Pig. 6. Several lines foi-ming an orthogonal paltcrn in a spring bni-ley f'ield ca\t of Bi-nmminge. Pattern is seen as white lines with matui.cd plunls against immalure p l a ~ ~ t s outside lines. Photographtd 1st August, 1970.

BOREAS 3 (1974)

20

0

r--1 ri-7 MONTHLY AMOUNTS OF PRECIPITATION RECORD : 1966

I 1

[ / MEAN PRECIPITATION I RECORD: 1931 - 60 L A

that the roots from taller barley plants in polygonal patterns continued deeper into the soil, in broad wedge-shaped structures, than in the surrounding areas (Figs. 9, 10). The excavations also revealed additional subsurface structures, some of which have narrow wedge- shapes from 1.5 to 2 m deep, and between 10 and 20 cm wide at the top (Figs. 9, 11).

Between the polygonal patterns (Fig. 3A) a grey mottling is seen. Here cereals have alter- nating conditions of growth, with the plants being taller or shorter. Excavations have revealed the existence of involutions below such mottled areas (Fig. 9). In July the root system of spring barley penetrates down below thc surface to a maximum of from 2.0 to 2.5 m in wedge matcrials (A in Fig. 9) and about 40 to

Fig. 7.4. Mean monthly precipitation I-ecol-d 1931-1960, and monthly record of precipitation 1966 for the meteorological station at Brammingc, West Jutland.

60 cm in areas with involutions (C in Fig. 9). A downward penetration to only 35 cm has been observed at the top of involutions. Out- side broad wedges (A) and involutions, root penetration to about 25 to 35 cm has been observed. The samc depth below the surfacc occurs in areas with narrow wcdge structures (B on Fig. 9).

Textural composition of soil inaterials

The tcxtural compositions of wedge-fill materials from the two wedge structures in Figs. 10 and 11, together with adjoining host materials, are shown in Figs. 12A, B, and Table 2. The grain size parameters used are those developed by Folk & Ward (1957). At the top of

IJORhAS 3 (1974) Img t - sca le ptitteriicd ground structures 161

-~~ ~~

b- MAY- + JU N E ~- +JULY :I: AUGUST+ SPANGSBJERG

D o ~ l y m o x 011 t e m p e r a t u r e

D o l l y meOn u , , t e m p e r a t u r e

a l l y m l n 01, tempera lure

35

30

25 -. a

2 0 : a

15 6

10 g 5:

" 1

Z*';g. 711. Daily maximum, minimurn, i~ i i t l mean a i r lcmpeiuturcs together with recorded precipitation and weather obsx\ations f o I the mctcorological stat,ioii Spangsbjcrg at Esbjel-g 1966. The timc 1-ange shown i n t h e figure is tha t par1 of thc growth s x b o i i o f eel-cnl plants when polygonal patterns appeared as ct-opiiiarkt in the ficltl\ n:imely from the beginning of May to ha rvcdng in August. Date shown in thc figuic refer^ to the date of aerial photography of cropmat-ks mentioned in t h e text.

the broad wcdge (sample 1) a coarser inaterial occurs, containing boulders with the same maximum diameter as the adjoining host material (sarnplc 6). Downwards through the middle and lower parts of the wedgc (samplcs 2 to 5 ) the maximum (9 diamcter of wedge-fill material increases, and the wedge-fill becomes morc sandy with a few gravel-sizcd grains. Median diameter (NIL:@) and mean diameter (Mzo) both increase downwards.

The sorting (or inclusive graphic standard deviation qo) is very poor in the upper part of wedge and host sample, and poor to moderate in the middle and lower part of the wedge.

Skewness ( S K , ~ ) changes from markedly negative in the upper part of the wedge and adjoining host materials, to almost symmetrical and positive in the middle and lower parts of

Thc textural composition of wedge-fill materials in the narrow wedge and the adjoining host materials shows the same maximum diameter, but a closer coincidence between wedge-fill materials and host materials from the upper part to the lower part of the wedge (Table 2). The variations in median diameter (MdO) and mean diamcter (Mz@) together with

the wedgc.

12 - norcas 4/74

the sorting qo show no systematic variation. A11 samples are poorly to rnoderatcly sorted. Skewness is negative to very negative and nearly symmetrical in the upper part of the wedge.

Morpiioin etric nuirly sis

Morphometric analysis of the stone fraction shows that the studicd wedges together with their adjoining host contain many rounded pebbles (Fig. 13).

Stirid grain siirfmc' tc>xtiLres

Examination of surface textures of medium size sand grains from wedge-fill and host materials by scanning electron microscopy (SEM) revealed two textural patterns. In wedge-fill and host materials, grains have a high relief, blocky conchoidal breakage patterns (quartz grains), and irregularly oriented V-shaped indentations (Figs. 14 A-E), the latter appearing with a smooth or rough surface in depressed area. The largest side of V's can range up to 70 microns, the normal being from about 2 to 10 microns. V-shaped indentations were described by Krinsley & Donahu

BOREAS 3 (1974)

160

110

120 z 9

a 100

+ + 'l

- V w n a

0 L 50

MONTHLY AMOUNTS OF PR EC IP ITAT I ON RECORD : 1970

i ' I

L -A

' MEAN PRECIPITATION RECORD: 193, - 60

(1968) and are considered to be diagnostic for high energy environments. The amount of energy in a certain environment is supposed to be the major factor i n dctermining the size of V formed.

