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DIVISION S-5—PEDOLOGY

Accelerated Soil Erosion of a Mississippian Mound at Cahokia Site in Illinois

^a Dep. of Natural Resources and Environmental Sciences, W-401c Turner Hall, 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL, 61801
^b Moscow State Univ., Faculty of Geography, Moscow, 119899, Russia
^c Dep. of Geography, Southern Illinois Univ., Edwardsville, IL 62026

* Corresponding author (k-olson1{at}uiuc.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS REFERENCES

 
About 1000 years ago prehistoric Indians built over 120 earthen mounds, including Mound 57, at the Cahokia site near Collinsville, IL. Loamy and clayey soil materials were apparently placed on the mound before 1100 CE by people of the Middle Mississippian culture. The objectives of this study were: (i) to determine the extent of soil formation during the past 850 yr, (ii) to determine the time period, extent, and direction of tillage, and (iii) to determine the erosion phase of soil on a mound and the extent of soil loss from erosion. The high amount and regular decrease in organic C with depth in 16 soil profiles suggested that since 1925 no tillage and little soil erosion had occurred on the sideslopes of M57. In the sideslope and footslope positions on the north, south, and east transects, the fine-loamy soil materials with fly ash were thickest suggesting that any tillage translocation and soil erosion must have occurred after 1864. This resulted in the current elongated south to north axis which was parallel to the tract boundary and the direction of plowing. The amounts and distribution of the fly ash in the upper 20 cm indicated that cultivation between 1864 and 1925 may have mixed some fly ash into the 20-cm deep tillage zone. Fly-ash content of soil layers to the 40-cm depth on the footslope was primarily a result of fly ash being deposited after the 1850s on the original soil surface and the subsequent deposition of sediment rich in fly ash.

Abbreviations: E, east • M57, Mound 57 • N, north • S, south • W, west

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS REFERENCES

 
ABOUT 1000 YEARS AGO, prehistoric (Mississippian) Indians built over 120 earthen mounds at a site named Cahokia located near Collinsville, IL. These earthen mounds, including M57, are near where the Illinois and Missouri Rivers enter the Mississippi River. Many of the earthen mounds left behind by the Mississippian Indians have been destroyed by two centuries of urban development and agricultural expansion. The centerpiece of the Mississippian occupation was the Cahokia Mounds (Fowler, 1997) now partially preserved as Cahokia Mounds State Historic Site. Cahokia was a site where elite rulers and religious leaders governed thousands of farmers, hunters, traders, and artisans who populated the vast Mississippi River floodplain around the site and who provided the labor to build the monumental earthen mounds. All were supported by the rich natural resources of the Mississippi River Valley and, especially, by the crops they grew, including maize (Zea mays L.). The area flourished between about 1000 and 1400 CE (Fowler, 1997); however, a short time later the Mississippian Indians suddenly abandoned their mounds. The final causes of the collapse of the Mississippian civilization are still the subject of considerable debate, but war, fire, food shortages, pollution of land and water, and diseases most likely contributed.

The soil materials on M57 are estimated to have been carried in baskets by the Mississippian Indians until 1350 CE (Fowler, 1997; Holley et al., 1993). The surrounding Grand Plaza surface (1100 CE) was used by Mississippian Indians for approximately two centuries (Holley et al., 1993). The nearby 30-m high Monks Mound was built during the same time period. Later, Indians from the Illini Confederacy occupied the Cahokia site, including the plaza surrounding M57 and Mound 48, from 1730 until 1752 (Fowler, 1997). M57 would have been vegetated by grass and trees for the 500 yr before the settlement by Trappist Monks in 1809. In 1864, the Ramey family purchased the land which included M57, the eastern side of Mound 48, and Monks Mound. About the same period of time the Merrell family acquired the adjacent Tract 57 (within a few meters) to the west of M57 which included most of nearby Mound 48 (Fig. 1) (Fowler, 1997). Both properties were sold to the state of Illinois in the 1925. M57 had a height of 3 m on the 1882 McAdams map, 1.5 m on the 1894 Thomas and 1906 Peterson–McAdams maps and 0.8 m on the 1966 University of Wisconsin–Milwaukee map (Fowler, 1997). There was no record of M57 being destroyed or deliberately truncated. Fowler (1997) suggested that intense cultivation (plowing down) had leveled the mound. An 1892 photo from the Missouri Historical Society showed cultivated fields (plowed in a south-north direction) to the south and west of Monks Mound (Fowler, 1997). The 1922 Goddard air photos of the east and west half of Cahokia pictured M57 in a cultivated field. Contrast lines in the photos suggest field operations were in a general south-north direction. This mound was apparently cultivated by the Ramey family (Fig. 2) between 1864 and 1925, the year it was acquired by the state of Illinois. Since its purchase, it had been primarily in grass.

