Ceremonial Road Discovered in the Ancient City of Cahokia.

A new study published in the Midcontinental Journal of Archaeology revealed a major ceremonial road running through the heart of Cahokia. Such a road has been the subject of debate and conjecture since the 1920s, and confirming its existence changes our understanding of the prehistoric city. 

A reconstruction of Cahokia with Monk's Mound in the distance.

The ancient indigenous people city of Cahokia near Collinsville, Illinois, is known to have been one of the most sophisticated pre-Columbian settlements north of Mexico. At its peak, it was home to 20,000 people and sprawled over 4,000 acres.

Cahokia was once composed of a collection of agricultural communities across the Midwest and Southeast starting around 800 AD and flourishing between the 11th and 12th centuries. It is a striking example of a complex chiefdom society, with many satellite mound centers and numerous outlying hamlets and villages. It was also a place where the indigenous people made pilgrimages for special spiritual rituals linked to the origin of the cosmos. At its peak, Cahokia boasted some 120 mounds, the largest of which is a ten-story earthen colossus known as Monks Mound. The giant mound is the largest prehistoric earthwork in the Americas, covering over 12 acres and is 100 feet high. An estimated 78,500 square feet of earth was used to build the mound between 800 and 1,350 AD, but not long after this time, Cahokia was mysteriously abandoned. 

The newly discovered ancient road, dubbed the "Rattlesnake Causeway," is an elevated embankment about 60 feet wide that stretches from Cahokia's Grand Plaza south through the center of the city, where it dead-ends in the middle of the burial feature known as Rattlesnake Mound.

Monks Mound with reconstructed stairs in a 2007 photo; repairs done to the mound at Cahokia, in Collinsville, Illinois, in 2005 shored up the mound and kept it from further collapse.

Dr. Sarah Baires of the University of Illinois has suggested that the causeway may have been a literal and symbolic centerpiece of the city, as it is aligned 5° east of north, forming a central "axis" around which the community seems to have been built. Previous research had indicated that the city's major mounds, plazas, and households were oriented along this 5° alignment. Now it appears the causeway marked the axis itself.

Previous research has suggested that Cahokia's buildings align with a celestial event known as the "major lunar standstill," when the moon rises at its southernmost point in the sky. The event occurs once every 18.6 years, and, as seen from Cahokia's Grand Plaza, it is visible over the bluffs south of Rattlesnake Mound, where the causeway ends.

Dr. Baires has suggested that the road's relationship to some of the city's most important mortuary mounds is a key to understanding its purpose. For example, Rattlesnake Mound is a major burial mound with at least 140 individuals buried there, and midway down the road's length is Mound 72, the site of hundreds of burials, including mass graves of sacrificial victims. Baires said that these spatial relationships suggest that the Rattlesnake Causeway served as a conduit between the realms of the living and the dead.

Compiled by Dr. Neil Gale, Ph.D.

How Waterways, Glacial Melt, and Earthquakes Realigned Ancient Rivers and Changed Illinois Borders.

From about 1673 until 1783, Illinois was known as the Illinois Country (Fig. 1) and the Illinois Territory from 1809 until statehood in 1818. 

(Fig. 1) Original proposed Illinois borders within the Illinois territory. A future addition to Illinois from the future state of Wisconsin.

In the 17th century, the French-built trading forts in the Illinois Country. Louis Jolliet and Father Pierre Marquette suggested a canal from the Illinois River to Lake Michigan to eliminate the portage at Mud Lake. But the canal was never built by the French. At the end of the French and Indian War in 1763, the area was ceded to the British and was then awarded to the new United States by the Treaty of Paris (1783). When the borders of Kentucky and Indiana were established, they formed Illinois' southern and eastern borders (the Ohio and Wabash Rivers and to 42°35" north latitude line, which extended between the Wabash River and the southernmost tip of Lake Michigan). The proposed northern boundary in an 1817 plan considered by the US Congress (derived from the Northwest Ordinance) was a straight line from the southernmost tip of Lake Michigan (in Indiana) to the Mississippi River just south of the Rock River confluence with the Mississippi River.

