Great Basin Naturalist

Volume 47 Number 4 Article 10

10-31-1987

Small-stone content of Mima mounds of the Columbia Plateau and Rocky Mountain regions: implications for mound origin

George W. Cox San Diego State University

Christopher G. Gakahu Moi University, Eldoret, Kenya

Douglas W. Allen San Diego State University

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Recommended Citation Cox, George W.; Gakahu, Christopher G.; and Allen, Douglas W. (1987) "Small-stone content of Mima mounds of the Columbia Plateau and Rocky Mountain regions: implications for mound origin," Great Basin Naturalist: Vol. 47 : No. 4 , Article 10. Available at: https://scholarsarchive.byu.edu/gbn/vol47/iss4/10

This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. SMALL-STONE CONTENT OF MIMA MOUNDS OF THE COLUMBIA PLATEAU AND ROCKY MOUNTAIN REGIONS: IMPLICATIONS FOR MOUND ORIGIN

George W. Cox', Christopher G. Gakalui", and Douglas W. Allen'

Abstr.\ct —Mima moundfields were investigated at the Lawrence Memorial Grassland Preserve, located on the Columbia Plateau in southern Wasco County, Oregon, and at three locations in the San Luis Valley and Sangre de Cristo Mountains, southern , to test the alternative hypotheses of mound origin by erosion, frost action, and soil translocation by geomyid pocket gophers. The concentrations of two size classes of small stones, gravel (8-15 mm diameter) and pebbles (15-50 mm diameter), were sampled along mound-to-intermound transects and at different depths within the mounds. Numbers and masses of small stones per unit soil volume increased from intermounds to mound tops at the Colorado sites and from mound edge to mound top at the Oregon site, where thin intermound soils lay directly on the weathering surface of basalt bedrock. Numbers and masses of small stones in the surface soil of mound tops were greater than or similar to concentrations in deeper layers. Mean masses of individual pebbles were greater in the intermound zone than in mound soils at the Oregon site, but did not differ along mound-intermound gradients at the Colorado sites. Ratios of gravel to pebbles varied significantly along the mound-intermound gradient at the Oregon site and at one Colorado site, being highest at mound edges or in intermounds. These observations support the hypothesis that mounds are formed by centripetal translocation of soil by geomyid pocket gophers, and are contrary to predictions based on theories assuming erosion or frost action to be the mechanism of mound formation.

In western North America, earth mounds, leys and basins and on plateaus and mountain which reach about 25 m in diameter and 2 m in meadows from eastern Idaho and southwest- height and are commonly known as Mima ern Montana south through northeastern mounds, occur in many locations from south- Utah and (R. Reider, personal com- ern Canada to northern Mexico (Cox 1984a). munication) to Colorado (Murray 1967, Vitek The density of ranges 1 to mounds from about 1978) and northern (J. D. Vitek, 3 per ha in localities in the Great Plains and to personal communication). more than 50 per ha in many localities in Three major hypotheses have been sug- . The material forming these gested for the origin of mounds in the interior

mounds consists largely of soil and small montane region of North America: (1) water

stones (up to about 50 mm in diameter) but erosion, (2) periglacial freeze-thaw dynamics,

includes few stones of larger size, although and (3) soil translocation by geomyid rodents. these may be abundant in intermound areas. Waters and Flagler (1929) postulated that Mounds of similar nature also have been re- the mounds of the Columbia Plateau resulted ported in East Africa (Cox and Gakahu 1983, from the erosion of a volcanic ash layer laid 1987), South Africa (Lovegrove and Siegfried down over the surface of basaltic rock, the 1986), and Argentina (Cox and Roig 1986). intermound zones constituting "erosion fur- In the interior montane region of western rows." Fosberg (1965) suggested that the North America, Mima mounds occur from stone nets often associated with Columbia southern British Columbia, Canada (O. Slay- Plateau mounds were formed by frost-sorting maker, personal communication), to central processes, and that soil material deposited Sonora, Mexico (Hill 1906). They are very over this system was eroded to leave mounds widespread on the Columbia Plateau of east- within the stone polygons. The erosional hy- ern , north central Oregon, and pothesis was also supported by Knechtel southwestern Idaho (Freeman 1926, Fosberg (1962) and Washburn (1980). 1965, Kaatz 1959, Malde 1961, 1964, Waters Others have regarded the mounds, as well and Flagler 1929). In the Rocky Mountain as the sorted stone nets often associated with region of the United States they occur in val- them, to be a periglacial phenomenon. Kaatz

'Department of Biolog> , San Diego State University, San Diego, California 92182. ^Department of Wildlife Management, Moi University, Box .3900, Eldoret, Kenya.