Wedge-fill materials also contain frostcd quartz sand grains with a low relief compared with the above type of grains. A characteristic feature of these grains is the presence of a pitted surface with scattered smooth grooves, and a blocky appcarancc in the bottom of the grooves (Figs. 3 5 A-F).

Moistwe coizteiit

The moisture content in wedge-fill materials from broad wedge-shaped structures is highcr compared to that in adjoining host materials at the samc level (Table 3). The wedge-fill materials from narrow wedge-shaped structures

Zig. 8 A . Mcan monthly piccipit:i(ion Iccord 1931-1960, and monthly rccoid of prccipitation 1970 for thc mctcorological station Bi-ammiiigc, Wcst Jutland.

have more or less the samc moisture content as adjoining host materials.

Tjzereborg region This location, north of Tjzreborg in southwestern Jutland, is situated in the central part of the Saale landscape of this region. Extensive commercial digging of glaciofluvial materials permit check observations in frcsh profiles. Aerial observation revealed lighter crop marks forming large-scale polygonal patterns in mature barley fields at the end of August 1970 (Fig. 16). The crop marks in thc field had been preserved throughout the growing season until harvesting, which took place a few days after the photography. Polygonal patterns are ir- regular random orthogonal patterns and the sides of patterns arc discontinuous. The pat-

ROREAS 3 (1974) Large-sctrle pcltterned ground structures 1 63

I-. M A Y --+JUNE &-JULY +. AUGUST+ SPA N GS B J E RG t' { Y , tT_> (' 0 &?>(I a (1 z3 0 E S B J E R G 1970

D a l l y ,71111 011 temperature

r 2 o

Fig. 8B. Daily maximum, minimum, and mean air temperatures togcther with recorded precipitation and wcather obsci-vations foi- the meteorological station Spnngabjei-g at Esbjerg 1970. Time range included in thc figurc is the part of growth season of cei-cal plants, when polygonal patterns appeared as cropmarks in the ficlds from thc beginning of May to harvesting in August. Diitcs shown in thc figure I-cfcr to aerial or field observations mcntioned in the tcxt.

Table 2. Gmin si7c paramctcrs of wcdge-fill materials and adjoining hosts a t Hramniinge, fi values.

Sample Median Mean Sorting Skew- no. diameter diameter ncss

MdO MZO 010

131 OJd wcdgc 1 0.65 -0.56 2.53 -0.52 2 1.25 I .27 1 .6X 0.18 3 1.30 1.28 0.96 -0.1 1 4 1.30 1.28 0.75 0.04 5 1.50 1 .51 0.88 0.28

I l o j t to hioad wedgc 6 1.05 -0. I8 2.14 -0.73

Narrow wcdgc 1 1.40 1.35 0.97 0.00 2 1.30 1.17 1.43 -0.32 3 1.30 1.18 1.16 -0.30

1 lost t o narrow wcdw 4 1.25 0.95 1.44 -0.46

terns in barley fields cnd abruptly at adjacent ones with beet crops. Tracing one side of the polygonal pattern to a neighbouring sand pit (just outside thc aerial photo) demonstrates that a polygonal side on thc aerial photo continues downwards in the sub-mil as a narrow wedge- shaped structure transecting horizontally layered sands and gravels (Fig. 17).

Varde region Crop Cercal crops grown on terraces composed of outwash materials along the Varde river clearly reveal marks of large-scale patterned ground structures (Figs. 3 SA, €4). On an aerial crop photo taken in late June 1967, the polygonal

A B C A

b

T<th/r 3. Moi\ ture content in \amples collectcd at Br;iinmingc, 27th Ju ly , 1970. Moisture contciit detc~-- mincd :is weight l o s s by wmplc\ at 105 C .

Sample iiitcinal i i i cin’s below the caith.; suIf;icc Brc1od Adjoining

Adjoining

Moistuic content, weight r j .

wedgc h m t N. ‘I I . I - ow wcdgc h O \ t

70- 80 100-1 I0 140-1 5 0 160-1 70 1 YO-100 60- 70 90-100

140-1 so

patterns show u p as darker patterns against a light or light-grey background. This contrasts with the colour pattcrns shown in other figures (c.g. Figs, 3A, 6). The reason for this may be

that the barley plants between the polygons were forced to ripen due to drought and so turned into a yellowish colour. The plants growing i n the polygonal patterns had a better supply of water, developed normally, and remained green.

As in the Bramminge rcgion, barley is herc the cereal crop best suited for revealiiig these polygons. Patterns are only traccable in ficlds situated a t levels above the raised Holocene sea floor (Fig. 2), and are mainly of the orthogonal type. Several generations of smaller polygons can often be seen in a large polygon. Size of polygons range in width from 10 to about SO m (Figs. 19, 20, and Table 1).

The same area was photographed during harvcsting on 25th August, 1970 (Fig. 21). Large-scale patterned ground structures are clearly revealed as crop marks in the barley field. Polygonal patterns arc blurred near the edge of the photo. The marks are not visible in the even, honey-coloured rows of threshed

BOREAS 3 (1974)

Fig . wcd Thc line (Fig

10. Profilc section of broad gc-\hapcd \tructure, Hramminge. structutc is observable a'i a pattern on Lhc aerial photograph

;. 3A).

straw around the edge of barley field where harvesting is in progress. As in the TjEreborg region, the crop marks have been preserved throughout the growing season until harvesting.