View larger version (25K): [in this window] [in a new window]   Fig. 1. Location of Mound 57 (M57) at Cahokia Site near Collinsville, Illinois.

View larger version (27K): [in this window] [in a new window]   Fig. 2. Mound 57 (M57) soil sampling locations and tract boundary at the Cahokia Site.

There has been is an increasing awareness that erosion, primarily responsible for the severe degradation occurring in topographically complex landscapes, is not only attributable to wind or water erosion, but also to tillage translocation or tillage erosion (Revel and Rouaud, 1985; Revel et al., 1990, 1993; Govers et al., 1994; Guineese and Revel, 1995; Poesen et al., 1997; Quine et al., 1997). Tillage translocation (or tillage erosion) is a progressive downslope movement of soil caused mechanically by tillage implements. It is considered a main cause of land degradation and abandonment in hilly cultivated areas. Availability of more powerful machinery after 1900 favored deep soil plowing at higher speeds in a direction, usually, perpendicular to the contour lines. This resulted in displacement of huge amounts of soil materials from the upper convex part of a hill (summit, shoulder, backslope) to the concave parts (footslope or toeslope). Tillage translocation exposes subsoil or subsurface layers that may be highly erodible by wind or water acting as a sediment delivery mechanism. Tillage translocation has often been considered the main cause of soil movement from the upper hill areas downslope to lower land. Lobb and Lindstrom (1999) reported that soil loss because of tillage erosion exceeded 15 kg m^-2 yr^-1 in the Great Lakes region. Factors affecting tillage erosion, include the tillage instrument, plow depth, wheel speed of tractors (or horses), soil moisture content, slope gradient, and direction of tillage operation (Mech and Free, 1942; Lindstrom et al., 1992; Govers et al., 1993, 1994). Gerontidis et al. (2001) found downslope displacement of soil during tillage increased with greater plow depth and slope gradient. In steep hillslope positions, the maximum soil displacement was 50% less after plowing the soil along contour lines instead of in the same slope direction. A 75% increase in soil displacement occurred when the depth of plowing was increased by 50%.

Magnetic susceptibility is the magnetizing ratio of material to the magnetic field inducing it. For soil, it is the result of contributions of susceptibilities from the large number of different substances in soil. Diamagnetic substances, such as organic matter and quartz have low and negative mass magnetic susceptibilities. The mass magnetic susceptibility in paramagnetic substances, such as clay minerals with transitional elements in their structure, are weakly positive. Antiferrimagnetic substances such as hematite, goethite, and lepidocrocite are examples of paramagnetic minerals with weak positive susceptibilites. Magnetite and maghemite are ferrimagnetic substances with strong positive magnetic susceptibility (McBride, 1986; Mullins, 1977). Magnetic susceptibility can also be used for detecting erosion or deposition on a soil surface. The magnetic susceptibility of sediment deposited is different from that of the original surface, as in the case of a buried soil (Fine et al., 1992).

Fly ash is particulate matter resulting from high temperature combustion of coal. It is produced in a variety of boilers, including those in steam locomotives and steam-powered farm machinery. The principal minerals in coal of the central USA are feldspars, pyrite, siderite, quartz, calcite, and clay minerals, mostly kaolinite (Harvey and Ruch, 1986). The minerals are vitrified above 1473 K, and a common product is spherules composed of glass, quartz, mullite, wustite, and magnetite (Huffman and Huggins, 1986). Until recent technologies allowed removal of fly ash from stack gases emanating from boilers, the ash was a component of smoke and was deposited over a wide area around the sources, especially near coal-fired power plants. Sphere occurrence and abundance have been used to identify sediments laid down during the industrial epoch (Locke and Bertine, 1986). The occurrence of ferrimagnetic minerals, usually embedded in the glassy phase, offers a convenient way to separate the spheres for identification and analysis.

