Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

sion in the Mayan of the Petén noticed that the soil was stripped down to
bedrock on newly cleared slopes in under a decade.
Rates of soil formation in the Central American jungle are far slower
than rates of erosion under Mayan agriculture. The region’s limestone
bedrock weathers about half an inch to five inches in a thousand years. An
average soil depth of about three inches developed on Mayan architecture
abandoned a thousand years ago indicates rates of soil formation similar to
the geologic erosion rate. Both are about a hundred times slower than erosion
from cultivated slopes.
The Mayan heartland was not the only place where soil influenced
Native American civilizations. Soils of central Mexico tell similar stories of
severe erosion on steep hillslopes undermining agriculture.
In the late 1940s UC Berkeley professor Sherburne Cook drove around
the central Mexican plateau and concluded that the land was in poorest
condition in areas that had supported the largest populations before the
Spanish conquest. The thick soil and sod covering uncultivated areas contrasted
with the truncated soil profiles, slopes stripped down to weathered
rock, and thick, artifact-rich valley fills derived from former hillslope soils
that characterized more densely populated areas. Cook saw evidence for
two periods of erosion, an ancient episode that stripped soils from hillsides
and a more recent episode that entrenched deep gullies into the valley bottoms.
“Evidently the entire range was once cleared for cultivation, abandoned,
allowed to become covered with young forest, and finally, the lower
portion again cleared.” 14 Despite Cook’s revelation, the timing of these
cycles remained uncertain until development of radiocarbon dating in the
1950s.
Analyses of sediment cores from Lake Pátzcuaro, east of Mexico City in
Michoacán, revealed evidence for three distinct periods of rapid soil erosion.
The first period accompanied extensive land clearance about 3,500
years ago shortly after maize cultivation began. The second period of high
erosion occurred in the late preclassic period between 2,500 and 1,200 years
ago. The third erosional period peaked immediately before the Spanish
conquest, when up to a hundred thousand people lived around the lake.
Despite the introduction of the plow, soil erosion rates dropped as diseases
decimated the region’s population after Cortez arrived in ad 1521.
Just as in ancient Greece and around the Mediterranean, cycles of soil
erosion in different parts of central Mexico occurred at different times and
so were not driven by changes in climate. For example, in the Puebla-
Tlaxcala area of the central Mexican highland, accelerated erosion of hill-
graveyard of empires 77