Nathaniel Pope, Illinois Territory Delegate in the United States Congress, proposed modifying the northern border by moving it 51 miles north for economic reasons and giving Illinois access to Lake Michigan, the Great Lakes, and the St. Lawrence Seaway. Another reason for the northern border move was unstated but was related to slavery. After the Missouri Comprise of 1820, Illinois would become a northern state and a vital part of the Union by 1860. While many in southern Illinois were sympathetic to the Confederate cause during the Civil War (1861-1865), most of the state of Illinois was not.

Many inhabitants living in the northern Illinois Territory (later Wisconsin) objected to the movement of the north boundary, the loss of the Lake Michigan waterfront and the location of a shipping port. The land, water, and population loss delayed Wisconsin's development for 30 years, and Wisconsin finally became a state in 1849. With the help of his brother Senator John Pope of Kentucky, Nathaniel Pope got Congress to move the northern boundary to its present-day location (Fig. 2).

(Fig. 2) Ancient Mississippi River location east of Quad Cites between the Rock and Green rivers to Illinois River and south to St. Louis. Location of the land additions to Illinois from the future states of Iowa and Missouri.

Adding 5,440,000 acres also raised the population to (nearly) 40,000, which was required for statehood. Illinois became a state in 1818. The port area on Lake Michigan became the future town of Chicago (Chicagou) in 1833 (Chicago incorporated as a city in 1837). It linked the two shipping routes with a portage between a small river that drained into Lake Michigan with the Illinois River. In 1848, the Illinois and Michigan Canal was completed, allowing the shipment of goods between the two waterway systems. With tensions rising and Civil War a possibility, the canal provided the Union with a northern route to ship goods without using the Ohio River. After the railroad and canal connected Lake Michigan to the rest of the state, Chicago grew incredibly fast. Chicago is the largest city in Illinois, and the greater Chicago area includes three-quarters of the state's population. The ceding of 8,500 miles of territory and the lakefront property on Lake Michigan by the US Congress to Illinois due to Nathaniel Pope's efforts altered the fortunes of Wisconsin and Illinois. Due to the northern boundary shift, the 5,440,000 acres added to Illinois include very productive soils.

During the Pleistocene Era (2.6 million years ago until about 11,700 years ago), numerous glacial advances covered most of Missouri and Illinois, with the two most recent designated as the Illinoian and Wisconsinan glaciations. Melt waters from these glaciers contributed to the re-alignment of the Mississippi River. The western boundary of Illinois was the Mississippi River (Fig. 2). However, before the Pleistocene glacial period, the ancient Mississippi River passed much farther to the east, as shown by the blue dashed lines. Today's lower Illinois River follows its course. The Wisconsin glacier eventually blocked the ancient Mississippi River, and its terminal moraine (point of furthest advance southward of a glacier) was about 12,000 to 15,000 years ago. The ancient Mississippi River then re-aligned itself to its current position, later used as the western border when Illinois became a state. If the Mississippi River had not been re-aligned, the 7.5 million acres (Fig. 2) would belong to the conditions of Missouri and Iowa. Before 1803, the French controlled the land west of the current Mississippi River and was part of the Louisiana Purchase that year. After Iowa and Missouri became states, they had a border dispute settled by the US Supreme Court. The border between these two states was primarily the 40°35" latitude line, which, if extended into the current area of Illinois between the Illinois and Mississippi rivers (Fig. 2), would determine the acreage each state would have gained if the ancient Mississippi River had not re-aligned. A total of 3.5 million acres would have gone to Missouri and 4 million acres to Iowa. This area includes some of Illinois's most productive soils for corn and soybean production.

Further to the south, the Mississippi River (just south of current Cape Girardeau, Missouri) was re-routed (Fig. 3) at the end of the Great Ice Age. After the last glacial advance, the melting ice flooded and altered the course of many channels and streams, including the Mississippi and Ohio rivers. Approximately 12 to 15 thousand years ago, scientists believe that the Ohio and Mississippi rivers changed course (Fig. 3) south of Cape Girardeau, Missouri.

(Fig. 3) The re-alignment of the Mississippi River south of Cape Girardeau, Missouri. The bedrock lined the Mississippi River channel near Thebes, Illinois.