The use of fly ash provides a time marker extending back to at least 1850, which in central and northern Illinois coincides with European settlement and initial cultivation. However, in the American Bottom (wide Mississippi River bottom located west of St. Louis, MO), the French established a mission and agricultural settlement at the village of Cahokia in 1699 after which there had been continuous European presence. Fly ash related to steam-locomotive use in Illinois was widespread, inasmuch as installed track increased from initial construction in the 1850s to 16000 km in the 1920s (Daniels, 2000). Some fly ash may be derived from steam-powered farm equipment. Threshers and self-propelled tractors, the latter coming into greatest use from 1880 to about 1920 (Wik, 1953), would produce magnetic fly ash, their boilers being fed by soft (bituminous) coal. For economic and supply reasons, however, many steam boilers on farms were fired with straw or wood which probably produced highly siliceous, glass-phase fly ash.

Jones and Olson (1990) found fly-ash spheres present in large amounts in soils and sediments near urban areas and in smaller quantities in rural locations in central Illinois. Fly ash had the potential of being an easily identified time marker in sedimentation studies. Fly ash present in soils can be used to identify sediments accumulated since construction of the railroads and to interpret the stratigraphy of sediments and their relationships to underlying soils. The usefulness of fly ash incidence in soil profiles will be aided by study of carefully chosen sites near sources of the ash, e.g., undisturbed areas in cemeteries first used in the mid-19th century that are adjacent to and downwind from railroads or rural power plants. Fly-ash particles, especially those in the silt- and sand-size range, are not prone to easy eluviation. Their distribution in the soil may give valuable information regarding biotic and abiotic mixing and pedoturbation of subsurface horizons since settlement. Knowledge of mixing characteristics will help interpret evidence of erosion, or accretion, on soil profiles, as well as the time frame for these events.

In southern Illinois, Hussain et al. (1998) found higher amounts of fly ash in A horizons at an uncultivated site than at a paired cultivated site for all landscape positions. These findings indicated the removal of approximately 47% or 10.6 cm of the original surface layer in 142 yr. Fly ash represented a better indicator of soil erosion than organic C, magnetic minerals, or magnetic susceptibility. Olson et al. (2002) used content of fly ash, magnetic susceptibility, magnetic minerals, and organic C as indicators of soil erosion at a site near Moscow, Russia. Replicated transects in cultivated and reforested sites were sampled and analyzed. The entire cultivated landscape contained 12% less fly-ash content and had 12% less magnetic susceptibility than the reforested site. These results suggested the loss of 2.4 cm of original A horizon and the pedon placed in the slightly eroded phase of the soil (Soil Survey Staff, 1993). Organic C and magnetic mineral data supported this placement.

The objectives of this study were: (i) to determine the extent of soil formation during the past 850 yr; (ii) to determine the time period, extent and direction of tillage; and (iii) to determine the erosion phase of mound soil and the extent of soil loss from erosion.

