THE BLUE MOUND STONE WALL:  A MINNESOTA STONEHENGE?


Mark Hollabaugh
Department of Physics and Astronomy
Normandale Community College
9700 France Avenue South
Bloomington, Minnesota 55431
Hollabaugh@aol.com

Copyright © 2002 by Mark Hollabaugh.  All rights reserved.  This article may not be copied, duplicated, or distributed without the permission of the author.


Abstract
    An east-west oriented stone wall at Blue Mounds State Park in southwestern Minnesota has evoked speculation as an equinox marker.  The origin of the wall is enigmatic.  Prior research has focused on the wall and its east-west orientation.  None of the prior research looked for anticipatory horizon sun watching markers.   A site visit did not reveal any obvious markers or locations that might have been used for sun watching.

Introduction
     Minnesota’s Blue Mounds State Park  offers a unique blend of topography, geology, plants and animals, and archaeology.  Located within the park is an east-west oriented stone wall of uncertain origin.  Because of the celestial orientation with the equinox sunrise and sunset azimuths, the wall has been dubbed the “Minnesota Stonehenge.”  Some research as been done, but the reports are generally not readily accessible to the archaeoastronomer.  The prior research tended to focus on the wall itself and not on the surrounding context.  In this paper I will discuss previous research on the wall and report on my own visit to the park at the 1999 autumnal equinox.  The central focus will be my examination of possible sites for horizon sun watching, including anticipatory observations.

Blue Mounds State Park
    Blue Mounds State Park is located in southwestern Minnesota about 5 km (3 mi) north of Luverne, Minnesota (Rock County), and 90 km (56 mi) northeast of Sioux Falls, South Dakota.  The “mound” is not associated in any way with the mound builders of the Mississippian culture.  The park takes its name from the appearance of a 30 m (100 ft) high Sioux quartzite cliff on the southeast edge of the park.  The rock itself is pinkish in color, but from a distance the ridge looks dark blue.  Although the Minnesota Department of Natural Resources managed land is known as Blue Mounds State Park, the area of the cliff is simply the “Blue Mound” or the “Mound.”  The park preserves a fragment of the vast tall-grass prairie that covered much of the Great Plains before the incursion of Europeans.  And although one does not think of cactus growing in Minnesota, prickly pear cactus flourishes in parts of the park, and blooms every July.  In 1961, bison were re-introduced to the park and today the Blue Mounds herd numbers about 45.  White settlers moved into the Blue Mound area about 150 years ago.  

    Blue Mound is but one of several archaeological sites in southwestern Minnesota.  About 34 km to the north is the Catlinite quarry at Pipestone National Monument.  Due to the importance of pipe making, this area was a crossroads of Plains Indian culture.  About 100 km to the northeast is Jeffers Petroglyphs State Historic Site.  These glyphs, carved on a Sioux quartzite outcropping flush with the ground, appear to range in age from 3000 B.C. E. to 1750 C.E.  I have visited the Jeffers site and did not see any glyphs of obvious astronomical symbolism.  

    The Blue Mound east-west oriented stone wall has become the source of much speculation.  The stone wall is located in the south end of the park and is approximately 375 m in length.  The approximate mid-point of the wall, where a hiking trail passes through the wall, is located at latitude 43° 41’ 37” N and longitude 96° 11’ 40” W.  (All positions reported in this paper were determined on 11 August 2000 with a Garmin GPS II+ using position averaging.  The “Figure of Merit” for all measurements was about ± 4 m.)  It is important to note the wall is not located on the highpoint of the mound, but about 400 m southwest of that location and 15 m lower in elevation.  The western end of the wall is about 20 m higher in elevation than the eastern end.  The wall is made of local field stone.  The height, now or in the past, is difficult to ascertain, but never is more than about .5 m.  The wall site has been designated 21RK8.  

    The entire mound in the area near the wall is covered with tall grasses, sometimes more than waist high in summer.  In some places, the wall is almost totally hidden by these grasses.  There are a few bushes of various species.  A large number of burr oak to the east and south of the wall cover the top of the cliff.   From the midpoint of the wall, grasses obscure the lower end of the wall, and the burr oak effectively obscure the distant eastern horizon.   The land on which the wall is located was first surveyed in 1870 (Barmore, 1985).   The western end of the rock wall is located about 20 m north of the Interpretive Center.  The Interpretive Center was formerly the home of Minnesota novelist and naturalist Frederick Manfred who speculated about the origin of the wall.
   