The 6-mile stretch of the Mississippi River near Thebes, Illinois, is unique. It is the only Mississippi River section in a narrow bedrock-lined valley with rock underlying the navigation channel. Some geologists believe heavy seismic activity along the Commerce Geophysical lineament (a northeast-trending magnetic and gravity feature that extends from central Arkansas to southern Illinois) about 12,000 years to 15,000 years ago created a fault that helped the Mississippi River cut through the "Thebes gap" [1] and made a new confluence 25 miles north of the current confluence, where the River switched from a braided, meandering river to one that cut through rock. The Mississippi River currently forms the state boundary between Missouri and Illinois. 

At Thebes, the Mississippi River is now located 30 miles to the east (Fig. 3) of where the ancient Mississippi River flowed. Before the 20th century, the Mississippi River migrated rapidly by eroding the outside and depositing on the inside of a river bend. Numerous oxbow lakes [2] mark old positions of the channel that have been abandoned. Early Holocene (the term given to the last 11,700 years of the Earth's history) to late Wisconsin liquefaction (conversion of soil into a fluid-like mass during an earthquake or other seismic events) features in western lowlands were induced by a local source, possibly by the Commerce fault (which is north of New Madrid Fault) as a result of earthquake upheaval along the Commerce Geophysical lineament running from central Indiana to Arkansas.

The New Madrid area has been the center of seismic activity for thousands of years, affecting the Mississippi River and perhaps the Ohio River re-routing. The land has rebounded by as much as 13 feet in 1,000 years after the last glacial period. The previous significant seismic activity resulted from an earthquake in 1450-1470 AD and another earthquake in Cahokia, Illinois, in 1811-1812.

Floodwaters of the ancient Mississippi River did not initially pass through this relatively narrow channel and valley. Instead, they were routed by the bedrock-controlled uplands near Scott City, Missouri, and north of Commerce and Benton, Missouri (Fig. 3) to an opening in the upland ridge 40 miles to the southwest. Then the River turned back to the south and merged with the ancient Ohio River near Morely, Missouri. Once floodwaters of the Mississippi River (from the north) and Ohio River (from the east) could cut a valley trench along a fault and through the bedrock-controlled upland west of Thebes. As a result of the Commerce fault, the distance the Mississippi had to travel was shortened from 50 miles to 6 miles. The two historic rivers also once joined at Malden, Missouri; however, the location of the confluence continued to change over time and is now located south of Cairo, Illinois, at Fort Defiance State Park [3]. The confluence of these two mighty rivers created a very rapidly changing channel. It appears that the bedrock-controlled upland was worn away by both rivers after seismic activity. The creation of the Commerce fault contributed to the opening of the bedrock-controlled channel (Fig. 3) after the last glacial advance, approximately 12,000 years to 15,000 years ago.

The modern-day Cache River Valley of southern Illinois (Fig. 4) has a string of tupelo-cypress (trees) swamps, sloughs, and shallow lakes, remnants of the ancient Ohio River whose confluence with the Mississippi River was once northwest of Cairo, Illinois. 

Cache River Valley on the Ohio River in Illinois.

The ancient Ohio River Valley, 50 miles long and 1½ to 3 miles wide, was formed by the meltwaters of northern glaciers as they advanced and retreated in numerous iterations over the last million years. The Mississippi River flowing southward from Minnesota was (and is today) a meandering river of oxbows and cut-offs, continuously eroding banks, re-depositing soil, and changing paths. Its historic meandering is particularly apparent in western Alexander County, Illinois, where topographical maps show oxbow swirls and curves, and Horseshoe Lake, where the ancient Mississippi River once flowed (Fig. 4).

(Fig. 4) The location of the ancient Cache River valley and ancient Ohio and Tennessee Rivers.

The upland hills of the Shawnee National Forest just north and west of the town of Olive Branch and north of Route 3 give way to a low-lying plain between the Mississippi and Ohio rivers. Historically this region has been a delta, confluence, and bottomlands dating back 30,000 to 800,000 years BP (Before Present: where "present" is defined as 1950 AD), with many of Illinois lands shown on the maps located on both sides of the Mississippi River as its channel changed positions over time. As a result, the fertile farmland soils of western Alexander County formed in alluvial (clay, silt, sand, gravel) and lacustrine (sedimentary rock formations which formed at the bottom of ancient lakes) deposits.

