AND PATTERNED GROUNDS IN - PERIGLACIAL OR SOMETHING ELSE

AIMO KEJONEN

AIMO KEJONEN, 1997: Permafrost and patterned grounds in Finland - periglacial or something else. Bull. Geol. Soc. Finland 69, Part 1-2, 97-108. Northernmost Finnish is periglacial. This area belongs to the zone of sporadic and alpine permafrost. Microclimatic permafrost can be found in eskers, caves and artifacts like mines everywhere in Finland. Periglacial per- mafrost occured in and during some early Weichselian interstadial and at Salpausselkä zone and in the supra-aquatic parts of North during Yonger Dryas. Nowadays the Southern and are no more periglacial. Patterned grounds occur in all parts of Finland. Their number and species vary in different parts of Finland. In Northern Lapland the variety of patterned grounds is most polymorphous and sorted and nonsorted patterns are equally common. In Southern and Central Finland the big, sorted types like boulder depressions and sorted nets account for over 90 % of patterned grounds. The processes forming patterned grounds are very active in Northernmost Lapland, but less active or fully fossilised in Southern Finland. The significance of local circumstances increases on the forming of patterned grounds in the south. Keywords: periglacial features, permafrost, , wedges, boulder depressions, genesis, Quaternary, Finland Aimo Kejonen: Geological Survey of Finland, Regional Office for Mid-Fin- land, P.O.Box 1237, F IN-70211 Kuopio, Finland.

INTRODUCTION permafrost and patterned grounds form (Lodz- inski 1909, V. Okko 1954, 1964, Taipale and In research, permafrost and patterned grounds Saarnisto 1994). The most comprehensive defin- are usually connected with periglacial condi- ing criteria were considered to be annual snow tions. However, the term "periglacial" is defined cover and freezing of the ground (Tricart & in many ways. Originally, it referred to the fore- Cailleux 1967). By strict definition, periglacial land conditions of a continental ice-sheet, where conditions are determined on the basis of cli- frost (congelifraction) is strong and matic factors. A climate in which periclacial 98 Aimo Kejonen conditions prevail can be defined by the upper contain ice layers and at times a frozen mineral and lower limit of the annual average tempera- soil core. The world's first artificial was ture, e.g. -1 15 °C, or by the upper limit of made in Finland (Seppälä 1982). Frost resem- the annual average temperature, e.g. under -2 bling that present in has been also found °C, -1 °C, 0 °C or +3 °C and the amount of from a bog in Lapland, south of the palsa area rainfall, e.g. 50-1250 mm/y, and/or according to proper (P. Hänninen 1995). the length of the thermal winter, e.g. over 210 Although there had already been earlier hints days/y (Peltier 1950. Williams 1961, Fairbridge of permafrost associated with mineral soils 1968, Embleton & King 1975, French 1976, (Lagercrantz 1974), alpine permafrost has been Washburn 1979). found in the summit areas of the mountains of According to different average temperature Finnish Lapland only during the last decade limits, the defined periglacial area in Finland (King and Seppälä 1987, 1988). The permafrost varies greatly. Using the under -1 °C limit, only layers are extensive in places. They comprise the bare mountainous area above the treeline in both unconsolidated earth and , and the frost Finnish Lapland is periglacial. Using under 0 °C, may be over 50 m thick. These observations the limit of the periglacial area runs through were confirmed when a well drilled at Yllästun- , Sodankylä and in Lapland. Using turi was frozen to a depth of 40 m in summertime under +3 °C, this area extends to Central Finland (Seppälä 1989). All sufficiently high sections and . According to Washburn above treeline in the mountains of Finnish Lap- (1979), the average temperature limit of -1 °C land have permafrost, from Saariselkä and Pal- best describes the border of the periglacial area. las-Ounastunturi northwards (Fig. 1). The lack Permafrost and patterned grounds are more com- of a snow blanket is important in the formation plicated phenomena than usually thought. There of alpine permafrost (King and Seppälä 1987). are several types of permafrost. Patterned Snowbank glaciers have been found at Enontekiö grounds are a visible indicator of frozen grounds, during recent years (Hirvas 1991). Based on the which are most abundant and best developed in previous, northernmost Lapland belongs to a permafrost areas. However, they also develop zone of sporadic and alpine permafrost (Emble- under suitable conditions as the products of an- ton and King 1975, Washburn 1979, Williams nual and, even, diurnal frost (Troll 1944, Wash- and Smith 1989). When the fall in average tem- bum 1979). perature