    My first visit to Blue Mounds State Park was in July, 1991, when I camped there for a weekend with my outdoor club.  The theme for the weekend was Minnesota archaeology and astronomy.  We also visited the Jeffers Petroglyphs State Historic Site and Pipestone National Monument.  During this visit, I became more curious, hopeful and yet dubious of the claims of an astronomical significance to the wall.  

Previous Research
    Archaeological surveys of the Blue Mounds State Park were undertaken in the 1970’s as a part of the documentation of the park’s resources.  The archaeologists approached the question of the wall’s origin and use with caution.  Hudak (1972) summarized his own work and the work of other archaeologists from the University of Minnesota who surveyed the Blue Mound for culturally significant materials.  He reported a 1971 autumnal equinox observation of the sunrise and notes the only result was to confirm the east-west orientation of the wall.  Hudak ends with a plea:  “More data and less fruitless conjecture is the answer to any possible reconstruction of the prehistory of the Blue Mound area.”

    Radzak (1977) suggested the wall is prehistoric in origin.  However, no cultural materials were found that would allow the wall to be dated or associated with any cultural group.  He notes that older residents in the area have no recollection of the wall being built by white settlers.  The wall points towards a steep drop off that may have been a bison drive area and Radzak says some bison bones were found in the area.  Radzak referred to Hudak’s 1971 equinox observations and agrees this observation only shows the wall lays east-west.  He also noted there are several prehistoric habitation sites within view of the top of the mound.  Site 21RK10, located about 2 km north of the wall near the present site of the park’s campground, did yield archaeological materials but nothing found permitted an identification of age or origin.

    A less academic discussion of the Blue Mound was published in the Minnesota Archaeologist in 1978 (Hudson, 1978).  The article is a reprint of an article that first appeared in the Worthington Daily Globe, Worthington, Minnesota, on 25 October 1975.  A postscript to this article is a lengthy letter to the editor by Frederick Manfred.  Manfred contemplated the origin of the rock wall in his backyard.  His discussion includes such seemingly unrelated observations as the growth of lichens on the rocks, suggesting that the growth patterns suggested something very old.  Noting the size of the boulders in the wall, Manfred proposed “that no farmer or homesteader would have been that crazy to move them.”   Manfred, apparently well-traveled, wrote the wall reminded him of walls built “in the southwestern United States and Maya land.”   On the possibility of the wall being a part of a bison jump, he thought the bison “were driven by strong winds over the cliffs during severe blizzards.”  Manfred was convinced long gone Indians built the wall.

    The notion that the Blue Mound stone wall is a Minnesota Stonehenge may be the result of a 1981 article in Minnesota Monthly , the magazine of Minnesota Public Radio (Novak, 1981).  Novak interviewed Tom Kehoe, a curator in archaeology for the Milwaukee Public Museum.  Kehoe notes that if there is astronomical significance to the wall, it is unlike any other he knows.

    The only thorough analysis of the astronomical significance of the wall was done by Frank Barmore, University of Wisconsin, La Crosse (1985).   Barmore meticulously measured all aspects of the wall with a transit.  He also examined historical records of the land in an attempt to determine if an early white settler in the area constructed the wall.  He notes the wall is constructed just 11 m south of and parallel to a modern fence line that was subsequently removed.  That fence line was assumed to lie along the boundary between two township sections.  In Figure 1 the section line is the brown dashed line lying just north of the wall midpoint position (“Midpoint”).  This section line then parallels Rock County Highway 8.

    Barmore’s most significant observation has to do with the accuracy of the wall as an equinox marker.  He reports that transit sightings along the wall gave an equinox sunrise azimuth between 89° and 92°:  “Since the point of sunrise moves along the horizon about 0° 32’ per day at the equinox at this latitude, an accuracy of about one quarter of this (0° 08’) would be required for adequate alignment.  Considering these points, I believe an accuracy of about one-tenth degree (0° 06’) is required if the equinox, as an astronomical event, is to be marked.  The wall orientation does not meet this standard.”

    Barmore concluded the wall was built between 1870 (when the first land survey failed to note the presence of the wall) and 1936 (when the wall appears on an aerial photograph of the area), and was not intended to mark any particular sunrise.  In descriptive material about the park, the Minnesota DNR avoids any direct suggestion the wall is an equinox marker:  “Who built it and why is unknown.  It is known that on the first day of spring and fall, the sunrise and sunset are lined up on this stone alignment.”

