Cahokia Mounds Photo Gallery
Earthworks of Eastern North America

Cahokia Mounds State Historic Site, National Historic Landmark, and World Heritage Site

monk's mound at cahokia
Monk's Mound, the largest-in-volume prehistoric earthen construction in the Americas. Monks Mound is one of 69
remaining mounds at Cahokia, most inside the 2,200 acre park preserving the central section of the largest
prehistoric city north of Mexico and the largest ancient settlment in North America in its time.

monk's mound at cahokia

Monument Location as Evidence of Geodesy

Monks Mound may be situated in relation to a geodetic attribute, the latitude tangent equals 0.80 (Monks Mound's geodetic axis is the hypotenuse of a 4:5 triangle). Considering why sites may have been positioned where they're situated arose in relation to Cahokia after an author attributed Cahokia's placement to the nearby 60 degree solstice angle. Cahokia was even termed, "The City of the Sun." While considering this idea, initially to quantify the temporal change in illumination angle, I assessed the geodetic properties of the Monks Mound latitude and discovered the '4/5 atan' latitude.

The latitude where tangent equals precisely 0.80 is 38.65981 degrees. Today atop Monk's Mound the latitude, GPS mean of four corner readings of upper terraces, equals 38.66052 degrees N. (tan = 0.800020). Given the formula for secular polar motion, I deduced the monument was positioned where latitude tangent equals precisely 0.80 around 1200 AD. This date is close to the radiocarbon date of the last constructs on the mound—the radiocarbon date and the polar motion dating agree. Current local plate motion modeling methods suggest less north-south motion. The current atan 4/5 latitude transects Woodhenge and matches the primary axis of E-W mounds.

Monk's Mound latitude

Tectonic plate motion alters the actual latitude of almost all geodetic positions on the planet, very slowly changing latitude at a local rate. The change in latitude due to plate and polar motion is very slow compared to the temporal change in solar and lunar illumination angles, a function of latitude combined with the angle of inclination of the axis of rotation (obliquity). These temporal changes provide possible date-reaching mechanisms. Monks Mound illustrates the possible utility of site latitude both as evidence of geodetic knowledge and as a date-reaching method.

The possibility that solar illumination geometry was the reason for Monks Mound's placement, contrastingly, is a very imprecise match at best. And the displaced position of the Woodhenge center post (discussed below) indicates the builders understood that the local solstice swing was greater than 60 degrees. The latitude where the solstice angle equals 60 degrees is south of Cahokia, and was moreso earlier in time. As earth's obliquity angle changes, the latitude at which the solstice angle equals 60 degrees moves about 150 feet a year and was even further south during the prehistoric period.

cahokia painting Cahokia Mounds State Historic Site, painting by William R. Iseminger.

cahokia latitude illustration

The south face of Monks Mound.
The south face of Monk's Mound.

Looking north across Cahokia's plaza to Monk's Mound.
Looking north across the plaza to Monk's Mound.

mound 72 at cahokia

cahokia mounds

Cahokia Mounds State Historic Site, National Historic Landmark, and World Heritage Site

woodhenge and monk's mound at cahokia
Looking east towards Monks Mound from Woodhenge.

woodhenge and monk's mound at cahokia

Woodhenge, a 125 m circle of evenly-spaced and cardinally-aligned posts, evidences an alignment to solstice sunrises with displacement of the center post. At the latitude of Cahokia, the solstice azimuth spans near 1/6th of circumference; the precise latitude of this coincidence is south of Cahokia. Woodhenge's center post is displaced eastward so the two posts at 30 degrees north and south of due east align to a wider angle than centerpoint angles, to the winter and summer solstice sunrises.

Warren Wittrey wrote articles in The Cahokian after discovering the even-spacing and circularity of the large Woodhenge pattern and doing further excavations and research. Study of post moulds in this area west of Monks Mound revealed five successive circles or partial circles build beginning around 1100 CE.

Interpretive Center. Cahokia Mounds State Historic Site, National Historic Landmark, and World Heritage Site.
The Interpretive Center. Cahokia Mounds State Historic Site, National Historic Landmark, and World Heritage Site.

Monument Coordinates, WASS-enabled GPS, WGS84
Code Location Latitude Longitude
cah28 Cahokia Mound 28 38.65897222 -90.05355556
cah41 Cahokia Mound 41 38.65980556 -90.06513889
cah42 Cahokia Mound 42 38.65994444 -90.06805556
cah44 Cahokia Mound 44 38.65977778 -90.07361111
cah48 Cahokia Mound 48 38.65791667 -90.06558333
cah49 Cahokia Mound 49 38.65758333 -90.06313889
cah51 Cahokia Mound 51 38.65833333 -90.05980556
cah55 Cahokia Mound 55 38.65625000 -90.05988889
cah56 Cahokia Mound 56 38.65588889 -90.06269444
cah59 Cahokia Mound 59 38.65444444 -90.06444444
cah60 Cahokia Mound 60 38.65433333 -90.06308333
cah67 Cahokia Mound 67 38.65275000 -90.06861111
cah68 Cahokia Mound 68 38.65216667 -90.06877778
cah72 Cahokia Mound72 38.65116667 -90.06372222
woodh Woodhenge Circle 38.65991667 -90.07502778
mm34c Monks Mound Terrace 3 & 4 38.66052083 -90.06207639
mm3c Monks Mound Terrace 3 38.66029861 -90.06213194
mm4c Monks Mound Terrace 4 38.66075000 -90.06204167

Cahokia Mounds Google Earth placemarks file has all GPS readings.

cahokia kmz file with GPS readings

Special thanks are due William R. Iseminger, archaeologist at the Cahokia Mounds State Historic Site and leading authority on the Cahokia civilization, for permission to use his painting of ancient Cahokia.

Cahokia Mounds State Historic Site - UNESCO World Heritage Centre


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