Lines of polygonal patterns are discontinuous and may narrow and even disappear for a few metres, appearing again along the strike line. More seldom a continuous, but non-uniform, width is seen. Considerable widening may occur where patterns intersect. The density of cereal plants is greater in a polygonal line, the plants also having broader leaves.

Clitmite

June 1967 had a precipitation of less than half thc mean for the month of June (Fig. 22A).

Detailed daily registration of air temperature and precipitation from May to August a t Karls- g;rde, 7 km ENE of Varde, is shown in Fig. 22B. I t is seen that half of the June precipitation was due to a thunderstorm preceding the photography of June 27th.

Wedge structures iii t l i (J soil

I t was possible to trace a polygonal crop line into a n adjacent sandpit. I n a profile vicw the polygonal line is seen to continue 5 m below the surface in the subsoil as a wedgc-shaped structure, deformed in the upper part (Fig. 23). A smaller and narrower wedge structure is seen close to the broad structure.

BOREAS 3 (1974)

/:ig, 11. I'rol'ile section of narrow' wedge structure, Bi-ammingc. Cast not obrcrvable on aerial photograph (Fig. 3A). 11-on and manganesc precipitations throw the wedge into I-elicf. Pciicil indicatcs scale.

Host , z n m p ~ e no 6 - Cast, s u , ~ , p ~ e 5 1,2,3,~,5 _ _ ~ _ _ _

Fig. 12.4. Grain fire dihhut ions , cumulative [requency curves, materials f rom the broad wedge-shaped structul-c (Fig. 10) arid I-elated host. Position of samplcr in cm hclow t h c surface: (I) 70-X0 cm, (2) 100-110 cm, (3) 140-150 cm, (4) 160-170 em, ( 5 ) 190-200 cm, (6 ) 160-170 cm.

HOREAS 3 ( I 974) Lorge-scale patterned grourzd structures 167

Evolution of polygonal patterns The polygonal patterns dcscribed in this paper all penetrate different sedimentary materials deposited during the Saale and Weichsel gla- ciations. Transection of wedges i n subsoil materials and downwarping of layers adjacent to the wedges indicate that polygonal patterns were introduced after deposition of materials. J t can be cxcluded that the polygonal patterns arc solution phenomena in underlying rocks. us downwarping of the sides of wedges and adjacent materials could not occur herc. The large-scalc polygonal patterns arc all very much like the polygonal patterns described as contraction cracks. Lachenbruch (1960a, b) applics the name to 'a reticulate system of intersecting contraction cracks on the surface of a body'. Contraction crack polygons form because of tensions resulting from a volume reduction which is in turn caused by desiccation or cooling. Size of polygons varies very much in different matcrials, from a few millimetres in cooling ceramics to about SO metres in permafrost. Sizes are attained of up to 300 metres in giant desiccation polygons of Great Basin clay playas from areas in the western United States (Neal ct al. 1968). The following modes of origin of large-scale patterned ground structures in western Jutland will be considered:

( I ) Giant desiccation polygons (2) Thcrmal contraction-cracks by cooling

(a) ice-wedge polygons (b) seasonal frost crack polygons (c) ground-wedge polygons.

Giotit desiccntiotz polygons

Giant desiccation polygons in western USA developed in materials composed of clay and silt favouring desiccation structures. Features formed by the orthogonal intersection of giant mud cracks arc described as 'giant' to distinguish them from shallower mud cracks which attain a size of several tens of cm and are only a few cms deep. But the content of coarser materials in wedge-fill from southwestern Jut- land and the lack of proof of a warm, dry climate during the Pleistocene of Denmark exclude a formation of large-scale polygonal patterns in southwestern Jutland as giant desiccation polygons.

Thermal contractiori-cr-cick polygons

It is most logical to interpret the origin of polygonal patterns in SW Jutland as thermal contraction-cracks formed by cooling and contraction in frozen ground. In this paper the term 'thermal contraction cracks' will be used for large-scale polygonal patterns observed both in the field and on aerial crop photographs as differential growth and ripening of cereal crops in southwestern Jutland. The mechanics of thermal contraction-cracks have

. . . I u 2 u L u 8 u 16" 3 2 u 60" U,125mmU,25mmO,Smm l m m 2mm L m m 8 m m 1 6 m m 32mm 6 L m m

'p 9 8 7 6 5 L 3 2 1 0 - 1 - 2 - 3 - L - 5 - 6 ? = - l a g d

j'ig. 128. Grain s i x tiisti-ihutions. cuinu1:itivc fiequcncy curves, mntcrinls f r o m thc narl-ow wedge structure (Fig. 11) and Ielatcd host. Position 01' anmplcs in cm below the earth's sui-facc: (1) 60-70 cm top of wedge, (2) 90-100 cm middle pait of wedge, (3) 140-150 cm lower part of wedge, (4) 90-100 cm host.

B i o a i l wc r lge

6o

L O -

N a r r o w w e d g e H o s t

60 “I.