609 610 Great Basin Naturalist Vol. 47, No. 4

(1959) suggested that moundfields were a climate of this region is cold and semiarid, thermokarst landscape, with the mounds rep- with annual precipitation averaging 280 mm. resenting the centers of former ice-wedge The vegetation of the mounds is dominated by polygons. Malde (1961, 1964) and Brunn- Idaho fescue {Festuca idahoensis) and blue- schweiler (1962) concluded that the mounds bunch wheatgrass (Agropyron spicatum), and were formed in the late Pleistocene by pro- that of the intermounds by Sandberg blue- cesses of freeze-thaw and solifluction. In the grass {Poa sondbergii), scabland sagebrush Sangre de Cristo Mountains of southern Colo- (Artemisia rigida), bitterroot (Lewisia re- rado, Frederking (1973) interpreted the diviva), and several species of biscuitroot {Lo- mechanism of formation of mounds in lower matium spp.). The northern pocket gopher alpine tundra areas to be frost heave, soil (Thomomys tolpoides) is abundant at this site. creep, and solifluction. This area was studied between 24 and 28 May Finally, the Dalquest and SchefiPer (1942) 1986. hypothesis that Mima-type mounds form by In southern Colorado three sites, all origi- the centripetal translocation of soil resulting nally investigated by Vitek (1978), were stud- from outward tunneling of pocket gophers ied. These sites span a wide range of altitudi- from their centers of activity has been applied nal and climatic conditions. Sampling of these to mounds of this region (Larrison 1942, Price sites was carried out between 30 July and 4 1949, Cox 1983a). August 1986. Cox and Gakahu (1986) derived alternative The Blanca South site (37°20'N, 105°33'W) predictions of the major hypotheses of Mima is located on the floor of the San Luis Valley, mound origin. These predictions pertained to about 13 km south of the community of the small stone content of mound and inter- Blanca, Costilla County, at an elevation of mound soils, and to moundfield geometry. 2,375 m. These mounds range from about 8.4 They tested these predictions against data to 16.8 m in diameter and from 11.4 to 42.5 cm from four Mima moundfields in western in height, and are developed on a residual Washington, central California, and southern sandy loam overlying extrusive basalt California. They concluded that the results bedrock. The arid climate has less than 20 cm strongly supported the pocket gopher hypoth- annual precipitation and is extremely cold in esis of mound origin. winter. The vegetation of the mounds is domi- Our studies extend this test to mounds of nated by winterfat (Eurotia lanata) and blue the Columbia Plateau of eastern Oregon and grama (Boutelouo gracilis), with snakeweed to mounds of valley floors, upland mesas, and (Giitierrezia sarothrae) and globemallow alpine tundra in southern Colorado. (Sphaeralcea coccinea) increasing in impor- tance in intermound areas. The valley pocket

Procedure gopher (Thomomys bottae) is common at this location. Study Areas The Mosca Flats site (37°46'N, 105°23'W) is In Oregon we investigated moundfields on located 9 km west of Red Wing, in western and adjacent to the Lawrence Memorial Huerfano County at an elevation of 2,800 m. Grassland Preserve (hereafter, Lawrence Mounds at this location range from 8.2 to 13.0 Preserve), a Registered National Natural m in diameter and from 20 to 71 cm in height. Landmark owned by the Nature Conser- Soils are sandy loams developed on Quater- vancy, near Shaniko, southern Wasco County nary gravels overlying Tertiary volcanics. (44°57'N, 120°48'W). The mounded portion Mean annual precipitation is probably 20-36 of this preserve is typical "biscuit scabland" cm, and the vegetation of both mound and