	MATERIALS AND METHODS

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Soil profile descriptions were made from three 3-cm diam. soil cores that were spaced 1 m apart and collected from each sampling site using hand soil augers to approximately a 1.2-m depth (ranging from 0.3 to 2.0 m). It should be noted that the soil sampling sites were on the Cahokia Mounds State Historic site and under terms of a sampling permit issued by Illinois Historic Preservation Agency, investigators were not allowed to dig soil pits, collect larger soil samples or take any soil cores with diameters larger than 3 cm. Figure 2 shows the location of the 16 sites that were sampled using three 3-cm diam. soil cores from each site. After describing, the three soil cores were cut into 5- to 10-cm segments and composited for laboratory analysis. Samples were dried, sieved to pass a 2-mm sieve and analyzed for pH, organic C, sand, silt, and clay contents, magnetic susceptibility, magnetic minerals, and fly-ash contents (Soil Survey Staff, 1984). Organic C was determined by the Walkley-Black procedure (Nelson and Sommers, 1982); the coefficient of variation for 20 duplicates was 7%. The magnetic minerals and fly ash content were determined using procedures developed by Jones and Olson (1990), updated by Hussain et al. (1998) and revised by Olson et al. (2002). Subsamples of fine-earth samples were split and then ground to <250 µm using a mortar and pestle. Magnetic susceptibility was subsequently determined with a modified Gouy balance (model MK I #6632, Johnson Matthey Wayne, PA). The coefficient of variation of the susceptibility determination for seven duplicates was 3%.

Statistical analyses were performed using Statistical Analysis System (SAS) computer software (SAS Institute, 1995). An LSD procedure was used at the P = 0.05 level to determine if there were significant organic C, pH, sand, silt, clay, magnetic susceptibility, magnetic mineral and fly ash differences between different landscape positions for the same depth.

	RESULTS

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Sixteen soil profiles (Table 1) from four transects [south (S), north (N), east (E), and west (W)] were described on M57 with profile M57-1N, M57-1S, M57-1E, and M57-1W on the summit (Fig. 2), M57-2S, M57-5N, M57-8E, M57-11W on the middle of the sideslope (in S, N, E, and W directions, respectively), M57-3S, M57-6N, M57-9E, M57-12W on the footslopes (in S, N, E, and W directions, respectively), and M57-4S, M57-7N, M57-10E, and M57-13W on the plaza (plain) (in S, N, E, and W directions, respectively).

View larger version (49K): [in this window] [in a new window]   Fig. 3. Cross sections of Mound 57 (M57) soil layers: (a) south to north cross section and (b) west to east cross section.

The depth of soil formation as expressed as the maximum extent of cambic horizon development (as indicated by Bwg or 2Bwg horizonation and subangular blocky structure in Table 1) were 75 to 120 cm deep on all landscape positions including the summit with 1% slope, sideslope with 4% slopes, footslope with 2% slopes, and plaza with <1% slopes. A cambic horizon (identified with the "w" subscript) is present in all pedons and all but two pedons fit in the fine family (Table 1) except for the north transect sideslope (5N) and footslope (6N) pedons that were fine-silty. Apparently, more of the original surface fine-loamy materials from the summit and upper sideslope were transported by cultivation (from south to north) and erosion by wind and water to the lower sideslope and footslope of the mound.

On the summit (1N), the 90- to 120-cm layer was identified as a buried (2Abg) horizon. Organic C content increased from 7 g kg ^-1 in the 90- to 100-cm layer to 8 g kg ^-1 in the 100- to 110-cm layer to 10 g kg ^-1 for the 110- to 120-cm layer (Table 2). The depth to the buried (2Abg) horizon when present is shown in Fig. 3. The buried (2Abg or Abg) horizon was observed under the summit at a depth of 90 cm (Table 1), the south sideslope (2S) at a depth of 100 cm, the south footslope (3S) at a depth of 70 cm, and the plaza (4S) at a depth of 70 cm. A buried (2Abg) horizon of M57 appears to be at a 75-cm depth on the west sideslope (11W), and at 82 cm on the footslope (12W), and in the current topsoil layer (elevated organic C levels) at the plain or plaza (13W) could include the previous soil surface layer. The buried (2Abg or Abg) horizon is absent on the north sideslope and the north and east footslopes and plain positions. Perhaps the original fine-textured surface topsoil on these positions was used in the construction of the mound (Fig. 3).