LO l r 20

0

~~~~ ~~~ ~~

I.‘ig. 13. Roundnc\s analysis o f pchhles From the studied wedges at 131-arnminge, logcthci- with adjoining host. 200 pebble.; in each sample.

been discussed by I x h e n b r u c h (1961, 1962, 1966).

From the sections investigated it seems oh- vious, based on the contraction theory first set forth by Left’ingwell (1915, 1919), that the wedgc structures probably formed as thermal contraction cracks in formerly perennially frozen ground. This theory has overwhelming evidence i n its favour and is generally accepted today - see for example Lachenbruch (1960a, b, 1962, 1966), Dylik (1966) and PCwC et al. (I 969). These authors distinguish between ice- wedges, fossil ice-wedges o r ice-wedgc casts, seasonal frost cracks, and ground-wedges or sand-wedges (1’6~6 1959).

Recent ice-wedges i n Alaska form during cold winters where frozen ground contracts and cracks i n B polygonal pattern (Lachen- bruch 1960a, b; Pdw6 et al. 1969). Thc cracks are presumcd to be a result of tensions in the surface of the earth (Fig. 24A, 1-4). In early spring, melt-water flows into fissures and freezes, a vertical ice-vein penetrating the permafrost being produced. Horizontal com- pressi on d 11 r i i i g t he f ol I o w i ng sum me r resu I ts in the distortion o f strata. Further cracking during the following winter in the fissures, now constituting weakness zones, cxpand the fis-

sures to an ice-filled wedge structure called an ice-wedge.

Climatic changes causing the disappearance of perennially frozen ground and melting of foliated ice in ice-wedges leave open spaces, into which sediments from above and the sides collapse. This is called a n ice-wedge cast (Wright 1961) o r a fossil ice-wedge (Fig. 24B, 1-3).

The mode of formation of seasonal frost cracks corresponds to the formation of ice- wedges and ground-wedges from pcrmafrost regions, though formation may take place i n seasonally frozen ground. Open cracks can he formed during strong drops in temperature during winter, with the subsequent filling of open cracks with sediments in spring. Seasonal frost cracks are known from areas with severely frozen ground outside pcrmafrost areas (Washburn et al. 1963; Thorarinsson 1964; Pataleiev 1955).

The formation of ground-wedges is very much like the formation of ice-wedges. Ground-wedges (Dylik 1966) or sand-wedges (PCwC 1959) are primary features formed by thermal contraction, which causes surface cracking under permafrost conditions. T h e open fissure polygons are, in contrast to icc-

BOREAS 3 (1974)

Fig. 14A-E Sc;inning elcctrnn microvxpy of sand grains from host materials, sample\ 4 and 6 (Figs. 12A-B). A. Well I-ountled quai-I/ sand grain, wmple 6. R . V-shaped indcntations on curving sul-face (cf. pic- tuic A). C. V-\hapcd indentations with :I smooth or I-ough sui-face i n dept-c\sctl at-ca (cf. pictut-c B). D. Rounded Icldspai- gi-ain, +ample 4. E. V-shaped indenlalion with :I smcoth sul-face in dcprersed aIca (cf. picture D).

wedges, formed during extremely arid and cold conditions without any available melt water. The open fissures are filled with sedimentary materials, often eolian sand (Pkw@ 1954). Only slight changes therefore take place in a transition to a fossil state.

The textural composition of wedge-fill materials in southwestern Jutland compared to the adjoining host materials suggest that the wedgc- f i l l formed in a different way than host materials. Lack of wind-facetted stones and the poor sorting of wedge-fill materials exclude a n orig- inal formation as a ground wedge. The range in sorting and skewness of wedge-fill materials and adjoining host materials, together with the close resemblance of roundness of pebbles in wedges and host materials, and large inclu- sions i n the upper part of wedgc, suggest formation as a n ice-wedge. As foliated wedge-ice

melts, adjacent and overlying materials drop into the space left by the melted ice. However, the rounding and frosting of quartz sand grains in wedges around Rramminge suggest active eolian processes in the area during the transformation of the wedge to a fossil state.

Recent structures intermediate between ice- wedges and ground-wedges are known from extremely arid areas in Canada (Pissart 1968).

Time of fornznficin of structures

The study of the distribution of fossil periglacial phenomena over wide areas and periglacial stratigraphy based on occurrences of different periglacial phenomena are not well established in Denmark. Therefore little is known about the time of formation of polygonal patterns and the later destruction and

BOREAS 3 (1974)

~~~~ ~ ~ ~

D E F Pig. 15A-k' Scanning electron microscopy of frosted yuirrtz sand grains from wedgofill matcrinls, samplc 3, Fig.

12A. A. Rounded grain with low relief. B. Sti-ongly smoothed ant1 gi-ound surt'acc (el. picturc A). C . Blocky appearancc i n bottom of gtoovc (cf. piclui-e H). D. Rounded grain with low relief. E. Detail of smoothed and ground surface (cf. piclutc D). F. UIocky appeal-ancc in boltom of groove (cf. picturc t).

rcplaccment of wedge-ice and subsequent filling of voids with wedge-fill materials such as sand, gravel, and boulders. A possible approach, therefore, is to attcmpt to comparc evcnts with climatic changes, though this is connected with considerable uncertainties. Climatic fluctua- tions in a cold climate will result in differcnt periglacial effects. Climatic oscillations during thc Pleistocene i n Dcnmark have been indicated mainly in studies of glacial tills and the changing of fauna and flora. Occurrences of loess may givc information about eolic activity. Solifl~iction phenomena and involutions indicate freeze and thaw conditions ncar the surface, while ice-wcdgc casts indicatc former permafrost conditions and give more exact information about the temperature at the time of formation of ice-wcdgcs (P6wC 1966a, b).