Site Visit
    Nowhere in any of the research reports did I find any investigation of other potential sun watching sites on the summit of the Blue Mound or sunrise markers on the distant eastern horizon.  I was struck by the focus on the wall itself and not on the larger context.  Most research emphasized the alignment of the wall and the search for cultural artifacts that might date it.  Barmore went a step further by examining land office records in an effort to determine if a farmer built the wall.  Were the previous researchers looking for the wrong kind of astronomical alignment?  Zeilik (1983) has pointed out the importance of anticipatory horizon sun watching in the prehistoric southwest.  My own research on Lakota ethnoastronomy suggests they practiced a type of horizon sun watching at the winter solstice (Hollabaugh, 1997).  All this made me wonder if the answer to Blue Mounds’ stone wall mystery lies not in the wall but along the horizon.  Thus, I decided to visit Blue Mound not only to observe the equinox sunrise, but also to see if there were any noticeable horizon sun markers or other possible sun watching stations on the mound.  I did not take a transit to make precise, detailed measurements because I saw this as an exploratory visit.
    
    I made two visits to the Blue Mound in order to satisfy my curiosity.  The first was at the autumnal equinox in 1999.  I visited the stone wall at sunset on 23 September and again at sunrise on 24 September.   I revisited the park on 11 August 2000 in order to obtain more accurate GPS coordinates after the Department of Defense discontinued selective availability.  

    Selective availability is a security technique that introduces errors into the GPS signal so that hostile parties may not obtain accurate position data.  Military issue GPS receivers contain the software and security codes to correct for these errors.  The Garmin GPS II+ uses position averaging to determine a location.  Essentially, the software continuously solves the equations to find the location of the receiver.  The “Figure of Merit” (FOM) represents the standard deviation of the estimated position during this averaging process.  Typically after about 15 minutes of use, the FOM stabilizes at a particular value determined by the complexity of the receiver, number of satellites visible, and the use or non-use of selective availability.  During the equinox visit to Blue Mound, when selective availability was on, the FOM was ± 10.5 m.  When I returned in August, 2000, the FOM was ± 4 m.  Most hand-held GPS receivers used in conjunction with a compass and topographical map should be adequate for a preliminary site survey when this level of accuracy is acceptable.

    Figure 1 is a portion of the Luverne USGS topographical map.  GPS waypoints were downloaded into Maptech® 4.02 Terrain Navigator.  The wall midpoint, the highest point in the park, and the glacial erratic known as Eagle Rock are shown on the map.
I calculated the solstice sunrise and sunset azimuths using “SunTimes for Windows” (Zepher Services, 1995), and the results are shown in Table 1.  I estimate that with a hand held compass, I could observe these directions to the horizon to ±1°.  


Figure 1.  The Blue Mound

    I arrived at Blue Mound on 23 September in time to observe the sunset at 7:21 PM, CDT.  Observations of the sunset at the equinox or solstices are very uninteresting from the wall.   From the midpoint of the wall, the sunset is unrewarding because one is looking uphill.  From the western end of the wall, the view of the western horizon is obscured by nearby shrubs and grasses and a further rise in hill.  Sunset from Eagle Rock, where one has a clear view of the western horizon is more dramatic.  The flat western horizon is interrupted only by constructs of modern civilization.  Figure 2 shows a sunset view of the eastern horizon with the wall in the foreground.  The drying summer grasses effectively obscured the wall.


Figure 2.  Looking east along the wall at the equinox sunset.

    I arrived at the wall to observe the sunrise at 6:30 AM CDT.  The most dramatic aspect of the sky was the presence of Jupiter and Venus in the morning sky.  Early morning clouds obscured the eastern horizon at the moment of sunrise (7:14 AM CDT).  Had the clouds been absent, I still could not have clearly seen the rising sun from the west end of the wall due to trees growing along the eastern edge of the mound.  These trees date from the late 19th to early 20th century.  An elderly local farmer, who has lived within view of the Blue Mound his entire life, told me the burr oak growing on the mound are not native and were planted by local farmers.  He offered this information after seeing my GPS receiver, compass, notebook, and cameras and asking me if I was an astronomer or archaeologist.  Evidently I was not the first curious visitor.