(Copeland 1980) and lies at an elevation of intermound areas is dominated by blue grama 1,036-1,060 m on the Shaniko Plateau, and pasture sagebrush (Artemisia frigida). formed of Columbia River basalts. The nu- The northern pocket gopher (T. talpoides) is merous Mima mounds range up to about 1 m abundant at this site. in height and about 20 m in diameter. The The Alpine Ridge site (37°39'N, 105°29'W) mound soils are classified as Condon aeolian lies at an elevation of 3,615 m in a saddle of silt loams and the shallow intermound soils as the main ridge of the Sangre de Cristo Moun- Bakeoven residual, very cobbly loams. The tains on the border of Alamosa and Huerfano October 1987 Cox ETAL.: Mima Mounds 611

counties. Mounds range from 8.4 to 16.8 m in tend to return more gravel than pebbles to- diameter and from 11.4 to 39.4 cm in height. ward the intermounds. Mean pebble masses Soils are residual sandy loams, in this case should be greater in the intermound zone developed on Precambrian metamorphic than in the mounds, but values for mound rocks. Precipitation at this lower alpine tun- edge and mound top should be similar be- dra site is probably in excess of 50 cm. The cause the major transportational bias should vegetation of mounds and intermounds is be exerted during movement of pebbles from dominated by Kobresia myosuroides, with intermound to mound edge. plant cover in the intermounds being sparser Methods and richer in mosses and lichens. The north-

ern pocket gopher is abundant at this site. Four (Alpine Ridge) to six (other sites) mounds were selected at each site for sam- Hypotheses pling small-stone content of mound and inter- Based on the analysis of mound-formation mound soils. The diameters and maximum hypotheses by Cox and Gakahu (1986), we heights of these mounds were measured. postulated the following patterns for small These mounds were chosen because they rock content of mound and intermound soils: were among the largest available and were

Erosion hypothesis. — The concentration surrounded on all sides by intermound flats. of both gravel and pebbles will be greater for On the top of each mound, a 2-m square was intermound and mound edge than for mound marked out, with sampling locations desig- tops because some concentration of these ero- nated at each corner. From these corner sion-resistant elements should occur as the points, transects were paced outward toward fines are removed to reduce the intermound the centers of the four widest intermound surface level. Because the smaller gravel frac- zones and sampling locations designated at tion should be carried away more than the the mound edge (0.5 m inward from the edge pebble fraction by such erosion, the gravel/ proper) and at a point one mound radius be- pebble ratio should be lower for the inter- yond the edge. A total of 12 locations were mound and mound edge than for the mound thus sampled for each mound. At each loca- top. Mean pebble mass should be least on the tion, 1,980 cm samples of the surface (0-10 mound top and greatest at the mound edge cm) material, including stones less than 50 and in the intermound zone. mm in maximum diameter, were collected. At Frost-sorting hypothesis. —The concen- the mound-top locations, pits were dug and tration of gravel and pebbles should increase similar samples taken at 30-40 cm (Colorado from mound centers to the center of the inter- sites) or at 40-50 and 80-90 cm (Oregon site). mound zone because of transport of these Samples were dry-sieved in the field to retain stones to the margins of convectional cells all stones greater than 8 mm in minimum (intermound centers). Because the larger diameter. In the laboratory the small-stone pebbles should be moved more actively, the fraction was separated into two size classes, ratio of gravel to pebbles should be greatest on arbitrarily termed gravel (8-15 mm) and peb- mound tops. The mean size of pebbles should bles (15-50 mm), and the numbers and also increase progressively from mound top to masses of eafch of these components were de- mound edge and intermound center. termined. Because of heavy deposition of FOSSORIAL rodent HYPOTHESIS. —Both caliche in the Blanca South soil, samples of gravel and pebbles should be more concen- small stones were washed for 24 hr in concen- trated on mound tops than at mound edges, if trated HCl before sorting and analysis. This soil and small stones are moved moundward was done to obtain the concentration of ele- by animal activity and if fines are selectively ments influenced by the mound-forming returned toward the intermounds by erosion. mechanism, rather than by the pattern of Concentrations should also be greater at caliche deposition. mound edges than in intermound areas, un- At the Oregon site samples for fine textural less the intermound zone is a strong source analysis were also taken at the three sampling area of weathering rock fragments. Gravel/ depths at one mound-top location on each pebble ratios should not be greatest on mound mound. These samples were analyzed by the tops, however, because erosion should also standard Bouyoucos technique (Cox 1985) to 612 Great Basin Naturalist Vol. 47, No. 4