Polygonal patterns in southwcstern Jutland

arc found at stratigraphically diffcrcnt lcvcls and in lithologically different matcrials (Fig. 2). Pattcrns occur both in deposits from the Saalc stage and in outwash materials from thc cnd of the Weichscl stage, but not in Holocene sea floor deposits. This time distribution inay assist in fixing the chronology for the formation of structurcs. As solifluctionary proccsscs during the Weichselian stage havc strongly rc- modcllcd large areas in southwcstern Jutland, the structures seen in the ficld and on aerial crop photographs cannot bc dated to the Saalc stage; the oldest structurcs therefore date from the beginning of the Weichseliaii stage. As several patterns occur i n late Wcichsel outwash deposits thc processes responsible for thc formation of structures existed after thc deposition of these materials. As formation ot patterns and growth of recent activc ice-wcdges

BORLAS 3 ( I 974) Lnrge-scnlc ptrttcrned ground structures 171

I.7g. 15. Low-level oblique cclour acrinl photo of large-scale oi-thogonnl, disconlitiuous pattern rcvcaled as cl-op-mai.!..; i n mature 1x11-Icy licltl N of Tiarchot-g. 1,ightci- ct-op-markr nhtuptly end at ti-ansition lo atljaccnl ficltl with bcct crops a1 thc left. PhotIigr-aplicd ZGth August, 1970, by Lcif Christcnscn.

BOREAS 3 ( 1974)

assume permafrost conditions and cold cli- matics (Pew6 1962, I966a; Lachenbruch 1962, 1966), the patterns in the Tjxreborg and Bram- minge regions can stratigraphically be placed in any time interval between Early Weichsel and Late glacial, while patterns in the Varde region stratigraphically are assumed to be placed in the Late Weichsel.

Though fossil pcriglacial phenomena occ~i r at different stratigraphic levels in SW Jutland, it is so far neither possible to give an ovcrall indication of different periglacial stages in southwestern Jutland nor an exact age distribution between different structures. Different climatic oscillations are known from the Weichsel glaciation. From early Weichscl the Rodebzk (Amersfoort) and Brgrup interstadials are known at 64,000 and 59,000 years B.P. respectively (Andersen 1961). From the late glacial a t the end of the Weichsel, the interstadials Bqjlling (12,500 to 12,000 years B.P.) and Allcrgd ( 1 1,700 to 3 1,000 years B.P.) occur (Iverscn 1954). From the maximum of the Weichsel glaciation, only glacial and fluvioglacial deposits are recorded in Denmark.

/-'ig. / 8 A . I.ow-lc\el chliquc aci.iiil p!ioto o f C I op-ma1 ks I evenling a dai-k hi-ge-scale polygonal pattern a lightel- hockgt-ound in se\eInl fi , SW of VaIdc. Photographcd 27th June, 1067, b y J . K . St. Jo>eph. Copyt~ight.

011 elds

P(iILiPO(.IIiiiiiliC .cigiiifi~rrrzcc

Icc-wedge casts are the most positive evidence o f former permafrost conditions. Recent permafrost occurs and may be formed in areas where the mean annual air temperature is about - 1'C to - 2 'C (Pkwk et al. 1969). Between James Bay and the westcrn Cordillera in Canada the present southern limit of permafrost rough- ly coincides with the - ' I C mean annual air isotherm (Brown 'I 967). Investigations from Alaska have shown that niean annual air temperatures from - 6 ° C to -8°C arc necessary for growth of active ice-wedges (Lachenbruch 1962, '1966; PCwC 1966 a, b, 1973; PkwC et al. 1969; Brown & PCwe 1973).

I t is very difficult to relate areas in southwestern Jutland bearing indications of fossil permafrost conditions to areas in Alaska and Canada having recent continuous and discontinuous perniafrost (PCwC 1969, 1973; Brown 1969; Brown & PCwC 3973). Assuming the same climatic conditions during formation of polygonal pattcrns in southwestern Jutland as in Alaska today i n areas with continuous permafrost, a mean annual air temperature of

BOREAS 3 (1 974)

F i g . IRR. Sketch of IaIge-xcnlc polygonal pattern on A. Polygonal pnttcrn is best displayed i n bat-ley fields, though it may also bc trace- able in gt-assfields. Distance between ar rows is 26 m.

Largescale patterned ground structures 173

~

a few degrees below freezing point is inferred. AF thc present mean annual air temperature for southwestern Jutland is between 8 C and 8.5 C, it must be concluded that the average annual air temperature a t the time o f formation of polygonal patterns was a t least 14’C to 16.5 C lower than present.

Compared with existing air temperature conditions for the Weichsel glacial stage based on pollen examinations (Andersen et al. 2960), the mean July air temperature during the Weichsel glacial stage was below 10 C, and only above 10 C in the Rodebzk and Brorup interstadials in the beginning of the Weichsel glacial stage and in Belling and Allcrod interstadials in the Late glacial at the end of the Weichsel glacial stage.