Thickness of the fine-loamy material in the mound (Table 1) above the buried fine-textured material was 60 cm on the summit (one pedon), 59 cm on the sideslope (mean of four pedons), 74 cm on the footslope (mean of four pedons) and 52 cm on the plaza (mean of four pedons). The summit (128.5 m) is 90 cm higher than the surrounding plaza 127.6 m (Fig. 3). The north plaza (7N) is higher (Fig. 3) than the south, east, and west plazas (4S, 10E, and 13W) and may have received some sediment from Mound 48 which is to the north of M57 (Fig. 1). M57 appeared to have been built on a Darwin soil (fine, smectitic, mesic Fluvaquentic Vertic Endoaquoll). The surrounding soil may have been excavated from north and east of M57 (Fig. 3) since the buried topsoil horizons are missing and the mound material immediately above the positions with a buried (Ab) horizon was fine-textured. At a later time (1100 CE), the Grand Plaza (Holley et al., 1993) was apparently constructed by covering the remaining soil surface with 45 cm of fine-loamy materials (Fig. 3). The fact that the footslopes have thicker fine-loamy materials present than the summit, sideslopes and plaza could be a result of tillage (prior to 1925), erosion and soil deposition. The sediment would then increase the thickness of fine-loamy material (on the footslope) over the buried fine-textured material (Darwin soil) under the plaza. Closer examination of the fine-loamy materials above the buried fine-textured material (Table 1) on the summit (100 cm), the sideslope (120+ cm on north transect, 60 cm on the south transect, 90 cm on the east transect, and 60 cm on the west transect) and footslope (93 cm on north transect, 80 cm on the south transect, 80 cm on the east transect, and 40 cm on the west transect) suggests that the fine-loamy material was moved as a result of tillage from the south to north direction. The exposed surface soil layers would have been eroded by water or wind action with sediments deposited on footslope. The build up of sediment would have elongated the mound (Fig. 3) by approximately 6 m (59.1 m vs. 52.9 m). Elongation of M57 (Fig. 2) in the general south to north direction is consistent with this direction of plowing and the location of the tract boundary. Figures 2 and 3 show the shape of M57 and the approximate tract boundary between the Ramey and Merrell tracts. The property boundary was oriented 16° to the east of magnetic north. This tract boundary and the direction of plowing paralleled the axis of M57 (Fig. 2).

Moldboard plows can also translocate soil materials up the slope if the furrow slice of soil was rolled up hill from a lower position. Plowing mostly from the south to north direction would cause soil displacement in the east direction from the summit, east sideslope, and east footslope. However, when plowing the west sideslope or footslope, soil materials would be returned to a higher elevation. Plowing of the west side of the mound in a south to north direction (16° east of magnetic north) would transport the fine-loamy materials higher up on the west side of the mound and reduce the amount of fine-loamy sediment accumulated on the west sideslope and footslope (Table 1). The presence of the boundary between Ramey and Merrell tracts (Fig. 2) could have restricted sediment movement from M57 to the west since most tract boundaries often included a fence that resulted in a low berm. The 1922 Goddard photo (Fowler, 1997) showed the boundary with shrubs or small trees on the tract boundary.

Any evidence in the profile of the previous abrupt plow layer boundary (between soil layers) (Table 1) was altered by rooting during the last 75 yr. The organic C content (Table 2) decreased substantially with depth (for the 0- to 5-, 5- to 10-, and 10- to 15-cm layers) in all 16 soil profiles which is consistent with a lack of recent tillage (most likely since 1925).

In the four summit soils (Table 2) organic C decreased regularly for the upper 20 cm. This same trend occurs for the four soil layers of the four sideslope, four footslope, and four plaza soils. Elevated organic C levels could most likely only be achieved under grass vegetation for many years (probably 75 yr) without tillage translocation and erosion. Also, the organic C level of the 40- to 50-cm layer of all 13 soils averaged 9 g kg^-1 which supported evidence of the presence of grass roots for many years.

Table 3 shows values for the 0 to 20 cm of each transect observation and the mean of the landscape position. Organic C levels were equivalent by landscape position for the summit, sideslope and footslope, the pH is similar (5.6–5.8 range). The sideslope had a slightly higher mean clay content and slightly lower mean sand content than the other two positions.

