Nonsorted Circles on Plateau Mountain, S.W. Alberta, Canada
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NONSORTED CIRCLES ON PLATEAU MOUNTAIN, S.W. ALBERTA, CANADA Stuart A. Harris Department of Geography, University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada T2N 1N4 e-mail: [email protected] Abstract Since 1979, raised, flat-centred, nonsorted circles averaging 1.5 m in diameter have been forming in felsen- meer at sites disturbed and then reclaimed, on top of Plateau Mountain. They take three to four years to form, spreading by gelifluction during thawing of the active layer. The surface becomes stable, but telescoping heave probes reveal that there is a winter circulation underneath them taking place in the upper part of the active layer. Annual movement averages 12 mm/a and the material cycles in 300-400 years. The soil is fairly dry, but development of ice segregation under the raised centre appears to be the driving force. The width to depth ratio of the circulation is about 2.57, since it extends under the surrounding soil. A typical diameter for a cell is 240 cm. The name, xeric nonsorted circle, is suggested to differentiate them from nonsorted circles described from the wet arctic lowlands. Introduction Suggested causes of circulation of material in circular patterned ground Washburn (1956, p. 829) introduced the term nonsor- ted circles for circular patterns of ground where there is Washburn (1956) summarized 19 processes that have negligible sorting of the coarse and fine soil compo- been suggested to be causes of the development of pat- nents. The centre is usually bounded by areas with a terned ground. Of these, several are concerned with greater vegetation cover. Washburn (1956) summarized segregation of coarse and fine materials which is not the earlier theories regarding their origin and subse- applicable to nonsorted patterns. Others deal with the quently new theories have been developed, e.g. Mackay development of ice wedge polygons. Eliminating these (1980). It is also possible that the new theories suggest- leaves differential heaving, cryostatic movement, circu- ed for the circulations found in sorted circles (e.g. lation due to ice thrusting, expansion due to absorption Hallet and Prestrud, 1986; Ray et al., 1983) might be of water by colloids, contraction due to drying, convec- involved. tion due to temperature-controlled density differences, moisture-controlled changes in intergranular pressure, The top of Plateau Mountain, S.W. Alberta, is marked and the artesian hypothesis. Washburn provides the by excellent examples of sorted patterned ground with history of each concept, together with a critique. mud boils in the centres. In 1950, oil exploration occurred on the top of the mountain, resulting in the As noted in French (1996), several new theories have construction of drilling pads and roads. When these been developed in recent years. He suggests cryoturba- were abandoned, these disturbed areas were bulldozed tion due to seasonal freezing and thawing to be the flat and left to revegetate naturally. After 1978, circular cause of the circular patterns. In a study of earth hum- tabular surfaces of bare ground rose 2-5 cm above the mocks, Mackay (1979, 1980) concluded that freezing surrounding soil. That shown in Figure 1 formed in less and thawing of ice lenses at the top and bottom of the than four years and then appeared to stabilize. active layer are aided by a gravity-induced cell-like movement, resulting in net upward movement in the This paper reviews the suggested origins of nonsorted centre and outflow near the surface. At depth, this is circles, and then provides data on the characteristics counter-balanced by a flow from the outside towards and nature of probe displacement in them at Plateau the centre. This fits well with the observations by other Mountain. These in turn provide a test case for some of workers of nonsorted circles in wet arctic lowland sites the theories under the environmental conditions found in fine-grained soils. on a xeric mountain top overlying permafrost. A similar progressive circulatory motion has been dis- covered in the coarse and fine components in sorted cir- Stuart A. Harris 441 Figure 1. The nonsorted circle studied at PM#1, Plateau Mountain, in 1996. cles in west Spitzbergen (Hallet and Prestrud, 1986; and Brown, 1978, 1982; Harris, 1990, 1995), and the Hallet et al., 1988), but the causes are uncertain. In this mean air temperature at 2500 m has decreased about case, the movements occur in summer. Ray et al. (1983) 1.5¡C during this period. Mean annual air temperature and Gleason et al. (1986) have also suggested summer is approximately -2.35¡C at that elevation with a mean displacement of material in sorted circles due to per- winter snow cover of 12 cm (Harris, 1995). The latter is colative pore convection through thawed