Cereal plants revealing subsoil st r u ct u r es The trench profiles in fields investigated clearly displayed different structures in subsoil materials below crop marks in fields with spring

barley and oats. T h e structures comprise ice- wedge casts and involutions. Evidently a key to the understanding of the crop marks are the subsoil structures, the texture of soil materials, and the climate during spring and early summer. I t must be stressed that profiles have been made only across patterns observed in the field. As large-scale patterns could be explained by farming procedures o r by soil factors and are only revealed as crop marks, misinterpieta- tions may occur as it is impossible to make as many trench profiles as necessary in all the fields.

The close resemblance of drainage patterns can be excluded, because test profiles have been made only in areas without draiilagc and drainagc patterns have more regular structures.

Observations of root penetration in the areas studied show that the root system of spring barley penetrates downwards in ice-wedge casts to a maximum of 2.5 m (Fig. 24C). The main part of the root system outside structures was only found to depths of 25 to 30 c m in July 1970. The poor stratification of wedge-fill materials is conducive to root penetration com-

BOREAS 3 (1974)

N = $ 1

Class i n l e r v a l s i n m e t r e s

Fig . 19. 1 lisiogrom showing nwnsuicmciits o l maximum mc\h in rcgular oricnlcd orthogonal lat-gc-scdc pat- tcimcd gouiitl stiuctuics 4 km southwe\t o f Val-tic nu \ccti o n vel-lical ;ici.i;iI cr-cip photos. See Tahlc 1 fol- \unim;ii y \t:iti\tic\.

pared to the horizontal layering of adjoining host materials. A slightly higher content of fines, such as clay and silt, in wedge-fill materials compared to the adjoining host leads to a relatively greater amount of retained water available for the plants during growth. As growth conditions change, yield structure varies much between plants in and outside crop marks within a year, and from year to year, dcpend- ing on the amount o f precipitation during the growth season.

During springtime i n Denmark precipitation is often insufficient t o cover all the potential evajiotranspiration. It is therefore necessary for the plants to extract available moisture from the subsoil, and spring-barley and oats growing above ice-wedge casts can obtain enough moisture for growth until caring, while surrounding plants outside ice-wedge casts suf- fer from shortage of water (Fig. 24C, 1-2). Conscyuently growth is here delayed and plants can even wither. In May and June the pattern can he observed at the surfacc as a darker pattern against slightly lighter surroundings, and clearly visible on an aerial photo.

If the whole growing season is dry. water stress can accelerate $he matur’;ttion of plants growing on sandy host materials. Growth continues above wedges and maturity occurs later

here. On black and white aerial photographs taken in July darker polygonal patterns are seen against lighter surroundings. When all cereal plants i n a field have matured, non- uniform colours and darker shades of brown are seen on colour aerial photographs of the poor crops outside polygonal patterns. This indicates that the observer loc~ks through the vegetation to the ground (Fig. 21).

Precipitation in June causes a new tillering pha:ie in withered arcas, and these new, late tillers ripen later. Plants above ice-wedge casts grow so well that the ability to de- velop new tillers is not released. The crop above ice-wedge casts ripened normally, where- as the crop in the withered areas remained green due to new tillers (Fig. 24C. 3-41. This phenomenon can be seen at the surface as a yellow pattern in a green field. On a n aerial photo in black and white it can be seen as a light pattern on a darker background.

30

25

20

15 c

c u

: : 10 a

E 0

- 0

L 5 QJ D

5 0 _iI -.

Z~ N

0 0 N

N - 8 5

Class i n t e r v a l s i n me t res

I.’ig. 20. F1 i j t o g r m show iiig mcaw rcinent s o f maximum mc\h in I-cgular oricnlcd orlhogonal Iargc-scolc pat- tcl-net1 ground s~i~uctui-cs 4.5 k in \outhwc\t of V31-dc a s ‘,cc11 on vcrticnl ncrial ciop photo\. Scc Tnhlc I for summary statisticu.

f ; ig . 2 / . I ~ o w - I c ~ c l oblique actial photo of Iai.ge-~cnle iion-ol-thogoii;il crop pattems in field.; of mature barley w u t h w c s t of Vai-tlc. Lal-ge-\calc cIop patterns ai-c most clearly rcvcalcd i i i harley f'icld adjoining the fLii-rn Iiouws a t thc top o f the picture. The iion-uiiifol-m coloui- and mottling w i l h darkel- tones of brown iii harley field indicates tha t the ohsei-vcr looks through ihc vcgciation to thc gl-ound. In barley field (lower right) stiucturcs aic not visible in the even, honey-colouicd r o w s of thrcshcd straw aruuiid thc outer edge where hainciting i s iii p!-ogi-css. N o crop polygons arc visible in thc green fields with gm\s and beet crop\. Phologiaphcd 25th August 1970 by 1,cif Chi-istciiwi.

I t is suggested that such crop patterns described above should be designated as positive potygotzcrl pcrftcrris in the field.

Summary and conclusions Subsoil large-scale patterned ground structures from southwestern Jutland arc prcscntcd. By means of some detailed cxamples i t has been possible to show scveral characteristic featurcs of these patterned ground structurcs, as being much more widespread in Denmark than hitherto realised and particularly common in southwcstcrn Jutland. It is assumed that the large-scale polygonal patterns in southwestern Jutland arc fossil ice-wedge polygons which probably formed in permafrost during the Wcichselian glaciation, when the ice sheet was

stagnating at the Main Stationary Line ir, central Jutland. The composition of wedge-fill materials suggests multiple sources. Voids left by melted wedge-ice were filled by materials washed down from the earth surface and the walls of the wedge. An aeolian component has also been found in wedge-fill materials in the Bramminge region.