In 1925, after 65 yr of cultivation, the sideslope pedons would most likely have fit in a moderately eroded phase of the mound soil (fine, smectitic, mesic Typic Endoaquoll) as a consequence of the increased clay content (Table 3) and deposition of sediment rich in fly ash on the footslope soil profiles (Table 5). When a topsoil layer is thinned as a result of erosion, moldboard plowing can move subsoil materials higher in clay to the surface and increase the clay content of the plow layer. The higher clay content of the 0- to 20-cm layer of the sideslope (Table 3) most likely resulted from plowing subsoil materials into the topsoil. The current sideslope profile would be in the slightly eroded phase of the mound soil. Apparently, 75 yr of grassland vegetation was sufficient to restore the amount and vertical distribution soil organic C (Tables 2 and 3). The amount and distribution of fly ash and magnetic minerals in the surface layer are similar for the summit, sideslope, and footslope profiles (Table 4) which supports the placement of the current sideslope pedon in the slightly eroded phase of the mound soil. The fly-ash content and magnetic susceptibility of the plain (plaza) position (Table 4) was lower than in other positions as a result of the 13W site having a low fly-ash content, possibly because of soil removal during construction of surface ditch along the current west field boundary. The fact that the fly-ash content of all pedons on the summit, sideslope, and footslope have similar fly-ash contents was surprising. The surficial 20-cm of sediment rich in fly ash found on the footslope would have been eroded from the summit and sideslopes of the mound. During the 65 yr of cultivation, soil material rich in fly ash was translocated from the summit to the sideslope and from the sideslope to the footslope. This movement of topsoil and sediment rich in fly ash by tillage translocation and erosion could have resulted in a net loss of fly ash on the original summit, maintained the fly-ash content on the sideslope, and increased the fly-ash amount on the footslope. The size of the summit would have been reduced and the footslope would have enlarged by sediment building up on the surrounding plaza. Between 1925 and the 1960s, all of the landscape positions were in grass and would have received the same amount of fly ash from the air. This resulted in the upper 10 cm of the mean surface layer having more fly ash (Table 5) than the underlying 10 cm layer.

In summary, the high subsoil pH values between 6.0 and 6.2 (medium to slightly acid) suggested that the mound has been under grass vegetation for part of the last 850 yr. The high amount and regular decrease in organic C with depth in 16 soil profiles are consistent with the mound under grass vegetation since 1925 with no tillage and little soil erosion of the sideslope. Soils on the summit had thicker A horizon materials and mollic epipedons than soils on the sideslope, footslope, and plaza. Fly-ash presence indicated which soil layers were exposed at the soil surface since the 1850s and the extent of soil mixing. In the sideslope and footslope positions on the north, south, and east transects, fine-loamy soil materials with fly ash were thickest suggesting that any soil erosion and tillage translocation occurred after 1864 and resulted in the current elongated north to south shape. The field-tract boundary was oriented 16° to the east of magnetic north which is parallel to the current axis of M57 and the previous direction of plowing. Fly-ash amounts and distribution of the fly ash in the upper 20 cm suggested that cultivation by the Ramey (or Merrell) families between 1864 and 1925 may have mixed some fly ash into the 20-cm deep tillage zone. Fly-ash content of soil layers to the 40-cm depth on the footslope was primarily a result of initial fly ash being deposited after 1850s on the original soil surface and the subsequent deposition of sediment rich in fly ash. In 1925, after 65 yr of cultivation, the sideslope pedons would most likely have qualified in the moderately eroded phase of the mound soil as a consequence of increased clay content and deposition of sediment rich in fly ash on the footslope soil profiles. However, the current sideslope pedons would be in the slightly eroded phase of the mound soil. Apparently, 75 yr of grass land vegetation was sufficient to restore the amount and distribution soil organic C. The amount and distribution of fly ash in the surface layer was similar for the summit, sideslope, and footslope profiles supporting placement of the current sideslope pedons in the slightly eroded phase of the mound soil.

	ACKNOWLEDGMENTS

 
Published with the approval of the Director of the Office of Research at the University of Illinois. Authors gratefully acknowledge the funding assistance (linkage grant no. 972219) from the Environmental Security Division of NATO. Additional funds for this work were provided by the SIUE College of Arts and Sciences and Graduate School. We thank the Illinois Historic Preservation Agency for granting permission to collect soil samples from M57.

Received for publication November 1, 2001.

	REFERENCES

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