Hydrologically, the Ohio River is the main eastern tributary of the Mississippi River. Today it runs along the borders of six states 981 miles west from Pittsburgh, Pennsylvania, to the Mississippi River confluence at Cairo, Illinois and drains lands west of the continental divide from the Appalachian Mountains encompassing all or part of 14 states. The Ohio River, a southwestern flowing river, was formed between 2½ and 3 million years ago when glacial ice-dammed portions of north-flowing rivers.

About 625,000 years ago, the ancient Ohio River, fed by Kentucky's Green and Cumberland rivers, flowed through the Cache River Basin and was smaller than the current Ohio River. The Wabash River (Indiana) had yet to form at that time. The Tennessee River was not a tributary of the Ohio River but formed the main channel before the later Ohio River appeared.

During the Woodfordian period (75,000 to 11,000 years ago), the floodwaters from the historic Ohio River watershed drained into eastern Illinois via Bay Creek (Fig. 4) to the northwest and then west through the Cache River Valley through present-day Alexander County, Illinois, where it converged with the Mississippi River near Morely, Missouri, located west of the Horseshoe State Conservation area. The middle Cache River Valley is 1.3 miles wide due to the previous River having been much larger since it carried waters from the ancient Ohio River Valley and the local waters from the upper Cache River Valley to the Mississippi River.

Extensive deposits of gravel and sand, some as deep as 160 feet, rest on the bedrock floor of the middle and eastern portions of the valley and offer evidence of glacial flooding which carved the valley deeply into the bedrock and then, as the water receded, back-filled the valley with sediments. With increasing sediment fill and climate changes, the ancient Ohio River shifted away from the Cache River Valley and into its present course. This event probably took place between 8,000 and 25,000 years ago. As a result, the Cache River became a slow-moving stream with extensive isolated, low swampy areas with a water table that ebbed and flowed with seasonal precipitation.

The upper and middle sections of the Cache River Valley, the Main Ditch, and Bay Creek are located in the ancient Ohio River Valley, where river water crossed through the state of Illinois approximately 10-20 miles north of the present Ohio River position. The Cache River Valley is deeper at a lower elevation (between 320 and 340 feet) than expected in a slow-moving swampy river system. The New Madrid Fault runs under and near Karnak and Ullin, Illinois, and the Cache River Valley elevation does not fit with the rest of the area. Steve Gough, a land-use change-over-time expert, has suggested a large section under the Cache River Valley sank during a significant earthquake in about 900 AD. The cypress trees in the Cache River Valley swamps are up to 1,000 years old, which would be consistent with this time estimate.

(Fig. 5) The additions and subtractions to Illinois. The orange area is the net border of Illinois without all the Mississippi and Ohio rivers re-routing and the decision to provide Illinois with Lake Frontage on Lake Michigan and connecting waterways.

If all these waterway-related changes had not occurred, the state of Illinois would only have 22 million acres, much smaller than the current 35 million acres (Fig. 5). All but one of the changes would have made Illinois 40% smaller and reduced the current population by more than 80%, since Chicago and Rockford would be in Wisconsin, Cairo, and Metropolis in Kentucky, Quincy in Missouri, and Rock Island, Moline and Peoria in Iowa. Borders such as the Mississippi and Ohio rivers, which were naturally re-aligned, dramatically changed the size and shape of Illinois. Clearly, the location of these waterways matters.

Compiled by Dr. Neil Gale, Ph.D.

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Additional Reading:

The History of Ancient Lake Chicago and Today's Lake Michigan.

Illinois on the west side of the Mississippi River - Missouri on the east side of the River.

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[1] Just south of Cape Girardeau, Missouri, the Mississippi River cuts a seven-mile gorge through the thick limestone of the Shawneetown Ridge. The gorge, known as the Thebes Gap or the Grand Chain, is as narrow as 3,000 feet in places and was notoriously difficult to navigate.

[2] An oxbow lake is a U-shaped lake that forms when a wide meander of a river is cut off, creating a free-standing body of water.

[3] Fort Defiance, known as Camp Defiance during the American Civil War, is a former military fortification located at the confluence of the Ohio and Mississippi rivers near Cairo in Alexander County, Illinois.