Comparison with reccnt air ternpcratures in Alaska, where ice-wedges arc actively growing, shows that the mean annual air temperature of southwcstcrn Jutland during ice-wedge growth was at least 34 C to 16.5 C lower than at present.

In sandy arcas the patterns manifest them- selves on the surface or on aerial crop photographs as crop marks in different cereal fields during growth seasons with low precipitation in May and June. Spring barley and oats seem

BOREAS 3 (1974)

120 I 7 6 1 I

MONTHLY AMOUNTS OF PREC I PI TAT ION RECORD: 1967

i i f I MEAN PRECIPITATION I I RECORD: 1931 - 60 L J

Fig . 22.4. Mean monthly precipitation I-ecot-d 1931-60 aiid monthly record of precipitation 1967 for the meteorological htation Varde.

F i g . 226. Daily maximum, minimum, and mean air temperatures togcthcr with recorded precipitation and wcathci- ohset-vations for the meteorological >,tation Karlsgst-de 7 km castnortheast of Varde 1967. Timc range included in the figurc is the part of gt-owth season of cereal plants, when polygonal patterns appwred a s crop-marks in the fields. Dates shown in the figure refer to aerial ohscrvations mentioned in the text.

t o be the best cereals for rcvealing fossil- patterncd ground structurcs in southwestern Jutland, since grasses rarely disclose the polygonal pattcrns. Rye and whcat do not sccm to be as suited for displaying fossil subsoil patterns in southwestern Jutland, and bcct crops at certain times completely hide patterns which exist. Crop marks appear as lines in the cereal ficlds with taller and better growing cereal plants compared to the surroundings, and features thus demonstrated are called positivc

Trench profiles havc revealed that the root net of cercal plants growing above broad icc- wedge casts can be traced downwards in the subsoil. However, narrow ice-wedge casts hav-

polygonrrl [ l t l l l ~ ’ r m .

ing a width in the upper part of wedge of 30 to 20 cm are not recognized as crop marks in thc fields. Differences in stratification and grain size composition between subsoil wedge- f i l l materials and adjoining host materials are assumcd to be responsible for the differential growth. As precipitation in Denmark is insufficient to cover all the potential evapotranspi- ration during some weeks in springtime and early summer, the plants must extract moisture which is available in more finely grained materials i n the subsoil. The roots penetrate deepcr in poorly layered wedge-fill materials than in the coarser, horizontally layered host materials, and the slightly higher content of fines (clay and silt) in wedge-fill materials have

FIRST W I N T E R

2

FIRST A U T U M N

- 3

BOREAS 3 (1974)

I N' th WINTER

P

N' th AUTUMN

6 Transformation to a

fossil state: ice-wedge cast

C l i m a t e ge t t i ng warmer

I Col lapse above ice-wedge S i l f ond sand woshed I n by su r face water

Ac t i ve lover-

I r regular s lumping and f i l l i n g of i ce - wedge void

A c t i v e layer,

3

Ac t i ve laver-

ice-wedge cas t

C Growth of sprinq barley and oats in cu t i va ted areas. Dry period in early summer

Humid tempera te c l imate

I Moy

Germrno t ing ce reo l plants

~

2 June

matured m a i n shoots

3. J u l y , R e t i l l e r i n q

L A u g u s t - S e p t e m b e r Late

Matu red ce rea l p l a n 3 r t l l l e r s

Fig. 23. Schcmatic i . c p r c w i t a t i o i i of (A) l o i m n [ i o n of an ice-wcdgc; (U) later transformation into an ice-wcdgc ca\t; and (C) crop-marks in spring bxrley abovc an ice-wcdgc cast. MatcIials in ice-wedgc cast arc moi-c f i n e g rn i i i c t l than m n t c h l s of adjoining host. A and B after I >achcnh i -uch (1960n, h) and 1'6~6 Cl al. (I9)hY).

3 greater water capacity. Wedge-fill materials i n this way function as a potential moisture reservoir, preventing cereals from suffering heavily during low precipitation. This results in better growing conditions above the ice- wedge casts, while plants outside ice-wedge casts may be severely stressed. This can result i n a normal maturation of cereals in the polygonal patterns, while precipitation at the end of June or the beginning of July promotes a re- growth of withered cereal plants outside the polygonal patterns. A polygonal pattern formed by lack of moisture in early spring and summer remains all through the growth season until harvesting.

The appearance of crop marks, growth, and form i n different ccreal fields in Denmark can give val~iable iiiforination about relative com- positional variations i n subsoil materials. How- cvcr, during mapping i n the field it is necessary to bear in mind the amount and distribution of prccipitation during the early growth scason from April to June.

Thc different crop marks can be used as ex- ccllent diagnostic criteria in field mapping of fossil-patterned ground structures at the surface or on aerial crop photographs in humid temperate zones.