caused by altitude is considered, the permafrost's border follows the limit quite well, with an annual average temperature of -1°C. PERMAFROST IN FINLAND During the last decade observations were made of frost surviving over the summer. In Permafrost has been known in Finland for 100 North Finland, winter 1987 was exceptionally years (Kihlman 1890). Finland's only permafrost cold with little snow. While the frost layer in is considered to be ice inside palsas, which does North Finland is normally 2-2.5 m thick, its not melt in summer (V. Okko 1964). The age, thickness then was quite commonly 3—4 m. In formation and development of the palsas have September, frost preserved in the earth still continually been the subject of scientific discus- caused the freezing of pipes, south of Oulu sion (Auer 1927, Ruuhijärvi 1962, Kujansuu and in Central Lapland. In ore exploration frost 1969, Salmi 1970a, 1970b, 1972, Seppälä 1976, was found, in October - November 1987, at a 1979, 1983 and 1988). According to present depth of over 2 m at a locality where the land understanding the development of palsas is cy- surface already had a 0.2-0.3 m frost layer (K. clic. They form and melt all the time. Their Nenonen, oral communication). In the excep- development is regulated by the thickness of the tionally cold conditions, short-lived "perma- snow cover and the bog layer. The palsas 99 Aimo Kejonen

Figure 1. Finland's permafrost areas. 1. Area of sporadic and alpine permafrost. 2. Area of occasional frost surviving over one or some years. 3. Area of microclimatic permafrost. 4. Permanent permafrost in a cave. 5. Occasional frost surviving over one or some years in a cave. 6. Permafrost in an esker. 7. Palsalike permafrost outside the palsa area proper. 8. Uncertain area. 9. Permafrost in artifacts such as mines. 100 Aimo Kejonen frost" formed, lasting 1.5-2 years. However, 1974, Mäkinen 1985a, 1985b, 1996). The ice only frost layers that survive many years are wedges and involutions of the I and II Sal- considered to be permafrost proper (Washburn pausselkä delta sands, and the ice wedges of the 1979). eskers of North Karelia (Fig. 2) originate at the In South Finland, permafrost also occurs in end of the Weichsel glaciation. In North Karelia eskers, caves and mines. The first observation permafrost occurred in the supra-aquatic areas from an esker was made in Turku at the turn of during the Younger Dryas. I have found ice the century (Leiviskä 1914 and 1939). wedges at Jaamankangas and , on the Permafrost was found in eskers at Lahti and eastern side of Kohtavaara. Other finds are at Laukaa (M. Okko 1969, Tynni 1972), and Äetsä Lake Herajärvi on , at Kaurilankan- (Fig. 1). Initially the permafrost and ice layers gas and at Ruhka on Tohmajärvi, and at were considered to be remnants of a continental Raatevaara on the eastern part of ice-sheet. At present they are explained by "esker (H. Rainio, oral communication). The late-gla- breathing", in other words the product of the cial landslides of Lapland (Fig. 1) are connected seasonal flow of air occurring in the eskers (V. to the melting of permafrost (Kujansuu 1972). Okko 1957). Sub-zero temperatures penetrating from the side into the esker form an internal cold store, which does not become warm enough PATTERNED GROUNDS IN FINLAND in summertime. Enclosed earth materials and freezing leakage water form permafrost. V. Okko Patterned grounds are structures visible at the (1957) found 'breathing eskers" at 28 localities. earth's surface, which are connected with proc- Ice and permafrost layers formed in caves and esses that produce frost action. Their size and mines originate in a similar manner. In winter, shape depend on the processes that form them, sub-zero temperatures flow to their deepest parts, on their component material, and on inclination. which do not get the opportunity to warm up Based on their morphology and material they during the summer (Kejonen et al. 1991). Ob- are divided into sorted and nonsorted patterned servations over summer of preserved ice and grounds (Table 1). In sorted patterned grounds, frost were first made in the Aijala mine (Leiviskä stones and boulders are separate from more 1939). In past years, a permanent natural ice- finer-grained earth. Earth materials, cracking of cave, a few caves and a Salpalinja WWII for- the ground, or vegetation, shape nonsorted pat- tress, where during cold and dry years, ice and terned grounds. There is no consensus on what frost are preserved throughout the summer, have formations belong to patterned grounds. Some been discovered (Fig. 1). Permafrost originating consider that sorted and nonsorted stripes, ta- in these types of protected places could be called luses ( slopes), earth hummocks, solifuction microclimatic