soil. In this in contrast to the deeper snow in the adjacent forest. case, the movement should be in the opposite direction Strong winds occur throughout the year. to that observed by Mackay and Hallet. Finally, van Vliet-Lanše (1988, 1991) has suggested a combination of Permafrost is continuous to discontinuous above tree- differential heave and ice segregation working with line at 2250 m, but includes relict permafrost beneath density instability to explain cryoturbation and hum- the mountain top (Harris and Brown, 1978, 1982). The mock formation in Spitzbergen. active layer of the permafrost in equilibrium with the present climate varies from 3 to 15 m in thickness, while Most of these theories have been developed to explain permafrost thickness exceeds 200 m. circulations in essentially moist or wet soils at lowland sites in a shallow active layer. There is a lack of studies Since 1958, a network of roads has been developed on of circulations in patterns at well drained sites in a thick the mountain to support drilling operations. The latter active layer such as occur on mountain tops at lower latitudes. W 114 Jasper Study area 118 W 118 Plateau Mountain is a flat-topped mountain lying Red Deer approximately 80 km southwest of Calgary (Figure 2) with a summit elevation of 2519 m. The mountain is formed from the core of an anticline which is tilted gen- tly downwards to the north and west. The cap rock con- sists of Upper Carboniferous siliceous dolomite of the Spray Lakes Group. Banff 51 N Tree-line varies in height, depending on aspect, but Calgary averages 2290 m elevation on the west side (Bryant and Scheinberg, 1970). Above tree-line, the alpine zone is Location dominated by a grassy alpine meadow with lichen- of covered rocks on the alpine tundra on the summit. PLATEAU MOUNTAIN in Southwest Alberta The area lies in the rainshadow east of the Continental Divide. As such, the top of the mountain lies well below Plateau Mountain Coleman the glaciation limit (¿strem, 1966) and the top of the mountain was not glaciated during the Wisconsin glaciations. The limited snowfall blows off the top to 0 50 miles accumulate in the back of cirques on the south and east 49 N sides of the mountain (Figure 3). Air and ground tem- perature data have been collected since 1974 (Harris Figure 2. Location of Plateau Mountain in southwest Alberta. 442 The 7th International Permafrost Conference Brown, 1978) at latitude 50¡ 12' 28.3" N and longitude yyyyy yyyy PLATEAU MOUNTAIN: 7500 114¡ 31' 15.6" W (Figure 3). The active layer is about 3 m yyyyyy Patterned Ground Type thick at this site. The new pattern was carefully mapped yy and the edges marked with 10 cm nails. Remapping was carried out each year until 1983. Since surface yyyyyy 7000 yyy expansion ceased in 1981, telescoping heave probes 7000 were emplaced to 80 cm depth in August 1983, one near k e yyyyyy yyy the centre of a pattern and one in the adjacent undis- e N r C turbed soil. With them were installed access tubes for a neutron probe and ground temperature cables. The fol- u a yyyyyy e yy t lowing monthly measurements were made for the next a l P 10 months: moisture content (neutron probe), soil den- sity (gamma probe), soil temperature, snow depth, and yyyyy 7500 y 8000 relative height of the segments of the telescoping 7500 probes. Thereafter, the relative positions of the telesco- yyyy ping probes were measured each July for three years to Sorted Circles track the movements taking place. yy Sorted Nets yyyy yy yy y Block Slopes Results Sorted Stripes yyyy yy y Disturbed Material Where the ground was disturbed by human activity, yyy yyyyy Alpine Meadow Tundra nonsorted circles appeared. Since disturbances only 8000 Roads started in 1950, these are new landforms that formed yyyyy under existing climatic conditions. They show the usual Roads yyy characteristic decrease in mean diameter and elonga- 0 1 km tion with increasing slope, but the average diameter on yyyyy yy a 0-1% slope is only 150 cm (mean of 51 samples) com- yyy yy pared with 320 cm (mean of 51 samples) for sorted pat- 7000 7500 7000 8000 terns in the same area. The material in which the non- yyyy sorted circles develop shows no signs of sorting. 7500 yy yyy 7500 yy yy Between 1974 and 1978, the nonsorted circles appeared to be inactive, but new ones appeared in 1979 Figure 3. Distribution of the different types of sorted patterned ground in the as the mean annual air temperature decreased. They alpine zone of Plateauyyy Mountain (modified from Woods, 1977). develop as tabular surfaces of bare ground rising about have resulted in several producing wells and under- 2-5 cm above the surrounding disturbed soil. Often se- ground gas pipelines, and the disturbed sites have been veral centres coalesce to form one large composite pat- smoothed out by bulldozers and left to revegetate natu- tern (Figure 4).