, 4 r ~ / , i r o ~ ~ Z ~ ~ ~ I y ~ ~ / r i ~ ~ i i / . s . - I am deeply indchtcd to Pro- I'c\\or Nils Spjcldnxs for supporting the project, and to D r . Roll Feyling-Ilan\scn foi- mnliing facilities avail- :hlc a~ the Department of Mici'opalncontology, Gco- logi\k Institiit, Aarhus Univei-sity. Thank.; ~ I K due to l'i-ol'c,sor Sigurd Andcrwi, Royal Vctcrinary and Agi-i- culrurnl Collugc, Copcnhngcn, i 0 1 - tliscu.;\ions and wit- iciun o f the nianusct-ipt i n compnny wi th Dr. Svend Andct-\cti. 1 pmlited gi.catly ii-om thcii- commentu. MI. J lit-o\hi I:nami, JcnI Scandina\ia, pi-epar-cd the scanning elect ion niictogl-nph\; Dt-. A. S . Ilorowitz (C;cologisk lnh.litut, Ani-hu\) made \nluablc cnmmcnts 0 1 1 the stitn- tistic\; MY';. C.iisxl Niclwn skilfully cxccutcd [he maps iintl tli-uwing\; and DI-. J. R. Wil\on kindly implovcd t h e E:ngli\h text . Financinl support for this study wils provided h y Stntcns Nat~i'~ide1l~l\;~hc1igc Forskningsrdd.

in Denmark in the F,arly Wcichsclinn Glacial. Darzm. gcol. Uiitlws. 11:75.

Andei-sen, S . T., dc Vries, €4. I,. 82 Zagwijn, W. €1. 1960: Climatic chnngc and i.adiocaihon dating in the Weichsclian glacial o f Denmark and the Netherlands. Geol. ('II Mijir. 39, 38-42.

Baniid, 1'. 1971: 111 Larscn, G., Villumsen, A,, Lihorius- sen, J . ;G Bxind, P.: Ekskursion pii Rnndci's-hladct og t i1 Vihorg-Knl-up cgncn. I)(iiislc gcol . Poreii. Rrs- skri/ t 197J, 105-1 11.

Bet Ihclscn, A. 1970: Yotogenlogiskc o g fcltgcologiske undci";c)gclrer i NV-Sjallnrid. I)crii.sli geol. Foreii. Arsslcrift 1970, 64G69.

Borncbusch, C . H. & Milthers, K . 1935: Jordhundskort over Danm:irk 1 : 500.000. D i z i i r t i . gcot. lirirlers. Ill: 24.

Llrown, R. J. E. 1967: P(wr/tijrost ir i Caii~rilei. Map NRC 9760. National R irch Council of Cannda, Di\i\ion ol Uuilding Rcscarch, Ottawa, and Geo- logical Survey oC Canada Map 1246A.

UI-own, R. J. E. 1969: Factom influencing discontinuous pcrmafrost i n Canada. ITi Pew&, T. L. (Ed.), The f'wig/oc,icil EriL*irorir>irvi/: I'tist u i i d Prrseizt, 11-53. h1 cG i I1 U t i iv. PI-css, Mont Ica I.

F3i-own, R. J . E. & F i w i , T. L. 1973: Disti-ihution of pcrni;il'~-ost in N o r l h Amci-ica nnd i t s relationship t o the cnvironincii(: a i.evicw, 1963-1973. Proc. of S e c . o r r r 1 l r i / i w w i . I'errriczfrost Coiif., Yuliictsli, 71- 100. U.S. Nat. Aciid. Sci.

Caillcux, A. 1942: Lcs actions Colicnncs p6riglaciaire~ en I . i ~ I o p e . Mtwi. SOC. G 4 d . Frciiice 21, 1-2, 46.

Cnillcux, A. 1957: I,cs mai'es du sud-mt dc Sjaclland (DenmaI-k). Compt. Keritl. Aced. Sci. 245, 1074-1076.

Chi-i\tcnsen, I,. 1968: An occui rerice of pcriglacial st ruclurcs a t Laogi, Jylland. Medrlr. dciitsk geol. I , 'o , .er i . 18, 46-54.

Chi-istcnwn, L. l973n: Fo\\ilc polygoiiinonsti-c i Jyhkc 1,aiidbrugs.joi-der. f lgcskr i / t for cigrurmirzer og iivrto- I I O I H W 2, 102-107. (Summary in English.)

ChIi\ten\en, L. 1973h: I ' lai itenvl og gcologi: Gcologisk tolkning at' ;nfgrodcnicmitrc i lundht-ugsjorder. Det

Dylik, J . 1966: Pi-ohlemr of ice-wcdgc structures and E~-oit-liswi-c polygons. Niril. I'rrj,ylcic.. 15. 241 -291.

Folk, R. L. & Ward, W. C. 1957: Bt-nms River bar: A \tutly i n the significnncc o f gi-ain size parameters. .loiir. S c d . / 'e /rolog): 27, 3-27.

41 I pp. Karl V/;ichholtz Vcrlag, Ncumunstcr. Gruhn, R. & Hry;in, A. L. 1969: Fossil ice-wedge poly-

goiis i n muthcn\t E\\cx, England. 111 N w C , T. L. (Ed,) I 96Y, 7'lir 7'1~riglrrc~icil EiivirorrriiiJri/: I ' m / mrtl J ' y ( , s ~ i i t , 351-363. hlcC;ill Univ. P r c s ~ , Montreal.

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crop-marks revealing large-scale patterned ground structures in cultivated areas, southwestern...

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