permafrost. sheets, tongues and steps and sedimentary struc- In different parts of Finland, as signs of an- tures, all produced by slope processes, are not cient permafrost, are casts, fossilised patterned grounds. Others consider these and sorted polygons (Fig. 2), and layer disturbances lands originating from the melting called involutions (Aartolahti 1970, 1980, Risti- of permafrost as patterned grounds (Embleton luoma 1974, Saarnisto 1977, Mäkinen 1985a, & King 1975, Fairbridge 1968, Troll 1944, 1985b, 1996). The ice wedges of Lapland and Washburn 1956, 1979). This study uses a broad Ostrobothnia are in till covered glaciofluvial for- classification of patterned grounds for simplifi- mations. They originated during some inter- cation of the problem. stadial of the Weichsel glaciation (Ristiluoma 101 Aimo Kejonen

Figure 2. Finland's patterned ground areas. 1. Mountain area of Finnish Lapland, in which there is a complete variety of patterned ground types and their formative processes. 2. Southern Lapland, where there is a rich variety of patterned ground types, and in which their formative processes are partly active and sorted patterned ground types are emphasised. 3. The area of boulder depressions and other sorted patterned grounds. 4. Individual mountains, which belong to the mountain area of Finnish Lapland. 5. Ice wedge casts. 6. Land slides in Lapland. 7. Limit of area of raised and fen bogs. 102 Aimo Kejonen

Table 1. Classification of patterned grounds. tions are earth hummock nets (pounikko in Finn- Circles Sorted Nonsorted ish) and earth islands belonging to nonsorted Nets Sorted Nonsorted circles. Patterned grounds are concentrated in Polygons Sorted Nonsorted wet and weakly wooded areas such as on the Terraces Sorted Nonsorted edges of bogs, annual drying ponds and lake Stripes Sorted Nonsorted shores. The snow blanket and abundant vegeta- Steps Sorted Nonsorted tion are the most important factors that limit Tongues Sorted Nonsorted frost thickness and the occurrence of patterned sheets Sorted Nonsorted grounds. Separate high treeless mountains such Boulder depressions as Pyhätunturi form isolated areas in which pat- Scree slopes (taluses) terned grounds are comparable to those of the Others Sorted Nonsorted treeless mountain areas of Northern Lapland (M. Hänninen 1994). On the coast of the Gulf of Bothnia and from There are patterned grounds everywhere in Kuusamo southwards (Fig. 2), the portion of Finland. Their number and species vary in dif- fossilised, or at least very slowly developed pat- ferent parts of the country. In some areas pat- terned grounds, grows. Simultaneously the type terned grounds form continually while elsewhere variation of the patterned grounds changes. they, or some specific types, are fossils (Aarto- Sorted patterned grounds become dominant. In lahti 1980). In the treeless mountain areas of , North Karelia and North Ostroboth- Finnish Lapland (Fig. 2) the variety of patterned nia, they account for over 90 % of patterned ground types is nearly complete. This variety grounds (Aartolahti 1969, 1971, 1980, Karlsson includes forms typical of permafrost areas, such 1979, Kejonen 1978, 1992, Lappalainen 1984). as ice wedges, solifluction lobes and sorted In favourable places, such as on shores, in caves stripes and nets (Tanner 1932, 1938, Okko, V. and in places where snow and the vegetation are 1954, Ohlson 1964, Piirola 1969, 1972, Seppälä removed by man, nonsorted patterned grounds, 1966, 1983, Kejonen 1979, 1984, 1994, Söder- earth hummocks, earth islands, frost cracks and man 1980, Teider 1984, Hietaranta 1989, M. gelifuction tongues may develop (Aartolahti Hänninen 1994). In general, sorted and non- 1969, 1970, 1971, 1980, Kejonen 1978, Kejonen sorted patterned ground types are as extensive; et al. 1991, Lappalainen 1984). From Lapland however, local conditions cause noticeble alter- southwards the assortment of patterned grounds nation of patterned ground types (Piirola 1969, changes. Boulder depressions, talus slopes and 1972, Söderman 1980). The processes that form other large, sorted forms become more common these types are active all the time. It has been at the expense of other types (Kejonen 1978 and possible to study the formation of patterned 1992, Karlsson 1979). This may be apparent grounds in detail (Söderman 1980, Kejonen because the vegetation easily covers small pat- 1979, 1994). terned grounds making them impossible to ob- A transition zone outside the treeless moun- serve. tain area of Lapland, southern Lapland and the Aartolahti (1980) considers the +3 °C annual Inari basin contains abundant patterned grounds average temperature limit as the southern limit formed by processes active under suitable con- for the formation of patterned ground. It is ap- ditions. The