Table 1. Characteristics of Mima mounds from which soil and small stone samples were collected in north central Oregon (southern Wasco County) and south central Colorado (San Luis Valley and Sangre de Cristo Mountains). October 1987 CoxETAL.: Mima Mounds 613

El Gravel Pebbles 150-

120-

E 90- 3

60-

30- *:

600f-

500-

B 400- w ^ 300-

til 200-

100

Inter- Edge Top Top Top mound 0-10 cm 40-50 cm 80-90 cm

Fig. 1. Total numbers and masses of gravel and pebbles in 1,980 cm^ soil samples from Mima mound tops (0-10, 40-50, and 80-90 cm depths), edges, and intermound zones at the Lawrence Memorial Grassland Preserve, Wasco County, Oregon. Four replicates were taken at each location on each of six mounds.

intermediate depth than at either the surface at Lawrence Preserve, Oregon (Table 2), or the greatest depth. Again, this variation varied significantly along the mound- was more pronounced for pebbles than for intermound gradient (Fgjo =12.8 and 15. 8 for gravel (F^io = 18.1, P < .001), with pebbles numbers and masses, respectively; P < .01 showing more than a 1.5X increase in mass and < .001, respectively), with the highest from intermediate depth to surface. ratios being at the mound edge. Variation Ratios of gravel numbers and masses to along the depth gradient at mound tops was — pebble numbers and masses in the surface soil significant only for ratios of masses (F2jo 614 Great Basin Naturalist Vol. 47, No. 4

Table 2. Mean gravel/pebble ratios for numbers and masses and mean masses of individual gravel and pebble elements from mound and intermound sites at the Lawrence Memorial Grassland Preserve, Oregon (n = 24 in all

Gravel/pebble ratio ± SE Mean mass (g) ± SE Location Depth Mass ratio Number ratio Gravel Pebbles

Mound top 0-10 cm 0.316 ± 0.028 3.544 ± 0.244 0.792 ± 0.019 8.838 ± 0.517 Mound top 40-50 cm 0.499 ± 0,0,58 6.1.58 ± 0.586 0.758 ± 0.029 9.795 ± 0.556 Mound top 80-90 cm 0.564 ± 0.087 6.805 ± 0.806 0.724 ± 0.031 9.396 ± 0.597 Mound edge 0-10 cm 0.4.54 ± 0.036 5.232 ± 0.432 0.725 ± 0.023 8.575 ± 0.346 Intermound 0-10 cm 0.221 ± 0.021 3.340 ± 0.279 0.820 ± 0.027 12.837 ± 0.525 October 1987 Cox ETAL.: Mima Mounds 615

T.-VBLE 3. Results of ANOVA tests of variables relating to small-stone content (gravel and pebbles) of mound and intermound soils at three locations in the San Luis Valley and Sangre de Cristo Mountains, Colorado. Surface positions are mound top, mound edge, and intermound; mound-top depth positions are 0-10 and 30-40 cm. Six mounds were sampled at Blanca South and Mosca Flats, four at Alpine Ridge.

Test 616 Great Basin Naturalist Vol. 47, No. 4

Table 5. Mean masses of individual gravel and pebbles in soil samples from Mima mound tops (0-10 and 30-40 cm depths), edges, and intermound zones at three localities in the San Luis Valley and Sangre de Cristo Mountains, southern Colorado. Six mounds were sampled at Blanca South and Mosca Flats, four at Alpine ridge. Values are derived from four replicate samples from each mound location.

