1491_ New Revelations of the Americas Before Columbus - Charles C. Mann [149]
I call Cahokia a city so as to have a stick to beat it with, but it was not a city in any modern sense. A city provides goods and services for its surrounding area, exchanging food from the countryside for the products of its sophisticated craftspeople. By definition, its inhabitants are urban—they aren’t farmers. Cahokia, however, was a huge collection of farmers packed cheek by jowl. It had few specialized craftworkers and no middle-class merchants. On reflection, Cahokia’s dissimilarity to other cities is not surprising; having never seen a city, its citizens had to invent every aspect of urban life for themselves.
Despite the nineteenth-century fascination with the mounds, archaeologists did not begin to examine Cahokia thoroughly until the 1960s. Since then studies have gushed from the presses. By and large, they have only confirmed Cahokia’s status as a statistical outlier. Cahokia sat on the eastern side of the American Bottom. Most of the area has clayey soil that is hard to till and prone to floods. Cahokia was located next to the largest stretch of good farmland in the entire American Bottom. At its far edge, a forest of oak and hickory topped a line of bluffs. The area was little settled until as late as 600 A.D., when people trickled in and formed small villages, groups of a few hundred who planted gardens and boated up and down the Mississippi to other villages. As the millennium approached, the American Bottom had a resident population of several thousand. Then, without much apparent warning, there was, according to the archaeologist Timothy R. Pauketat of the University of Illinois at Urbana-Champaign, what has been called a “Big Bang”—a few decades of tumultuous change.
Cahokia’s mounds emerged from the Big Bang, along with the East St. Louis mound complex a mile away (the second biggest, after Cahokia, though now mostly destroyed) and the St. Louis mounds just across the Mississippi (the fourth biggest). Monks Mound was the first and most grandiose of the construction projects. Its core is a slab of clay about 900 feet long, 650 feet wide, and more than 20 feet tall. From an engineering standpoint, clay should never be selected as the bearing material for a big earthen monument. Clay readily absorbs water, expanding as it does. The American Bottom clay, known as smectite clay, is especially prone to swelling: its volume can increase by a factor of eight. Drying, it shrinks back to its original dimensions. Over time the heaving will destroy whatever is built on top of it. The Cahokians’ solution to this problem was discovered mainly by Woods, the University of Kansas archaeologist and geographer, who has spent two decades excavating Monks Mound.
To minimize instability, he told me, the Cahokians kept the slab at a constant moisture level: wet but not too wet. Moistening the clay was easy—capillary action will draw up water from the floodplain, which has a high water table. The trick is to stop evaporation from drying out the top. In an impressive display of engineering savvy, the Cahokians encapsulated the slab, sealing it off from the air by wrapping it in thin, alternating layers of sand and clay. The sand acts as a shield for the slab. Water rises through the clay to meet it, but cannot proceed further because the sand is too loose for further capillary action. Nor can the water evaporate; the clay layers atop the sand press down and prevent air from coming in. In addition, the sand lets rainfall drain away from the mound, preventing