amount of the sorted patterned proximately the same spot as the border between grounds, and their visibility increase southwards fen and raised bogs (Fig. 2). However, on the (Virkkala 1955, Kena 1957, Söderman 1980). border's southern side, around Kullaan Isojärvi, Boulder depressions, taluses, sorted nets, circles besides other boulder field types, there are nu- and polygons are common. They are usually merous boulder depressions. Nevertheless, the active. The most easily found nonsorted forma- area only rose from the sea during the Ancylus- 103 Aimo Kejonen and Litorina periods, when there was little frost places in the mountain areas of Finnish Lapland action. According to Söderman (1982), on the there are arctic soils both on the present land areas covered by the ancient great lakes Suur- surface and buried (cf. Tedrow 1977). These Saimaa and Muinais-Päijänne, there are many differ from the podzols usual in Finland. young boulder depressions that are less than The oldest patterned grounds in Lapland and 5000 years old. In contrast, the most active frost Ostrobothnia developed during some Veiksel in- action occur in the treeless mountain areas of terstadial or early Veiksel stadial (Mäkinen Finnish Lapland, which has an annual average 1985a, 1985b, 1996). They are in the glacioflu- temperature under -1 °C (Kejonen 1979, 1994, vial material of till-covered eskers and marginal Söderman 1980). formations. Next patterned ground formed in The significance of location increases in the Lapland during deglaciation. and for some time south. On low shores, stone- and earth islands after it. The climate warmed about 8000 BP. No form and shore boulder fields develop into boul- patterned grounds formed, or their birth was rare, der depressions (Aartolahti 1969, 1970, Kejonen 5000-8000 years ago (Kejonen 1979, 1994, 1978, Söderman 1982, Lappalainen 1984). Earth Söderman 1980). Then, most of the present tree- hummocks may form on fields and logging ar- less area was covered by forest (Eronen 1979, eas. During spring and autumn, solifluction, nee- Eronen and Huttunen 1987). When the climate dle-ice creep and boulder creep, and land and cooled, about 5000 years ago, patterned grounds rock-slides occur in road cuts (Aartolahti 1969, began to form again and the summit areas be- 1980, Lappalainen 1984). Taluses develop in the came treeless. Relatively small falls in tempera- Kuopio area at least (Kejonen 1978, 1993). To- ture, such as the Little Ice Age, already sped up day, scree slopes develop in the Viipuri rapakivi patterned ground formation (Hietaranta 1989, area more than in adjoining areas of other rock Söderman 1980, Kejonen 1994). The present types. A weathering rock type and the resulting formation of patterned grounds is so fast that precipices are favourable for the formation of their development can be followed by measure- scree slopes. Frost cracking develops in areas in ment. Besides the cold climate, patterned which the snow covering is weak, or from which grounds formed in the treeless mountain areas it has been removed (e.g. roads) (Aartolahti of Finnish Lapland are affected by the wind 1969, 1970, 1972, 1980, Lappalainen 1984). regulating the thickness of the snow cover, vege- Around Kuopio, the rise of the road surfaces tation, vegetation destruction (e.g. by forest fires, caused by frost is usually 5-20 cm, even 70 cm insect destruction, or human activity, and by frost in extreme cases (Lappalainen 1984). Annual weathering (Seppälä 1981, Seppälä and Rastas layered frost may cause the formation of small- 1980, Mattsson 1987). scale thermokarst (Kejonen 1984). Knowledge about the development of the pat- terned grounds of Central Finland and South Finland is incomplete. There was permafrost in DEVELOPMENT HISTORY OF PAT- the Salpausselkä zone and in the supra-aquatic TERNED GROUNDS areas of East Finland during the Younger Dryas. During this period ice-wedges and involutions In Lapland patterned grounds have been dated formed (Aartolahti 1972, 1980, Saarnisto 1977). by separate methods. Their development history These are missing from subaquatic areas. New has beem established by studying patterned land rose from the sea at the end of the Ice Age, grounds and layered sucessions formed during but permafrost no longer exists. Later, lake trans- their formation (Kejonen 1979, 1994, Mäkinen gressions again drowned areas that were already 1985a, 1985b, 1996, Ohlson 1964, Söderman dry. Patterned grounds still formed in these areas 1980, Vliet-Lanoe et al. 1990). Frost action has quite late. On the shores of the lakes Saimaa also affected soil development (). In and Päijänne, boulder depressions occur in areas 104 Aimo Kejonen that were still covered by water 4000-5000 years eral tens metres thick and tens of hectares