Mean mass (g) ± SE

Locality Rock component Top (0-10 cm) Top (30-40 cm) Edge Intermound

Blanca South Gravel * 0.714 ± 0.008 0.684 ± 0.009 0.706 ± 0.010 0.677 ± 0.010 (n = 24) Pebbles 6.866 ±0.198 7.460 ± 0.292 6.544 ± 0.238 6.683 ± 0.330

Mosca Flats Gravel 0.752 ± 0.056 0.684 ± 0.009 0.711 ± 0.008 0.728 ± 0.012 (n = 24) Pebbles 6.673 ± 0.310 6.420 ± 0,312 7.156 ±0.355 6.355 ± 0.347

Alpine Ridge Gravel 0.707 ± 0.013 0.715 ± 0.009 0.718 ± 0.015 0.758 ± 0.026 (n = 16) Pebbles 7.490 ± 0.205 8.084 ± 0.290 7.117 ±0.360 7.269 ± 0.317

'DF 3,15, F ,3.56, P<. 05

Table 6. Soil textural data for mound depth profiles at the Lawrence Memorial Grassland Preserve, Oregon, and for mound and intermound locations at three sites in the San Luis Valley and Sangre de Gristo Mountains, southern Colorado. October 1987 CoxETAL.: Mima Mounds 617

intermound soil having the highest concentra- mals. However, an argillic B horizon is evi- tion of silt and the surface soil of the mound dent in at least some mounds of this region (R. the highest concentration of sand. Reider, personal communication). Our data are very similar to those obtained by Johnson Discussion (1982) at this same site. Johnson (1982), how- ever, noted that the intermound soils were With respect to predictions of the three somewhat sandier than those of the mounds. hypotheses of mound origin, the trends of The texture of the intermound soil probably increase in total concentrations of gravel and reflects the contribution of coarser compo- pebbles from mound edge to mound top at all nents by weathering of the basaltic bedrock. locations support the fossorial rodent hypoth- At the Colorado sites texture was quite similar esis. The low concentrations of gravel and for both intermounds and mounds. The pebbles in intermound areas at the three Col- higher concentration of sand in mound-top orado sites also support this hypothesis. The soils at Alpine Ridge and the higher concen- very high concentrations of small rocks in the tration of clay in the deeper mound soils at intermound areas at the Oregon site reflect Mosca Flats, however, suggest that some dif- only the shallowness of these soils over the ferential removal of the finer textures occurs weathering surface of the basalt bedrock. by wind and water erosion from the mounds. The increases in small-stone concentration Thus, many of the observed patterns of from deep to surface layers of the mounds at small-rock composition support the fossorial the Colorado locations likewise support the rodent hypothesis, and none supports the ero- fossorial rodent hypothesis, as does the in- sion or frost-sorting hypothesis. Both small- crease in mass of small rocks from intermedi- stone concentration and soil textural patterns ate depth to the surface of mounds at the are also consistent with the hypothesis that Oregon site. The high surface concentrations erosion is presently a mechanism of mound of small stones suggest that movement of soil degradation rather than development. and small stones to the tops of mounds is being Other evidence also argues strongly against offset by erosional removal of soil fines. Thus, freeze-thaw dynamics as a cause of mound erosion now appears to be an agent of mound formation. The suggestion that mounds may destruction. be remnant centers of ancient ice-wedge