    Later in the morning, I hiked along the edge of the cliff and also followed a trail system (4.5 km) that circles the top of the mound.  The most striking feature on the top of the mound is Eagle Rock, a glacial erratic about 2 m high and 25 m in circumference.  Although the base of the rock is lower than the highest point (elevation 517 m) on the mound, when one stands on the rock, about 2 m in height, the vantage point is indeed the highest point on the Blue Mound.

    I carefully surveyed the horizon from the highest point on the mound, first with my naked eye, then with binoculars, for anything that might have been used as a horizon sun watching marker.  Except for a few silos and trees located on farmsteads, there is nothing dramatic along the horizon between azimuths 55° and 122°.  In fact the most notable horizon feature is the row of wind generators on the Buffalo Ridge (know to geographers as the Coteau des Prairies) some 45 km to the north-northeast.

                                                                       Summer Solstice    Winter Solstice
                                            Sunrise Azimuth            55°                        122°
                                            Sunset Azimuth            304°                       237°

Table 1.  Solstice Sun Azimuths at Blue Mounds State Park


    I also stood on Eagle Rock and took compass bearings to obvious rock outcroppings and the high point of the mound.  I moved to other the outcroppings and “mini-mounds” and repeated the procedure.  Again, there were no lines of sight that seemed significant.   Would a further examination of the top of the Mound reveal anything interesting?  Possibly, but in the absence of other corroborating cultural evidence, I doubt such a study would be fruitful from an astronomical standpoint.

    I wish to report on one interesting coincidence.  As seen from the top of the Blue Mound, the azimuth of the summer solstice sunrise and the direction to the Jeffers petroglyph site are the same.  This is, in all likelihood, only a coincidence.  I report it here hopefully to deter any interpretations to the contrary in the popular mind.

Conclusions
    I find no suggestion at Blue Mound of any horizon sun watching.  There are locations on the top of the Mound that would be much better locations for an east-west equinox marker, yet the rock wall is not located there.   I support Barmore’s conclusion that the east-west rock wall is not astronomically significant.  

    Could someone have stood on Eagle Rock or the high point of the Blue Mound and watched the sunrise?  Probably.  Did a Paleo-Indian, or in more modern times the Lakota or Dakota, observe the moon, the stars and planets from the Mound?  Most likely.  After all, the night sky from the Mound is spectacular.  Did they sight to points on the horizon in order to anticipate the solstice or equinox sunrise?  Maybe, but not likely.  

    If you want to find an alignment you will find it.   Many people wanted to find an astronomical alignment at Blue Mounds State Park.  Many of the local populace still believe they have a Minnesota Stonehenge in their backyard.   But, sometimes a rock wall is just a rock wall.
Acknowledgments

    I wish to acknowledge the helpful comments of Katharine Salter-Godell, Department of Anthropology, Normandale Community College.  Dr. Scott Anfinson, Minnesota Historical Society, provided copies of unpublished materials in the files of the Historical Society.  
 
References

Barmore, Frank L., (1985), “The Blue Mounds Stone Wall:  Astronomical Significance and
Antiquity,” Minnesota Archaeologist, 44:2; 41-49.

Hollabaugh, Mark, (1997), Lakota Celestial Imagery:  Spirit and Sky, presented at the Dakota History
Conference, May, 1997, Sioux Falls, South Dakota.  Paper published in conference Proceedings, published by the Center for Western Studies, Augustana College.

Hudak, Gary J., (1972)  “Notes on the Archaeology of the Blue Mounds State Park,” unpublished
manuscript in the files of the Minnesota Historical Society, St. Paul, Minnesota.  File number RK-72-01.

Hudson, Lew (1978), “Blue Mound’s Stone Fence,” Minnesota Archaeologist , 37:2; 50-55.

Novak, Jay (1981), “Rock County Stonehenge,” Minnesota Monthly, April, 1981, 15:4;8-9,43.

Radzak, Lee, (1977), “Historical Sketch of the Blue Mound Area,” unpublished manuscript in
the files of the Minnesota Historical Society, St. Paul, Minnesota.  

Zeilik, Michael (1983), “Anticipation in Ceremony:  The Readiness is All,” in Astronomy and
Ceremony in the Prehistoric Southwest, Papers of the Maxwell Museum, No. 2, University of New Mexico, 25-42.