wide. ago (Söderman 1982). Patterned grounds form The range of patterned grounds is almost com- under favourable circumstances in conditions plete. It corresponds to the variety of patterned still warmer than Finland's climate, such as in ground types met in other periglacal areas. Gotland (Königsson 1966). They are caused by Sorted and nonsorted patterned grounds are annual frost and also present-day activity. equally common. Processes forming patterned Based on observations at Kivijärvi, at Kinnula grounds are continually active. and at Toholampi, part of the boulder prints 2. On the southern side of the periglacial depressions of may be older zone is a zone comprising South Lapland and than assumed. In places in which till has been part of North Ostrobothnia, in which, after ex- excavated in construction of forestry roads, or ceptionally cold winters with little snow, in which a ditch was made through a boulder periglacial features form. The area's bogs may depression, a boulder bed is seen sagging under contain palsa-like frost formations. In exception- the till. In the buried boulder bed, the order of ally cold winters with little snow, permafrost layers is typical of a boulder depression. The layers that last over the following summer, or largest boulders are uppermost and the smallest for a couple of years, form in mineral earths. stones are underneath. They have initially devel- These can not be considered to be actual per- oped sometime during the Weichsel interstadial mafrost, rather they are a form intermediate be- and have been preserved despite the glacial ero- tween permafrost and annual frost. The propor- sion. They are in the area of passive glacial tion of sorted patterned grounds increases with , from which abundant interglacial- and southward movement. In places suitable for frost interstadial deposits have been found (Nenonen action, annual frost develops patterned grounds 1995). In Sweden and Norway, boulder depres- in the manner of permafrost. sions, boulder blankets and sorted nets have been 3. Central and South Finland do not belong described, which the glacier covered during the to the present-day periglacial zone, although in Veiksel glaciation (Nesje et al. 1988, Kleman places the area contains abundant patterned and Borgström 1990, Hättestrand 1994). Glacial grounds. The patterned grounds locally abundant erosion has also varied by area in Finland, since in some areas are mainly unsorted types. Most there are tor-like weathering remnants older than of the patterned grounds are fossilised, at least the Ice Age in mountains in Lapland, in Os- in the southern part of the area. Annual frost trobothnia, and even around Kuopio (Kaitanen forms patterned grounds, in the manner of per- 1969, 1979, 1989, Söderman 1980, Söderman et mafrost, on areas very favourable for frost ac- al. 1983, Kejonen 1993). tion. 4. Throughout Finland, microclimatic perma- frost may form in suitable places such as in CONCLUSIONS eskers, in caves and in mines. This permafrost is not periglacial, rather it is the result of the The information presented above on Finland's retention of winter cold in the earth, caves, or permafrost and patterned grounds presentations mines. This permafrost type is not significant in can be summarised in the following six state- the formation of patterned grounds, and can be ments: called microclimatic permafrost. 1. Northernmost Lapland and the treeless 5. In Finland, there was periglacial perma- mountain zone in particular are periglacial ar- frost during the interstadial period of the last eas, with sporadic and alpine permafrost. Palsas ice age and on supra-aquatic areas during the occur in the area and there are permafrost layers melting phase of the continental ice. Ice wedge in mineral earth and rock in the summit area of casts and involutions representing signs of an- the mountain zone. The layers can even be sev- cient permafrost occur at Salpausselkä and in 105 Aimo Kejonen

North Karelia, Ostrobothnia and Lapland. The and other slope processes were at a standstill ice wedge casts of Salpausselkä and North Kare- during the temperature optimum. In cold periods, lia are late glacial. The ice wedge casts of Lap- frost action restarts quickly. Even in Lapland, land are covered by till blankets formed during but particularly in Central and Southern Finland, some interstadial of the Veiksel glaciation. patterned grounds form in places from which 6. The formation of patterned grounds is to- some factor (e.g. coastal forces, forest fire, man tally bound to climatic change; however, local or animals) has destroyed the snow cover, or conditions have a significant effect. Solifluction vegetation, preventing frost action. 106 Aimo Kejonen

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