The significantly greater change in concen- polygons (Kaatz 1959) is not supported by evi- tration of pebbles than of gravel along the dence of former permafrost, such as ice- mound-intermound gradient at Lawrence wedge casts, from the vicinity of any present Preserve, Blanca South, and Mosca Flats, to- moundfield (Washburn 1980). The hypothesis gether with the significant variation in the that Mima mounds represent some sort of gravel/pebble number ratio at Lawrence Pre- frost-sorting phenomenon is likewise not well serve, also supports the fossorial rodent hy- supported by observations in any present-day pothesis. In no instance, as predicted by the periglacial environments. Most active sorted erosion and frost-sorting hypotheses, did the polygons lack central mounds of appreciable highest values of this ratio occur at mound height and are less than 4 m in diameter tops. The trend of mean pebble mass at (Washburn 1980). The largest sorted stone Lawrence Preserve also agrees with the pre- nets may reach 5-20 m in diameter and have a diction of the fossorial rodent hypothesis. In mounded center up to 1 m above the border- no case was a significant difference in mean ing gutter, but such nets require that the com- pebble mass noted between mound top and mon large clasts in the soil system be 0.5-3.0 mound edge, as predicted by the erosion and m in diameter (Goldthwaite 1976). Even frost-sorting hypotheses. these net dimensions are exceeded commonly Soil textural data from Lawrence Preserve in Mima mound fields, even though clasts of indicate that the mound soils are very high in such size are rarely present. Recent models of silt and clay content, which reflects the high the development of sorted polygons (Gleason loess component of the mound parent mate- et al. 1986), as well, suggest that the width of rial. The lack of strong textural sorting with such polygons should be about 3.6X the depth depth suggests that the mound soils are kept of the active layer of the soil. For the very well mixed by the activities of burrowing ani- shallow soils of most Mima moundfields, this 618 Great Basin Naturalist Vol. 47, No. 4 relationship does not permit the formation of Blanca South, and perhaps Mosca Flats, must mound-intermound units of the order of be somewhat different, however. Blanca 20-30 m or more in diameter. Finally, even South is the driest site at which Mima mounds the large sorted stone circles and nets associ- have been recorded in North America. Al- ated with Mima mounds on the Columbia though the mounds at this location are the Plateau have recently been attributed to the lowest of those at the three Colorado sites soil-mining activities of pocket gophers (Cox examined in this study, they are as sharply and Allen 1987). Thus, we conclude that defined and numerous as those at other Colo- periglacial hypotheses of Mima mound origin rado sites. This suggests that the impetus for are conclusively falsified. their formation is strong, but that their devel- Data on small-stone concentrations in opment in height is limited more severely by mound and intermound soils at these Colum- erosion. Wind erosion appears to be intense at bia Plateau and Rocky Mountain sites are sim- this site, as suggested by the difference in ilar to those of Cox and Gakahu (1986) for sites sandiness of the upslope and downslope sides on the Pacific Coast from southern California of the mound from which texture samples to the Puget Lowlands of Washington. In all, were obtained (Table 6). data from eight Mima mound sites, spanning a It is unlikely that waterlogged conditions wide range of climatic and geological settings, are prevalent for significant periods at the show a consistent pattern of concentration of Blanca South site, which receives less than 20 the small-stone fraction in mound soils. In cm of precipitation annually. This site pos- addition, all of these sites are consistent in sesses a thick, shallow caliche layer. In two showing highest gravel/pebble ratios at inter- intermound pits the surface of this layer lay at mound or mound edge locations, rather than 29-36 cm. On unmounded alluvial flats im- on mound tops, as predicted by physical hy- mediately below the study area, rock-free, potheses of mound origin. friable soil extended to a depth of 60 cm in a Data for the Colorado sites differ from those single test pit. Similarly, at Mosca Flats low of the Pacific Coast sites (Cox and Gakahu annual precipitation and good drainage proba- 1986) and our Oregon site in showing no varia- bly prevent prolonged waterlogging of the soil Shal- tion in mean pebble mass along the mound- (J. D. Vitek, personal communication). intermound gradient. This apparently reflects lowness of the surface soil, per se, seems to the deficiency of heavy rock fragments with favor the formation of Mima mounds at these maximum diameters less than 50 mm in the sites. The shallowness of intermound soils intermound soils at the Colorado sites. Mean may expose pocket gophers to high predation masses of pebbles at these locations ranged risk by animals such as badgers and coyotes, from 6.4 to 7.3 g (Table 5), compared to values or to exposure to severe winter cold, which of roughly 9- 14 g for intermound soils at other characterizes the San Luis Valley. Because sites. Mima mounds are absent from shallow desert The impetus for formation of Mima mounds soils within the range of pocket gophers in by pocket gopher activity at Lawrence Pre- much of the Southwest, we suggest that the

serve and Alpine Ridge sites is probably wa- primary advantage of mounds at sites on the terlogging of the shallow intermound soils floor of the San Luis Vafley is reduction in during wet periods of the year. Intermound exposure of pocket gophers to cold. In the soils at these locations are shallow, and pre- deeper soils of mounds, these animals can cipitation levels are high enough that wet con- locate their nests at deeper, more insulated ditions are frequent, especially in spring. In levels. these locations, as well, water erosion proba- bly exceeds wind erosion and may be the pri- Acknowledgments mary physical factor limiting height develop- ment of the mounds. Selective erosional Catherine MacDonald, Oregon Land Stew- transport of silt and clay fractions from ard for the Nature Conservancy, gave permis- mounds to intermounds probably accounts for sion for studies at the Lawrence Memorial the sandier texture of mound-top soils at Grassland Preserve and furnished back- Alpine Ridge. ground information on this site. Annan Priday The impetus for Mima mound formation at gave permission for sampling of mounds on October 198' CoxETAL.; Mima Mounds 619

property adjacent to the preserve. Donald B. Dlxon, W J , AND M B Brown, eds 1979. BMDP-79. Biomedical computer P-series. Univer- Lawrence provided valuable advice on the programs sity of California Press, Berkeley. work at the Lawrence Preserve. D. John Freeman, OW 1926. Scabland mounds of eastern Wash- Vitek provided extensive background infor- ington. Science 64: 450-451. mation and unpublished data on soil textures Fosberg, M a. 1965. Characteristics and genesis of pat- terned ground in Wisconsin time in a soil for the Colorado sites and, together with Mark chestnut in southern Idaho. Soil Sci. 99: 30-37. S. Gregory, assisted with field work at these Frederking, R L 1973. Spatial variation of the presence locations. Malgorzata Zalejko carried out and form of earth mounds on a selected alp sur- much of the laboratory analysis of samples face, Sangre de Cristo Mountains, Colorado. Un- from the Colorado sites. Richard Reider and published dissertation, University of , Iowa City. John D. Vitek gave criticism and suggestions Gleason. K. G.. W B Krantz. N Caine. J H George, on an earlier draft of the manuscript. We and R. D Gunn 1986. Geometrical aspects of thank all of these individuals for their help. sorted patterned ground in recurrently frozen soil. This study was supported by NSF grant INT- Science 232: 216-220. 8420336 and by grant 211 187 from the San GoLDTHWAiT, R P 1976. Frost-sorted patterned ground: a review. Quaternary Res. 6: 27-35. Diego State University Foundation. Hill, R T 1906. On the origin of small mounds of the lower Mississippi Vallev and . Science 23: 704-706. Lite MTU RE Cited Johnson, C. B 1982. Soil mounds and patterned ground of the Lawrence Memorial Grassland Preserve. Brunnschweiler, D 1962. The periglacial realm in Unpublished thesis, Oregon State University, North America during the Wisconsin glaciation. Corvallis. Biuletyn Peryglacjalny 11: 15-27. Ka.\tz, M. R. 1959. Patterned ground in central Washing- CoPELAND, W. N. 1980. The Lawrence Memorial Grass- ton: a preliminary report. Northwest Sci. 33: 145- land Preserve: a biophysical inventory with man- 156. agement recommendations. Unpublished report Knechtel, M M 1952. Pimpled plains of eastern Okla- (revised 1983), Oregon Chapter, Nature Conser- homa. Geol. Soc. Amer. Bull. 63: 689-700.

vancv, Portland, Oregon. Larrison, E J. 1942. Pocket gophers and ecological suc- Cox, G W 'l984a. Mounds of mystery. Nat. Hist. 9.3(6): cession in the Wenas region of Washington. The 36-45. Murrelet 23: 35-41.

1984b. The distribution of Mima mound grass- LovEGROVE, B G , AND W R. SIEGFRIED 1986. The distri- lands in San Diego County, California. Ecology bution and formation of Mima-like earth mounds 65: 1397-1405. in the western Cape Region of South Africa. S. Afr.

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